razeri_plugin_contract/

renderer.rs

// Copyright (C) 2026 Gabriel Shea
// Licensed under either of the Apache License, Version 2.0 or the MIT
// license, at your option.

//! Layered rendering traits for SQL generation.
//!
//! The hierarchy is designed so that implementing [`SqlRenderer`] (4 methods)
//! gives you ANSI-standard defaults for all higher-level traits.

use crate::DataType;
use crate::bulk_load_form::BulkLoadForm;
use crate::constraint_enforcement::ConstraintEnforcement;
use crate::date_format::DateFormat;
use crate::evolution::{ColumnEvolution, Written, WrittenObject};
use crate::flatten::{Flatten, FlattenRequest};
use crate::large_object::{HeldAs, TextLengthUnit};
use crate::number_format::NumberFormat;
use crate::procedure_log_level::ProcedureLogLevel;
use crate::routine_form::{ProcedureForm, ScalarFunctionForm, TableFunctionForm};
use crate::routine_spec::RoutineSpec;
use crate::sid_spelling::{SidHelperBody, SidSpelling, UNSPELLED_SID};

/// **Where the aggregated expression goes** in a form returned by
/// [`SqlRenderer::median_aggregate_form`].
///
/// The form is a template rather than finished SQL so that *which function*
/// and *how a column is written* stay in different hands — the dialect answers
/// the first, the query renderer the second. A form must carry this token
/// exactly once.
pub const AGGREGATE_ARGUMENT: &str = "{argument}";

/// **What kind of value an aggregate would have to collapse** — the axis
/// [`SqlRenderer::render_single_row_aggregate`] varies on, held to three arms
/// because three is what the measurements distinguish.
///
/// Not the full type lattice: `MAX` behaves identically across every numeric,
/// textual and temporal type on every platform razeri reaches, and splitting
/// them would invite per-type arms nothing has measured.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum AggregableKind {
    /// Numeric, textual and temporal — every platform orders these and every
    /// platform has `MAX` for them.
    Orderable,
    /// A truth value. `PostgreSQL` and SQL Server have no `MAX` for it and each
    /// spells the collapse differently.
    Boolean,
    /// `uuid`, JSON, binary, interval, array, map — the families with no
    /// ordering to borrow. Availability is per platform and is measured, never
    /// assumed.
    Opaque,
}

/// **A clock bound once for a whole load script** — what
/// [`SqlRenderer::render_inline_clock_binding`] answers on an engine whose
/// loads ship as flat statement lists rather than procedure bodies.
///
/// The `statements` run first and capture the engine's current moment under
/// one name; `reference` is the expression every later statement of the same
/// run reads it back by. One binding, many readers, is what makes a version's
/// close and its successor's open the same instant by construction.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct InlineClockBinding {
    /// The statement(s) that bind the moment, in run order, each terminated.
    pub statements: Vec<String>,
    /// The expression that reads the bound moment back.
    pub reference: String,
}

/// **One delimited source file, in every spelling an engine's file relation may
/// ask for it in** — what
/// [`SqlRenderer::delimited_file_relation_for`] is handed.
///
/// One struct rather than several arguments, because a slot that grows an
/// argument is a slot every dialect has to be recompiled for; one that grows a
/// field is not. Each is composed once, by the caller that knows the answer,
/// so no dialect has to take a path apart to find a file name in it.
///
/// A dialect reads the fields. One that builds a value, in its own tests,
/// starts from [`DelimitedFile::new`] and the `with_` methods, which is also
/// why it cannot be written out field by field outside this crate: the next
/// thing Razeri learns to say about a file arrives as a field with a default,
/// and no dialect's tests break for it.
#[derive(Debug, Clone, Copy)]
#[non_exhaustive]
pub struct DelimitedFile<'a> {
    /// The whole path to the file, already a single-quoted SQL string literal
    /// and already escaped. Put it where the relation takes a path, and do not
    /// quote it again.
    pub path_literal: &'a str,
    /// The directory the file sits in, already a single-quoted SQL string
    /// literal — for an engine that resolves a file name against a directory
    /// rather than opening a path.
    pub directory_literal: &'a str,
    /// The file's own name inside that directory, bare: no quotes and no path
    /// in front of it.
    pub name: &'a str,
    /// **The file's columns, in the order the file writes them**, spelled
    /// exactly as its header row spells them and with no quoting applied.
    ///
    /// Empty where the caller measured no header — a bundle assembled by hand.
    /// An engine that cannot read a file without its columns declared answers
    /// `None` to an empty list rather than guessing at one.
    ///
    /// A file with no header line ([`header`](Self::header) `false`) has no
    /// names of its own, and its columns arrive as `column0`, `column1` and
    /// on, counted from zero: the names `DuckDB` gives such a file, and the
    /// names a header cell with no name is read under everywhere.
    pub columns: &'a [String],
    /// **How the file's lines end**, as Razeri measured it when it read the
    /// file: [`LineEnding::CrLf`] for a carriage return and a newline,
    /// [`LineEnding::Lf`] for a newline alone.
    ///
    /// Put it wherever the engine's read names a record delimiter. An engine
    /// told `\n` for a file whose lines end `\r\n` either keeps the carriage
    /// return on the last field, so a name is stored one character too long
    /// and a number cannot be read, or, where the last field is quoted, reads
    /// past the closing quote into the next line. An engine whose read finds
    /// the ending itself can leave this alone, and should say so where it
    /// states its read.
    pub line_ending: LineEnding,
    /// **The character between the file's fields**, as the design declares
    /// it. Never a guess: an engine whose reader would otherwise sniff the
    /// delimiter is told this one.
    ///
    /// The four ASCII separators ([`FileDelimiter::is_control`]) are bytes no
    /// text editor shows, and engines differ in how a statement may name
    /// them: an escape, a hexadecimal literal, a function call, or not at
    /// all. Spell the one your engine was measured to take. Where it takes
    /// none, answer `None`: Razeri then declines the file by name and says
    /// which delimiter your engine does read.
    pub delimiter: FileDelimiter,
    /// **The character a field is quoted with**: `"` unless the design
    /// declares another. A field inside two of them may hold the delimiter
    /// and a line break, and two of them together inside it stand for one.
    pub quote: char,
    /// **Whether the file's first line names its columns.** `true` unless the
    /// design declares `header: false`; a file without one has its first row
    /// of values on its first line, and its
    /// [`columns`](Self::columns) are named by position.
    pub header: bool,
    /// **The encoding to read the bytes as**, where they are not UTF-8: the
    /// one name the encoding standard files it under (`windows-1252`,
    /// `shift_jis`). `None` on the ordinary day, which means UTF-8.
    ///
    /// Razeri asks
    /// [`delimited_file_relation_in_encoding`](SqlRenderer::delimited_file_relation_in_encoding)
    /// whether your engine has a word for the encoding before it hands one
    /// over here, so a dialect that answers that one `None` never meets an
    /// encoding in this field. One that answers it, and also overrides
    /// [`delimited_file_relation_for`](SqlRenderer::delimited_file_relation_for),
    /// reads this field there.
    pub encoding: Option<&'a str>,
}

impl<'a> DelimitedFile<'a> {
    /// **A file at `path_literal`, read the way every engine reads a file by
    /// default**: no columns measured, lines ending `\n`, split on commas,
    /// quoted with `"`, a header on its first line, and UTF-8.
    ///
    /// `path_literal` and `directory_literal` are finished single-quoted SQL
    /// string literals; `name` is the file's own name, bare.
    #[must_use]
    pub const fn new(path_literal: &'a str, directory_literal: &'a str, name: &'a str) -> Self {
        Self {
            path_literal,
            directory_literal,
            name,
            columns: &[],
            line_ending: LineEnding::Lf,
            delimiter: FileDelimiter::Comma,
            quote: '"',
            header: true,
            encoding: None,
        }
    }

    /// The same file, with its columns in the order the file writes them.
    #[must_use]
    pub const fn with_columns(mut self, columns: &'a [String]) -> Self {
        self.columns = columns;
        self
    }

    /// The same file, its lines ending as `line_ending` says.
    #[must_use]
    pub const fn with_line_ending(mut self, line_ending: LineEnding) -> Self {
        self.line_ending = line_ending;
        self
    }

    /// The same file, its fields split by `delimiter`.
    #[must_use]
    pub const fn with_delimiter(mut self, delimiter: FileDelimiter) -> Self {
        self.delimiter = delimiter;
        self
    }

    /// The same file, its fields quoted with `quote`.
    #[must_use]
    pub const fn with_quote(mut self, quote: char) -> Self {
        self.quote = quote;
        self
    }

    /// The same file, with a header line or without one.
    #[must_use]
    pub const fn with_header(mut self, header: bool) -> Self {
        self.header = header;
        self
    }

    /// The same file, its bytes read as `encoding`, or as UTF-8 for `None`.
    #[must_use]
    pub const fn with_encoding(mut self, encoding: Option<&'a str>) -> Self {
        self.encoding = encoding;
        self
    }

    /// **Whether the file is written the way a read that is told nothing
    /// reads it**: split on commas, quoted with `"`, and headed.
    ///
    /// The default [`delimited_file_relation_for`](SqlRenderer::delimited_file_relation_for)
    /// answers only for such a file, because the path-only
    /// [`delimited_file_relation`](SqlRenderer::delimited_file_relation) it
    /// hands over to has no way to be told anything else.
    #[must_use]
    pub fn is_plain(&self) -> bool {
        self.delimiter == FileDelimiter::Comma && self.quote == '"' && self.header
    }
}

/// **The character between a delimited file's fields** — one of the eight a
/// design names with a word, or any other single ASCII character.
///
/// The words are `comma`, `tab`, `pipe`, `semicolon`, and the four ASCII
/// separators `fs`, `gs`, `rs` and `us` (bytes `0x1C` to `0x1F`). A character
/// one of the words names always arrives as that word, never as
/// [`FileDelimiter::Literal`].
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
pub enum FileDelimiter {
    /// `,`
    #[default]
    Comma,
    /// The tab character.
    Tab,
    /// `|`
    Pipe,
    /// `;`
    Semicolon,
    /// The file separator, `0x1C`.
    Fs,
    /// The group separator, `0x1D`.
    Gs,
    /// The record separator, `0x1E`.
    Rs,
    /// The unit separator, `0x1F`.
    Us,
    /// Any other single ASCII character, written as itself.
    Literal(char),
}

impl FileDelimiter {
    /// The character itself.
    #[must_use]
    pub const fn as_char(self) -> char {
        match self {
            Self::Comma => ',',
            Self::Tab => '\t',
            Self::Pipe => '|',
            Self::Semicolon => ';',
            Self::Fs => '\u{1c}',
            Self::Gs => '\u{1d}',
            Self::Rs => '\u{1e}',
            Self::Us => '\u{1f}',
            Self::Literal(c) => c,
        }
    }

    /// The character as the one byte a reader splits on. Every delimiter a
    /// design can declare is ASCII.
    #[must_use]
    pub const fn as_byte(self) -> u8 {
        // ASCII for every value a design can declare.
        self.as_char() as u8
    }

    /// The word a design writes for this delimiter, or `None` for a literal.
    #[must_use]
    pub const fn word(self) -> Option<&'static str> {
        Some(match self {
            Self::Comma => "comma",
            Self::Tab => "tab",
            Self::Pipe => "pipe",
            Self::Semicolon => "semicolon",
            Self::Fs => "fs",
            Self::Gs => "gs",
            Self::Rs => "rs",
            Self::Us => "us",
            Self::Literal(_) => return None,
        })
    }

    /// **Whether this is one of the four ASCII separators**, `0x1C` to
    /// `0x1F` — the delimiters no text editor shows, which an engine's
    /// statement names, if at all, by an escape of its own.
    #[must_use]
    pub const fn is_control(self) -> bool {
        matches!(self, Self::Fs | Self::Gs | Self::Rs | Self::Us)
    }
}

/// **One setting a statement was written with, for the engine it names.**
///
/// Razeri's statement builders carry settings written for one engine — a
/// view's `security_barrier` for `PostgreSQL`, a table's data compression for
/// SQL Server. They reach every dialect tagged with the engine they were
/// written for, as that engine's dialect names itself
/// ([`SqlRenderer::dialect_name`]), and a dialect spells the ones written for
/// it and passes over the rest. Nothing in Razeri reads an engine's name to
/// decide which settings apply.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[non_exhaustive]
pub struct EngineSetting<'a> {
    /// The engine the setting was written for, as its dialect names itself.
    pub engine: &'a str,
    /// The setting's name, in the engine's own words.
    pub name: &'a str,
    /// Its value, as the engine writes it.
    pub value: &'a str,
}

impl<'a> EngineSetting<'a> {
    /// One setting.
    #[must_use]
    pub const fn new(engine: &'a str, name: &'a str, value: &'a str) -> Self {
        Self { engine, name, value }
    }
}

/// **How the lines of a delimited file end.**
///
/// Measured, never assumed: [`LineEnding::of`] reads it off the file's own
/// bytes, and a file with no line break at all reads as [`LineEnding::Lf`],
/// which is what every engine reads by default.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq, Hash)]
pub enum LineEnding {
    /// A newline alone, `\n`: every system but Windows.
    #[default]
    Lf,
    /// A carriage return and then a newline, `\r\n`: a file written on
    /// Windows.
    CrLf,
}

impl LineEnding {
    /// **How the lines of this text end**, decided by its first line break.
    ///
    /// The first break ends the header row, so it is written the way the rest
    /// of the file is. Text with no newline at all is [`LineEnding::Lf`].
    #[must_use]
    pub fn of(text: &[u8]) -> Self {
        match text.iter().position(|&byte| byte == b'\n') {
            Some(at) if at > 0 && text[at - 1] == b'\r' => Self::CrLf,
            _ => Self::Lf,
        }
    }

    /// The ending's characters, as they stand in the file.
    #[must_use]
    pub const fn as_str(self) -> &'static str {
        match self {
            Self::Lf => "\n",
            Self::CrLf => "\r\n",
        }
    }

    /// The ending spelled with backslashes, `\n` or `\r\n`: the two or four
    /// characters an engine that reads backslash escapes in a string literal
    /// takes for it. Put it inside the quotes of a record delimiter, on an
    /// engine that reads them that way.
    #[must_use]
    pub const fn escaped(self) -> &'static str {
        match self {
            Self::Lf => "\\n",
            Self::CrLf => "\\r\\n",
        }
    }
}

/// **What a table's `CREATE TABLE` is about to say** — what
/// [`SqlRenderer::render_table_suffix`] is handed to decide the clause after
/// the column list.
///
/// One struct rather than several arguments, so the slot can learn more about a
/// table without changing its signature. Razeri builds it; a dialect reads it.
/// A dialect's own tests build one with [`TableSuffixRequest::new`] and the
/// `with_` methods, which is also why it cannot be written out field by field
/// outside this crate.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[non_exhaustive]
pub struct TableSuffixRequest<'a> {
    /// Whether the table is dropped at the end of its session, its transaction
    /// or its batch. A transient table that outlives its session is not
    /// temporary.
    pub temporary: bool,
    /// The table's primary-key columns in key order, unquoted. Empty on a table
    /// with no primary key.
    pub primary_key: &'a [String],
    /// The columns the design's storage plan asked this table be clustered
    /// by, unquoted, and only those this table has. Empty on most tables.
    pub clustering_columns: &'a [String],
    /// The column whose greatest value survives when rows share a key, on the
    /// one table Razeri versions that way: the target of an engine whose
    /// [`MergeShape`](crate::MergeShape) is
    /// [`InsertDedup`](crate::InsertDedup). `None` on every other table.
    pub version_column: Option<&'a str>,
}

impl<'a> TableSuffixRequest<'a> {
    /// A persistent table with no primary key, no clustering columns and no
    /// version column.
    #[must_use]
    pub const fn new() -> Self {
        Self { temporary: false, primary_key: &[], clustering_columns: &[], version_column: None }
    }

    /// The same table, temporary or not.
    #[must_use]
    pub const fn with_temporary(mut self, temporary: bool) -> Self {
        self.temporary = temporary;
        self
    }

    /// The same table, with these primary-key columns.
    #[must_use]
    pub const fn with_primary_key(mut self, columns: &'a [String]) -> Self {
        self.primary_key = columns;
        self
    }

    /// The same table, with these clustering columns.
    #[must_use]
    pub const fn with_clustering_columns(mut self, columns: &'a [String]) -> Self {
        self.clustering_columns = columns;
        self
    }

    /// The same table, with this version column.
    #[must_use]
    pub const fn with_version_column(mut self, column: Option<&'a str>) -> Self {
        self.version_column = column;
        self
    }
}

impl Default for TableSuffixRequest<'_> {
    fn default() -> Self {
        Self::new()
    }
}

/// Level 1 — Core SQL rendering. REQUIRED: every dialect must implement these 4 methods.
///
/// These methods define the absolute minimum surface for rendering SQL:
/// data types, identifier quoting, qualified names, and timestamps.
pub trait SqlRenderer: Send + Sync {
    /// Render a [`DataType`] as platform-specific SQL.
    fn render_data_type(&self, dt: &DataType) -> String;

    /// **The type text for a routine's PARAMETER**, which is not always the text
    /// a column carries.
    ///
    /// Most engines spell a type one way wherever it appears, and for those this
    /// hook is [`Self::render_data_type`] under another name — which is the
    /// default, so a dialect with nothing to say says nothing.
    ///
    /// **Oracle is the reason it exists.** A column there takes
    /// `NUMBER(19, 0)` and `VARCHAR2(100)`; the same types in a parameter list
    /// are a compile error (`PLS-00103`, reported at the opening parenthesis),
    /// and the bare `NUMBER` and `VARCHAR2` are what a parameter clause takes.
    /// The length and the precision belong on the local declaration instead,
    /// which keeps them. A dialect there answers:
    ///
    /// ```rust,ignore
    /// fn render_parameter_data_type(&self, dt: &DataType) -> String {
    ///     let column = self.render_data_type(dt);
    ///     match column.find('(') {
    ///         Some(at) => column[..at].trim_end().to_owned(),
    ///         None => column,
    ///     }
    /// }
    /// ```
    ///
    /// It is what fills
    /// [`RoutineParameter::data_type`](crate::routine_spec::RoutineParameter::data_type),
    /// so a dialect that answers it does not also post-process the string
    /// Razeri handed it.
    ///
    /// Parameters:
    /// - `dt`: the type the design declared.
    fn render_parameter_data_type(&self, dt: &DataType) -> String {
        self.render_data_type(dt)
    }

    /// **The type text for a column that ADMITS NULL** — the whole type, not a
    /// modifier appended to one.
    ///
    /// Most engines spell nullability by SILENCE: a column declared without
    /// `NOT NULL` admits one, and its type text is the same text a required
    /// column carries. That is the default here, and for those engines this
    /// hook is [`SqlRenderer::render_data_type`] under another name.
    ///
    /// **`ClickHouse` is the exception, and it is the reason this hook exists.**
    /// There a bare type is NOT NULL — nullability is a property OF the type,
    /// spelled `Nullable(T)` — so a renderer that expresses *nullable* by
    /// omitting a keyword expresses nothing at all, and every `nullable:` a
    /// design declares is discarded on that engine.
    ///
    /// # What a discarded `nullable:` costs, measured rather than assumed
    ///
    /// Not a loud refusal. `insert_null_as_default` **defaults to `1`** on
    /// `ClickHouse` (24.10.2, server default, `changed = 0`), so
    /// `INSERT … SELECT NULL` into a bare column does not raise — it writes the
    /// TYPE'S DEFAULT, silently: `''` for `String`, `0` for `Int64`,
    /// `1900-01-01` for `Date32` (which is razeri's own unknown-member date, so
    /// an unknown attribute comes back indistinguishable from a deliberately
    /// seeded one). Only with `insert_null_as_default = 0` does the engine
    /// answer `Code: 70 … Cannot convert NULL to a non-nullable type`. **A
    /// design's own data would be fabricated on that engine and honest on
    /// every other**, and the DDL is where that is prevented.
    ///
    /// # A dialect overriding this owes a measurement
    ///
    /// Not every type can be wrapped. `ClickHouse` refuses
    /// `Nullable(Array(…))`, `Nullable(Map(…))` and `Nullable(JSON)` outright,
    /// so its override declines those and says so in its own rustdoc. A
    /// wrapper that cannot be applied must be DECLINED by name, never applied
    /// and hoped for: an unwrappable type rendered `Nullable(…)` turns a silent
    /// wrong value into a `CREATE TABLE` that does not run.
    fn render_nullable_data_type(&self, dt: &DataType) -> String {
        self.render_data_type(dt)
    }

    /// **A comment, written as this engine reads one.**
    ///
    /// Every comment Razeri writes into a script goes through here: the note
    /// over a statement, the heading of each part of a load, the banner at the
    /// top of a file. The default is [`crate::comment::block_comment`], `/* … */`,
    /// which closes itself, so a script whose line endings a tool turns into
    /// spaces still runs. A line comment, `-- …`, would turn every statement
    /// after it into comment.
    ///
    /// Override it only for an engine that cannot read a block comment, and
    /// say which in the override's own doc comment. No engine Razeri carries
    /// does: each of them reads `/* … */`. What comes back stands on lines of
    /// its own; Razeri indents it with the statement it describes.
    ///
    /// Parameters:
    /// - `text`: the comment's words, one or more lines, with no comment marks.
    fn render_comment(&self, text: &str) -> String {
        crate::comment::block_comment(text)
    }

    /// Quote an identifier using platform conventions.
    ///
    /// `PostgreSQL`: `"name"`, `MySQL`: `` `name` ``, SQL Server: `[name]`
    fn quote_identifier(&self, name: &str) -> String;

    /// Render a fully qualified name (optional schema + name).
    fn qualify_name(&self, schema: Option<&str>, name: &str) -> String;

    /// Whether this dialect uses schemas in qualified names.
    ///
    /// `ClickHouse` uses only `database.object` (no schema level).
    /// Default: `true` (most platforms use `catalog.schema.object`).
    fn uses_schema_in_qualified_names(&self) -> bool {
        true
    }

    /// **The name an object takes on an engine with no schemas**, where this
    /// dialect keeps a design's layers apart by folding the schema into it —
    /// `sales_raw` and `customers` become `sales_raw_customers` — or `None`,
    /// the default, where the schema is dropped.
    ///
    /// It is asked only of a dialect whose
    /// [`DialectCapabilities::supports_schemas`](crate::DialectCapabilities::supports_schemas)
    /// is `false`. With `None`, every object of every layer lives under its
    /// own bare name, so a design whose Raw and Core layers both hold a
    /// `customers` is refused: each load would write over the other's rows.
    /// Answer with a fold and Razeri keeps each object's schema and writes
    /// what this returns wherever it names the object — its `CREATE`, every
    /// statement that reads or writes it, every reference to it — so the same
    /// design renders, and two objects are refused only where their folded
    /// names are the same.
    ///
    /// Return the name unquoted; Razeri quotes it with
    /// [`Self::quote_identifier`]. [`Self::qualify_name`] given a schema has
    /// to answer the same object, quoted, because some statements are named
    /// through it; the conformance report checks that the two agree.
    fn fold_schema_into_name(&self, schema: &str, name: &str) -> Option<String> {
        let _ = (schema, name);
        None
    }

    /// Whether this dialect is the "agnostic" pseudo-dialect.
    ///
    /// Agnostic rendering produces unquoted, platform-neutral DDL.
    /// Real dialect implementations should not override this.
    fn is_agnostic_dialect(&self) -> bool {
        false
    }

    /// Whether this dialect supports `SECURE VIEW` (Snowflake).
    fn supports_secure_view(&self) -> bool {
        false
    }

    /// Whether this dialect supports `GLOBAL TEMPORARY VIEW` (Databricks).
    fn supports_global_temporary_view(&self) -> bool {
        false
    }

    /// Render the platform's `CURRENT_TIMESTAMP` expression.
    fn render_current_timestamp(&self) -> String;

    /// **The schemas a catalog scan passes over** — the engine's own
    /// bookkeeping, never a warehouse's contents.
    ///
    /// Read by [`Self::render_catalog_columns_query`] and
    /// [`Self::render_catalog_object_times_query`] when no single schema was
    /// named, so a scan of a whole warehouse sees the tables somebody created
    /// and not the catalog describing them. The default names the one schema
    /// the SQL standard puts the catalog in; an engine that keeps more of its
    /// own next to it adds them.
    fn catalog_system_schemas(&self) -> &'static [&'static str] {
        &["information_schema"]
    }

    /// **The query that lists what the warehouse holds** — one row per column,
    /// as `(schema, object, column)`.
    ///
    /// Razeri asks it to answer a question it cannot answer from a design: how
    /// much of this warehouse razeri itself rendered. Columns rather than
    /// object names alone, because the answer is read off both — a name carries
    /// the tier razeri mints (`…_staging`, `…_current`), and a column carries
    /// the bookkeeping only razeri writes (`…_sid`, `_loaded_at`). A scan that
    /// saw names alone would either miss an entity whose target table is a
    /// plain noun or count a table razeri never touched.
    ///
    /// The three columns are positional and in that order: every caller reads
    /// row `0`, `1`, `2` and nothing reads a column by name, so an engine whose
    /// catalog spells the headings differently needs no special case.
    ///
    /// `None` means **this engine states no catalog razeri can scan**, which is
    /// an answer rather than an omission: the caller says so by name instead of
    /// reporting an empty warehouse.
    ///
    /// Parameters:
    /// - `schema`: the one schema to look in. `None` means the whole warehouse,
    ///   minus [`Self::catalog_system_schemas`].
    fn render_catalog_columns_query(&self, schema: Option<&str>) -> Option<String> {
        Some(format!(
            "SELECT table_schema, table_name, column_name\nFROM \
             information_schema.columns\n{}\nORDER BY table_schema, table_name, column_name",
            self.catalog_scope_predicate("table_schema", schema)
        ))
    }

    /// **The query that says when each object was created**, as
    /// `(schema, object, created_at)` — where the engine records it at all.
    ///
    /// A second, independent reading of the same question
    /// [`Self::render_catalog_columns_query`] opens: the earliest moment razeri
    /// can prove it was here. It is a `None` on most engines, and deliberately
    /// so — `PostgreSQL` and `DuckDB` keep no creation time for a relation, and
    /// inventing one would be worse than having none.
    ///
    /// Parameters:
    /// - `schema`: the one schema to look in, or `None` for the whole
    ///   warehouse.
    fn render_catalog_object_times_query(&self, schema: Option<&str>) -> Option<String> {
        let _ = schema;
        None
    }

    /// The `WHERE` clause both catalog queries share: one named schema, or
    /// every schema that is not the engine's own.
    ///
    /// Parameters:
    /// - `column`: the catalog's own name for the schema column.
    /// - `schema`: the one schema to look in, or `None` for the whole
    ///   warehouse.
    fn catalog_scope_predicate(&self, column: &str, schema: Option<&str>) -> String {
        if let Some(one) = schema {
            return format!("WHERE {column} = '{}'", one.replace('\'', "''"));
        }
        let excluded: Vec<String> = self
            .catalog_system_schemas()
            .iter()
            .map(|name| format!("'{}'", name.replace('\'', "''")))
            .collect();
        format!("WHERE {column} NOT IN ({})", excluded.join(", "))
    }

    /// Return a stable string identifier for this dialect.
    ///
    /// A human-readable label in diagnostics and error messages, for **every
    /// dialect** registered in the `PlatformRegistry` (e.g. `"postgresql"`,
    /// `"sqlite"`, `"cockroachdb"`). For the engines Razeri ships, two
    /// renderings are chosen by this name rather than asked of a trait method —
    /// a bulk load's statement and a query's own syntax. Any other name gets
    /// the generic form: the bulk load in ANSI, the query spelled through your
    /// renderer. Keep it stable once a dialect is published.
    ///
    /// Convention: lowercase, hyphen-separated (e.g. `"postgresql"`,
    /// `"sql-server"`, `"snowflake"`, `"sqlite"`).
    ///
    /// The default returns `"ansi"`.
    fn dialect_name(&self) -> &'static str {
        "ansi"
    }

    /// **How this engine spells a surrogate key.**
    ///
    /// Razeri derives every key one way — cast each part to text, escape and
    /// join the parts of a composite, hash the result, guard NULLs with a
    /// sentinel — and composes that derivation in exactly one place
    /// ([`crate::sid`]). What a dialect decides is the *vocabulary*: how this
    /// engine writes a replacement and a Unicode literal, how it joins
    /// strings, how it casts a value to text, and how it spells a 64-bit hash.
    ///
    /// The default is [`UNSPELLED_SID`] — ANSI's vocabulary under a name that
    /// says it is a guess. **A dialect that leaves it there is telling Razeri it
    /// has not decided**, and the emitted SQL will call an `xxhash64` its engine
    /// very likely does not have. Override it.
    ///
    /// If your engine has no portable 64-bit hash, say so rather than reaching
    /// for the nearest-looking function: a hash that is deterministic within one
    /// warehouse and equal to nothing outside it is the lock-in a portable key
    /// exists to end. Name an artifact your dialect's prerequisites install, or
    /// refuse the key and orient the reader.
    fn sid_spelling(&self) -> SidSpelling {
        UNSPELLED_SID
    }

    /// **The body of the surrogate-key helper, where SQL cannot spell it.**
    ///
    /// Razeri emits one function per warehouse — `generate_sid` — and normally
    /// builds its body itself, from the vocabulary
    /// [`sid_spelling`](Self::sid_spelling) states. The default here is `None`,
    /// which says exactly that: *compose the derivation in my own SQL*.
    ///
    /// Answer with [`SidHelperBody`] only when there is no SQL that could hold
    /// the derivation — the engine has no 64-bit hash under any name, so the
    /// vocabulary has nothing to spell the hash with and the definition has to
    /// be written in another language. `BigQuery` is the case Razeri ships:
    /// `GoogleSQL` has no XXH64, and the function that looks like the answer
    /// (`FARM_FINGERPRINT`) is a different number, so the body is JavaScript.
    ///
    /// **Do not use this to change the key.** The body computes the same number
    /// every other engine computes — XXH64, seed 0, over the UTF-8 bytes of the
    /// one argument, as given — and [`SidHelperBody`] says what it owes
    /// in full. Reaching for a native hash that is merely deterministic keys a
    /// warehouse to numbers nothing outside it can reproduce.
    ///
    /// A dialect that answers here should also state a
    /// [`SidSpelling::hash`](crate::SidSpelling::hash) that no engine has, so
    /// that any path which fell back to composing SQL fails loudly at deploy
    /// time rather than hashing nothing and handing back the value.
    fn sid_helper_body(&self) -> Option<SidHelperBody> {
        None
    }

    /// **How this engine reads a delimited text file as a relation**, or `None`
    /// where it cannot.
    ///
    /// A design can bind an entity to a source file, and Razeri then generates
    /// the staging fill: an `INSERT … SELECT … FROM <this>`. That only works on
    /// an engine that can open a file from inside a query. `DuckDB` can
    /// (`read_csv_auto`); most cannot — `PostgreSQL` wants `COPY`, Snowflake a
    /// stage, `SQLite` has nothing at all. Each of those is a bulk-load design
    /// decision rather than a rendering detail, so the default is `None` and
    /// the generated bundle says which platform declined and why instead of
    /// emitting SQL the engine would refuse.
    ///
    /// `path_literal` arrives as a finished single-quoted SQL string literal,
    /// already escaped — put it where the relation takes a path and do not
    /// quote it again.
    ///
    /// **What the returned relation owes**, because the fill is written against
    /// it and nothing checks these at render time:
    ///
    /// - the file's first row is its header, and the column names come from it,
    ///   spelled exactly as the file spells them;
    /// - every column reads as **text**, unparsed. The fill casts nothing: a
    ///   date's layout and a currency's prefix are the source's business, and a
    ///   reader that guessed a type would fail the load on its first row;
    /// - it is a relation usable in a `FROM` clause, not a statement.
    ///
    /// The path is embedded in the generated file as it was spelled when the
    /// bundle was built, so it must be one the engine resolves at run time.
    fn delimited_file_relation(&self, path_literal: &str) -> Option<String> {
        let _ = path_literal;
        None
    }

    /// **How this engine reads a delimited text file as a relation, told
    /// everything Razeri knows about the file** — or `None` where it cannot.
    ///
    /// The richer spelling of
    /// [`delimited_file_relation`](Self::delimited_file_relation), and the one
    /// the staging fill asks. The default hands the path to that method — or,
    /// for a file in a named [`encoding`](DelimitedFile::encoding), to
    /// [`delimited_file_relation_in_encoding`](Self::delimited_file_relation_in_encoding)
    /// — so a dialect that has stated those has stated this too, and its
    /// renders are byte for byte what they were.
    ///
    /// **The default reads only a plain file** ([`DelimitedFile::is_plain`]):
    /// split on commas, quoted with `"`, and headed. Neither method it hands
    /// over to can be told a delimiter, a quote or a missing header, so for
    /// any other file it answers `None`, and Razeri declines that file by name
    /// rather than reading a pipe-delimited file as one column. Override this
    /// method to read the others: put the file's
    /// [`delimiter`](DelimitedFile::delimiter),
    /// [`quote`](DelimitedFile::quote) and [`header`](DelimitedFile::header)
    /// where your engine's read takes them, and answer `None` for one it
    /// cannot be told.
    ///
    /// Override it, too, where the path alone is not enough. Two engines' file
    /// reads need more and cannot be given it any other way:
    ///
    /// - **the file's columns declared in the statement.** Oracle's external
    ///   table answers `ORA-00906: missing left parenthesis` without them, and
    ///   has no form that takes its column names from a header row;
    /// - **a directory rather than a path.** The same engine answers
    ///   `KUP-04125: no directory object specified for location 1 in LOCATION
    ///   clause` to an absolute path, and resolves a file name against a named
    ///   directory object instead — which
    ///   [`delimited_file_prerequisites`](Self::delimited_file_prerequisites)
    ///   is where such an object is made.
    ///
    /// Everything the returned relation owes, it owes for the reasons
    /// [`delimited_file_relation`](Self::delimited_file_relation) states:
    /// header from the first row, every column as text and unparsed, usable in
    /// a `FROM` clause. **Declaring the columns does not license declaring
    /// their types**: they are the source's text, and a reader that guessed
    /// would fail the load on its first row.
    fn delimited_file_relation_for(&self, file: &DelimitedFile<'_>) -> Option<String> {
        if !file.is_plain() {
            return None;
        }
        match file.encoding {
            None => self.delimited_file_relation(file.path_literal),
            Some(encoding) => self.delimited_file_relation_in_encoding(file.path_literal, encoding),
        }
    }

    /// **How this engine reads a parquet file as a relation of text** — or
    /// `None` where it cannot, which is the default.
    ///
    /// The relation owes what a delimited file's does: the file's own column
    /// names, every column as **text**, usable in a `FROM` clause. A parquet
    /// file carries its types, and the fill still reads each value as text so
    /// that one set of conversions, cleanings and checks serves every file;
    /// the text is the engine's own cast of each value (`2024-01-01 00:57:55`
    /// for a timestamp, `2.0` for a whole double), which is also the text
    /// Razeri counts and totals the file by before a build.
    ///
    /// `path_literal` arrives as a finished single-quoted SQL string literal,
    /// already escaped. An engine that answers `None` has its parquet sources
    /// declined by name, with the conversion that would load them.
    fn parquet_file_relation(&self, path_literal: &str) -> Option<String> {
        let _ = path_literal;
        None
    }

    /// **Statements this engine needs run once before any of its file relations
    /// resolve**, in order — and none by default.
    ///
    /// An engine that opens a path needs nothing here. An engine that opens a
    /// file only through an object somebody made — Oracle's `DIRECTORY` — needs
    /// that object made first, and the generated staging file is where it
    /// belongs: it is the one file that knows which directory the sources were
    /// read from.
    ///
    /// Each string is one statement without its terminator. Every one must be
    /// safe to run again, because the file it lands in runs on every deploy.
    ///
    /// `directory_literal` arrives as a finished single-quoted SQL string
    /// literal, already escaped.
    ///
    /// **Say what the statement costs.** An object like this is often
    /// privileged, and a statement an operator cannot run is better read than
    /// guessed at — put the privilege in a comment inside the statement, where
    /// it travels with it.
    fn delimited_file_prerequisites(&self, directory_literal: &str) -> Vec<String> {
        let _ = directory_literal;
        Vec::new()
    }

    /// **Statements this engine needs run before it can read THIS file**, in
    /// order — and none by default.
    ///
    /// The per-file companion of
    /// [`delimited_file_prerequisites`](Self::delimited_file_prerequisites),
    /// and the slot an engine with no file relation at all reads a file
    /// through. Where that method makes what every read shares — a directory
    /// object, a permitted path — this one makes what belongs to one file, and
    /// the staging fill emits it directly in front of the fill that reads it.
    ///
    /// Two kinds of engine answer it.
    ///
    /// - One whose file read is a **statement rather than a relation**. SAP
    ///   HANA's is `IMPORT FROM CSV FILE`, which writes into a table that
    ///   already exists; there is nothing to put in a `FROM` clause. Such a
    ///   dialect lands the file's rows in a table here and answers that table
    ///   from
    ///   [`delimited_file_relation_for`](Self::delimited_file_relation_for).
    /// - One whose relation needs a per-file object made first — a format, an
    ///   external stage, a descriptor.
    ///
    /// Each string is one statement without its terminator. **Every one must be
    /// safe to run again**, because the file it lands in runs on every deploy,
    /// and safe to run beside the others: a second deploy stages what the
    /// source holds now rather than adding a second copy of it.
    ///
    /// **Say what the statement costs.** A read like this is often privileged,
    /// and a statement an operator cannot run is better read than guessed at —
    /// put the privilege in a comment inside the statement, where it travels
    /// with it.
    fn delimited_file_statements_for(&self, file: &DelimitedFile<'_>) -> Vec<String> {
        let _ = file;
        Vec::new()
    }

    /// **The one-row table a query reads when it has nothing else to read**,
    /// or `None` on an engine that takes a `SELECT` with no `FROM` at all.
    ///
    /// Some of what Razeri measures is a value rather than a row of a table —
    /// the grand total a reconciliation compares against its sources is a sum
    /// of counts, and there is nothing to select it from. Most engines take
    /// `SELECT <value>` as it stands. SAP HANA refuses it at the next word and
    /// wants `FROM DUMMY`, the one-row table it keeps for exactly this; Oracle
    /// kept `DUAL` for the same reason until it began accepting the bare form.
    ///
    /// Answer the table's name, as a query would write it after `FROM`. Every
    /// engine that answers `None` renders exactly what it rendered before.
    fn render_one_row_source(&self) -> Option<String> {
        None
    }

    /// **How this engine reads a delimited text file whose bytes are not
    /// UTF-8**, or `None` where it states no way to.
    ///
    /// UTF-8 is the canonical form and every engine reads it, so
    /// [`delimited_file_relation`](Self::delimited_file_relation) answers for
    /// it and this is asked only about the other days. A design can name the
    /// encoding to fall back to when a file's bytes are not UTF-8, and razeri
    /// resolves that name once, to the one name the encoding standard files it
    /// under — `windows-1252`, `shift_jis`, `koi8-r`. That name arrives here,
    /// and an engine that has its own word for the same encoding translates it
    /// the way an engine translates every other name razeri hands it.
    ///
    /// Everything the returned relation owes, it owes for the same reasons
    /// [`delimited_file_relation`](Self::delimited_file_relation) says: header
    /// from the first row, every column as text, usable in a `FROM` clause.
    ///
    /// **`None` is a complete answer, and the common one.** An engine that
    /// reads no file at all says nothing here; so does an engine that reads
    /// files but has no word for this particular encoding. Razeri then declines
    /// that source by name and says what to do instead — convert the file to
    /// UTF-8, or write the load by hand — rather than pointing the engine at
    /// bytes it will read as some other language.
    ///
    /// **Answer only for encodings you have run.** An engine that accepts the
    /// name and reads the bytes as something else is worse than one that
    /// refuses the name: the first is a warehouse of plausible nonsense and the
    /// second is a sentence on a terminal.
    ///
    /// **Razeri asks it two ways.** First with a path of `''`, to learn
    /// whether your engine has a word for the encoding at all; a file it has
    /// none for is declined there. Then the read itself goes through
    /// [`delimited_file_relation_for`](Self::delimited_file_relation_for)
    /// with the file's [`encoding`](DelimitedFile::encoding) set, whose
    /// default hands a plain file back here.
    ///
    /// `path_literal` arrives as a finished single-quoted SQL string literal,
    /// already escaped — put it where the relation takes a path and do not
    /// quote it again. `encoding` does not: it is a bare name, and an engine
    /// that puts it in a string literal writes the quotes itself.
    fn delimited_file_relation_in_encoding(
        &self,
        path_literal: &str,
        encoding: &str,
    ) -> Option<String> {
        let _ = (path_literal, encoding);
        None
    }

    /// **The `CAST` target spelling for a [`DataType`] on this engine.**
    ///
    /// A column declaration and a `CAST` target are two grammars on most
    /// engines, so this is a separate answer from
    /// [`render_data_type`](Self::render_data_type) and not a synonym for it.
    /// `MySQL` takes `SIGNED` where it declares `BIGINT`; T-SQL takes
    /// `NVARCHAR(MAX)` where ANSI says `TEXT`.
    ///
    /// Where the two *are* the same on your engine — which is the common case
    /// on engines with one type vocabulary — say so by delegating:
    ///
    /// ```rust,ignore
    /// fn render_cast_type(&self, dt: &DataType) -> String {
    ///     self.render_data_type(dt)
    /// }
    /// ```
    ///
    /// The default is ANSI's cast vocabulary. Razeri's generated load opens
    /// with a CTE that casts every source column to its declared type, so a
    /// wrong name here fails the load on its first statement.
    fn render_cast_type(&self, dt: &DataType) -> String {
        crate::agnostic::render_cast_type_ansi(dt)
    }

    /// **Which stored-procedure grammar this engine speaks.**
    ///
    /// Razeri composes the procedure — its parameters, its steps, its comment —
    /// and spells it in the form this returns. The default is
    /// [`ProcedureForm::Agnostic`], which parses on no engine: it is the honest
    /// answer for a dialect that has not said, and it is what an unlisted
    /// platform has always been handed.
    ///
    /// It lives on `SqlRenderer` rather than on
    /// [`ProcedureRenderer`] because the composer
    /// holds the renderer as a [`DdlRenderer`], and a decision reachable only
    /// through a trait the caller does not hold is a decision the caller makes
    /// instead.
    fn procedure_form(&self) -> ProcedureForm {
        ProcedureForm::Agnostic
    }

    /// **Which scalar-function grammar this engine speaks.**
    ///
    /// The default is [`ScalarFunctionForm::PostgreSql`] — what an unlisted
    /// dialect has always been handed, and a dangerous inheritance on a near
    /// neighbour: `LANGUAGE SQL IMMUTABLE PARALLEL SAFE` is a *"syntax error at
    /// or near `PARALLEL`"* on an engine that descends from `PostgreSQL` and
    /// reports itself as one.
    fn scalar_function_form(&self) -> ScalarFunctionForm {
        ScalarFunctionForm::PostgreSql
    }

    /// **Which of Razeri's bulk-load grammars this engine speaks**, or `None`
    /// where it speaks none of them.
    ///
    /// The bulk-load builder writes its statement in the form named here. The
    /// default is `None`, and the builder then writes Razeri's own annotated
    /// form, which no engine runs: name a form only where your engine takes
    /// that statement as written.
    fn bulk_load_form(&self) -> Option<BulkLoadForm> {
        None
    }

    /// **The clause a view's settings are written in**, between its name and
    /// `AS` — `PostgreSQL`'s `WITH (security_barrier = true)` — or `None` where
    /// none of `settings` is this engine's, which is the default.
    ///
    /// See [`EngineSetting`]: spell the settings written for your engine and
    /// pass over the rest.
    fn render_view_options(&self, settings: &[EngineSetting<'_>]) -> Option<String> {
        let _ = settings;
        None
    }

    /// **The words a join's settings are written in**, in front of its join
    /// keyword — `ClickHouse`'s `GLOBAL ANY` — or `None` where none of
    /// `settings` is this engine's, which is the default.
    fn render_join_options(&self, settings: &[EngineSetting<'_>]) -> Option<String> {
        let _ = settings;
        None
    }

    /// **Which table-valued-function grammar this engine speaks, and — where it
    /// has none — the words it refuses in.**
    ///
    /// The default is [`TableFunctionForm::PostgreSql`], what an unlisted
    /// dialect has always been handed. If your engine has no routine a query
    /// selects `FROM`, answer [`TableFunctionForm::Unsupported`] with your own
    /// sentence: Razeri then declines to build the routine at all and prints
    /// **your** words rather than another engine's. A refusal written once per
    /// artifact kind tells every author about whichever engine happened to need
    /// it first, and the near form an author will otherwise find on their own
    /// is exactly what costs them a day.
    fn table_function_form(&self) -> TableFunctionForm {
        TableFunctionForm::PostgreSql
    }

    // ═══════════════════════════════════════════════════════════════
    // Routines this dialect spells itself
    // ═══════════════════════════════════════════════════════════════

    /// **Spell a whole stored procedure in this engine's own grammar** — the
    /// method [`ProcedureForm::Custom`] promises.
    ///
    /// Razeri composes the routine and hands it over as a [`RoutineSpec`]: the
    /// name, the parameters with their types already in your spelling, the
    /// declarations, the body's statements, the comment. What comes back is the
    /// complete `CREATE`, punctuation and all, and Razeri emits it unchanged.
    ///
    /// The default is `None`, which means *"I did not write one"*. It is only
    /// ever reached by a dialect that answered
    /// [`ProcedureForm::Custom`] and then did not override this — the
    /// conformance kit reports that as blocking, and the emitted SQL carries a
    /// comment saying the same thing where the procedure would have been.
    ///
    /// It lives on `SqlRenderer` rather than on [`ProcedureRenderer`] for the
    /// reason [`Self::procedure_form`] does: the composer holds the renderer as
    /// a [`DdlRenderer`], and an answer reachable only through a trait the
    /// caller does not hold is an answer the caller makes up instead.
    ///
    /// Parameters:
    /// - `routine`: the procedure Razeri composed.
    fn render_procedure(&self, routine: &RoutineSpec) -> Option<String> {
        let _ = routine;
        None
    }

    /// **Spell a whole scalar function in this engine's own grammar** — the
    /// method [`ScalarFunctionForm::Custom`] promises.
    ///
    /// The same contract as [`Self::render_procedure`]: a composed
    /// [`RoutineSpec`] in, the complete `CREATE` out, and `None` only where a
    /// dialect claimed a custom grammar and did not write the method.
    ///
    /// Parameters:
    /// - `routine`: the function Razeri composed.
    fn render_scalar_function(&self, routine: &RoutineSpec) -> Option<String> {
        let _ = routine;
        None
    }

    /// **Spell a whole table-valued function in this engine's own grammar** —
    /// the method [`TableFunctionForm::Custom`] promises.
    ///
    /// The same contract as [`Self::render_procedure`]: a composed
    /// [`RoutineSpec`] in, the complete `CREATE` out, and `None` only where a
    /// dialect claimed a custom grammar and did not write the method.
    ///
    /// Parameters:
    /// - `routine`: the function Razeri composed.
    fn render_table_function(&self, routine: &RoutineSpec) -> Option<String> {
        let _ = routine;
        None
    }

    // ═══════════════════════════════════════════════════════════════
    // Expression rendering extensions
    // ═══════════════════════════════════════════════════════════════

    /// Platform's `CURRENT_DATE` expression.
    ///
    /// `SqlServer`: `CAST(GETDATE() AS DATE)`, `ClickHouse`: `today()`, others: `CURRENT_DATE`
    fn render_current_date(&self) -> String {
        "CURRENT_DATE".to_string()
    }

    /// Render a boolean literal.
    ///
    /// SqlServer/ClickHouse: `1`/`0`, others: `TRUE`/`FALSE`
    fn render_boolean_literal(&self, value: bool) -> String {
        if value { "TRUE".to_string() } else { "FALSE".to_string() }
    }

    /// **Collapse a subquery that selects AT MOST ONE ROW into an aggregate**,
    /// for the engines that will not accept an unaggregated correlated scalar.
    ///
    /// Razeri's SCD-2 and SCD-1 re-opens assign `col = (SELECT … WHERE
    /// {correlation})`. Databricks refuses that shape by name
    /// (`MUST_AGGREGATE_CORRELATED_SCALAR_SUBQUERY`) and Snowflake stopped
    /// unnesting it the day the guard beside it became uncorrelated, so the
    /// emission wraps the projection in an aggregate. **Which** aggregate is a
    /// dialect fact and **whether one exists at all** is a dialect × type fact,
    /// which is why this hook takes the kind and may answer `None`:
    ///
    /// - `MAX` is the obvious choice and is wrong for a BOOLEAN on two of the
    ///   platforms razeri reaches. Measured on the running
    ///   containers: `PostgreSQL` and Citus raise
    ///   *"function max(boolean) does not exist"*, SQL Server raises
    ///   *"Msg 8117: Operand data type bit is invalid for max operator"*, while
    ///   `MariaDB` (`MAX(TRUE)` → `1`) and `ClickHouse` (`max(true)` → `true`)
    ///   answer it happily;
    /// - `PostgreSQL` has no `max()` for `uuid`, `jsonb` or `bytea` either
    ///   (measured, same run), and SQL Server has one for both
    ///   `uniqueidentifier` and `varbinary` (measured).
    ///
    /// **`None` means *this dialect cannot aggregate this kind* and is not a
    /// failure.** The caller then emits the bare correlated scalar — the
    /// prior shape, measured green on the whole
    /// `PostgreSQL` family — and says so on the output, because that shape is
    /// the one Databricks refuses.
    ///
    /// **Correctness does not come from the aggregate's meaning.** The subquery
    /// is under a guard that is false for the entire load if any member
    /// disagrees with itself, so it selects one row; `MAX`, `BOOL_OR` and
    /// `BOOLOR_AGG` of one value are all that value. The choice between them is
    /// availability, not semantics.
    fn render_single_row_aggregate(
        &self,
        expression: &str,
        kind: AggregableKind,
    ) -> Option<String> {
        let _ = kind;
        Some(format!("MAX({expression})"))
    }

    /// **Take any one value of `expression` from a group**, for the engines
    /// that cannot group on the column itself.
    ///
    /// A load collapses staged rows that repeat. Where the engine will not
    /// group on a nested column
    /// ([`crate::DialectCapabilities::groups_nested_values`]) the collapse
    /// groups on the remaining columns and takes one value of that one, which
    /// is what this renders.
    ///
    /// `ANY_VALUE` is the spelling `BigQuery`, Snowflake, Redshift, Databricks
    /// and `MySQL` all use, so it is the default; an engine that spells it
    /// differently, or reaches it through `FIRST`, says so here.
    ///
    /// Unlike [`Self::render_single_row_aggregate`] this one cannot decline:
    /// it is asked only where the dialect has already said it cannot group on
    /// the column, so there is no second shape to fall back to.
    ///
    /// Parameters:
    /// - `expression`: the column reference, already spelled by the renderer
    ///   that lays out the query.
    fn render_any_value(&self, expression: &str) -> String {
        format!("ANY_VALUE({expression})")
    }

    /// **The AGGREGATE median this engine has for `expression`, or `None` when
    /// it has none.**
    ///
    /// Aggregate is the load-bearing word, and it is why this cannot be
    /// answered from a function-name table. Four spellings are in the wild:
    ///
    /// - a plain `median(x)` aggregate — `duckdb`, `clickhouse`, `snowflake`;
    /// - the SQL ordered-set aggregate
    ///   `PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY x)` — `postgres` and its
    ///   family (measured on a running instance: it groups);
    /// - a two-argument `PERCENTILE(x, 0.5)` — `databricks`;
    /// - and a WINDOW function wearing one of those names, which is **not**
    ///   this. `mariadb` has `MEDIAN(x) OVER (…)` and `sqlserver` has
    ///   `PERCENTILE_CONT(…) OVER (…)`, and both are a syntax error in a
    ///   `GROUP BY` select list (measured: `MariaDB` raises `ERROR 1064` and
    ///   SQL Server raises *"Msg 10753: The function `PERCENTILE_CONT` must
    ///   have an OVER clause"*). A window function returns one value per ROW,
    ///   not one per group, so it is a different answer and not a fallback.
    ///
    /// **The default declines**, so a dialect that has never been asked the
    /// question is never assumed to have the answer, and a dialect added
    /// tomorrow opts in deliberately with a form somebody measured. The caller
    /// turns `None` into a build-time refusal naming the column and the target
    /// — razeri does not emit SQL its target cannot run.
    ///
    /// # Why a TEMPLATE rather than finished SQL
    ///
    /// The aggregated expression is a column reference, and how a column
    /// reference is spelled — quoted, bracketed, upper-cased — belongs to the
    /// renderer that lays out the query, not to this answer. Handing back a
    /// form with [`AGGREGATE_ARGUMENT`] where the expression goes keeps the
    /// two apart: this trait owns *which function*, the query renderer owns
    /// *how the column is written*, and neither has to reimplement the other.
    fn median_aggregate_form(&self) -> Option<&'static str> {
        None
    }

    /// Render a date literal.
    ///
    /// `ClickHouse`: `toDate('2024-01-15')`, others: `DATE '2024-01-15'`
    fn render_date_literal(&self, date: &str) -> String {
        format!("DATE '{date}'")
    }

    /// Render a timestamp literal.
    ///
    /// `ClickHouse`: `toDateTime('...')`, others: `TIMESTAMP '...'`
    fn render_timestamp_literal(&self, ts: &str) -> String {
        format!("TIMESTAMP '{ts}'")
    }

    /// Render an interval literal.
    ///
    /// `SqlServer`: `'P1Y2M3D'`, `ClickHouse`: `INTERVAL 1 YEAR`, others: `INTERVAL '1 year'`
    fn render_interval_literal(&self, interval: &str) -> String {
        format!("INTERVAL '{interval}'")
    }

    /// Render the string concatenation operator.
    ///
    /// `SqlServer`: `+`, others: `||`
    fn concat_operator(&self) -> &'static str {
        "||"
    }

    /// Render a NaN literal for a floating-point type.
    ///
    /// `PostgreSQL`: `'NaN'::float`, others: `NULL`
    fn render_nan_literal(&self, _dt: &DataType) -> String {
        "NULL".to_string()
    }

    /// Render an infinity literal for a floating-point type.
    ///
    /// `PostgreSQL`: `'Infinity'::float` / `'-Infinity'::float`, others: `NULL`
    fn render_infinity_literal(&self, _positive: bool, _dt: &DataType) -> String {
        "NULL".to_string()
    }

    /// Render the NULL-coalescing function name.
    ///
    /// `SqlServer`: `ISNULL`, `ClickHouse`: `ifNull`, others: `COALESCE`
    fn ifnull_function_name(&self) -> &'static str {
        "COALESCE"
    }

    /// Render an IIF expression (or CASE WHEN equivalent).
    ///
    /// `SqlServer`: `IIF(cond, then, else)`, others: `CASE WHEN cond THEN then ELSE else END`
    fn render_iif(&self, cond: &str, then_val: &str, else_val: &str) -> String {
        format!("CASE WHEN {cond} THEN {then_val} ELSE {else_val} END")
    }

    /// Render a `DATE_TRUNC` expression.
    ///
    /// `SqlServer`: `DATETRUNC(field, expr)`, `ClickHouse`: `toStartOfMonth(expr)`, others: `DATE_TRUNC('field', expr)`
    fn render_date_trunc(&self, field: &str, expr: &str) -> String {
        format!("DATE_TRUNC('{field}', {expr})")
    }

    /// **One part of a date or a timestamp, as a whole number.**
    ///
    /// A design's `date_part('<part>', <column>)` hands this one of ten words,
    /// and each must answer the number named here, the same on every engine:
    ///
    /// | `field` | the number |
    /// |---|---|
    /// | `YEAR` | the year, such as 2026 |
    /// | `QUARTER` | 1 to 4 |
    /// | `MONTH` | 1 to 12 |
    /// | `ISOWEEK` | the ISO 8601 week, 1 to 53, weeks starting on Monday |
    /// | `DAY` | the day of the month, 1 to 31 |
    /// | `HOUR` | 0 to 23 |
    /// | `MINUTE` | 0 to 59 |
    /// | `SECOND` | 0 to 59, the whole second |
    /// | `ISODOW` | the ISO day of the week, Monday 1 to Sunday 7 |
    /// | `DOY` | the day of the year, 1 to 366 |
    ///
    /// The default writes `EXTRACT(<field> FROM <column>)`, which most engines
    /// refuse for `ISOWEEK`, `ISODOW` or `DOY`: override it, spelling each word
    /// your engine's way — `DATEPART(ISO_WEEK, c)` on SQL Server,
    /// `toISOWeek(c)` on `ClickHouse` — and run each of the ten on your engine.
    fn render_date_extract(&self, field: &str, column: &str) -> String {
        format!("EXTRACT({field} FROM {column})")
    }

    /// Render a `TRY_CAST` expression.
    ///
    /// `ClickHouse`: `accurateCastOrNull(expr, 'Type')`, others: `TRY_CAST(expr AS Type)`
    fn render_try_cast(&self, expr: &str, target: &DataType) -> String {
        format!("TRY_CAST({} AS {})", expr, self.render_data_type(target))
    }

    /// Render a named parameter placeholder.
    ///
    /// `SqlServer`: `@name`, `PostgreSQL`: `:name`, Databricks: `{name}`
    fn render_named_parameter(&self, name: &str) -> String {
        format!(":{name}")
    }

    /// Keyword emitted for MERGE NOT MATCHED branch.
    ///
    /// SqlServer/Databricks: `WHEN NOT MATCHED BY TARGET`, others: `WHEN NOT MATCHED`.
    fn merge_not_matched_keyword(&self) -> &'static str {
        "WHEN NOT MATCHED"
    }

    /// Whether MERGE supports `WHEN NOT MATCHED BY SOURCE`.
    fn supports_merge_not_matched_by_source(&self) -> bool {
        false
    }

    /// Whether this platform takes a `MERGE` statement.
    ///
    /// **This is `DialectCapabilities::merge_shape().merges()`, restated where
    /// only a `SqlRenderer` is in hand** — the standalone `MergeStatement`
    /// builder takes this trait alone and cannot ask the capability surface.
    /// The two must answer identically; the capability/renderer consistency
    /// gate holds them to equality with no exceptions, so a dialect that
    /// overrides one and not the other is refused before it ships. An engine
    /// whose shape is an upsert statement, an engine-deduplicated insert, or a
    /// refusal answers `false` here.
    fn merge_supported(&self) -> bool {
        true
    }

    /// Whether `WITH SCHEMA EVOLUTION` is supported for MERGE.
    fn merge_supports_schema_evolution_keyword(&self) -> bool {
        false
    }

    /// Whether table hints are supported in MERGE target.
    fn merge_supports_table_hints(&self) -> bool {
        false
    }

    /// Whether OUTPUT clause should be used for MERGE result projection.
    fn merge_uses_output_clause(&self) -> bool {
        false
    }

    /// Whether RETURNING clause is supported for MERGE.
    fn merge_supports_returning_clause(&self) -> bool {
        false
    }

    /// **The longest line this engine's own client reads intact from a script
    /// file**, or `None` where it reads any line.
    ///
    /// A generated statement can put a long expression on one line. An engine
    /// whose client misreads a line past some length is handed its scripts
    /// with every longer line broken between two words, outside any quoted
    /// text and outside any comment, which changes nothing the statement
    /// says.
    fn longest_script_line(&self) -> Option<usize> {
        None
    }

    /// **How this engine writes a short name for a relation.**
    ///
    /// A composed load gives a table, a view, a common table expression or a
    /// subquery a short name and then addresses it by that name for the rest
    /// of the statement. Most engines take the word `AS` in front of it, and
    /// most of those also take the name on its own. One takes only the name:
    /// `FROM orders AS src` is refused outright, in a `FROM` and again in a
    /// `MERGE`.
    ///
    /// So the two words are the dialect's to write. `relation` arrives already
    /// quoted and qualified; `alias` arrives already quoted. Nothing here
    /// changes what the statement means — only how this engine spells it.
    fn render_relation_alias(&self, relation: &str, alias: &str) -> String {
        format!("{relation} AS {alias}")
    }

    /// **Whether this engine reserves `word`**, so a name spelled that way
    /// has to be quoted to be read as a name.
    ///
    /// Razeri writes a few short names of its own inside a statement: the
    /// marker column a load's anti-join selects (`1 AS matched`), and the
    /// relation that carries it (`standing.matched`). It writes them bare,
    /// and asks this first: a word the engine reserves is written through
    /// [`render_alias`](Self::render_alias), quoted.
    ///
    /// The default is the SQL standard's list,
    /// [`SQL_STANDARD_RESERVED_WORDS`](crate::reserved_words::SQL_STANDARD_RESERVED_WORDS),
    /// in any case. An engine that reserves a word the standard leaves free
    /// answers `true` for it too, and keeps the standard's answer for the
    /// rest:
    ///
    /// ```text
    /// fn is_reserved_word(&self, word: &str) -> bool {
    ///     word.eq_ignore_ascii_case("matched")
    ///         || razeri_plugin_contract::reserved_words::is_standard_reserved_word(word)
    /// }
    /// ```
    fn is_reserved_word(&self, word: &str) -> bool {
        crate::reserved_words::is_standard_reserved_word(word)
    }

    /// **A short name Razeri gives a column or a relation inside one
    /// statement, written so this engine reads it as a name.**
    ///
    /// Bare where [`is_reserved_word`](Self::is_reserved_word) says the word
    /// is free, and through [`quote_identifier`](Self::quote_identifier)
    /// where it is reserved. Every place a statement writes the name asks
    /// this, so the name and every reference to it are spelled alike.
    fn render_alias(&self, alias: &str) -> String {
        if self.is_reserved_word(alias) { self.quote_identifier(alias) } else { alias.to_owned() }
    }

    /// **How a statement that changes the shape of the warehouse is written
    /// inside a routine's body.**
    ///
    /// A generated load empties its landing table when it is finished, and on
    /// most engines that statement stands in the body exactly as it would
    /// stand on its own. One engine runs no such statement directly in a
    /// block: `TRUNCATE TABLE …` there is a parse error, and the same words
    /// handed back to the SQL engine as a string run. `statement` arrives
    /// terminated the way the rest of the body is.
    fn render_ddl_within_routine_body(&self, statement: &str) -> String {
        statement.to_owned()
    }

    /// **How this engine empties a table and keeps it.**
    ///
    /// A generated load empties the staging table it fills before it writes
    /// the next batch into it. Most engines do that with `TRUNCATE TABLE`, and
    /// that is the default. One has no such statement and answers a syntax
    /// error at the word; it empties a table with `DELETE FROM` instead.
    ///
    /// `relation` arrives already quoted and qualified. The answer is one whole
    /// statement, ended with `;`.
    fn render_empty_table(&self, relation: &str) -> String {
        format!("TRUNCATE TABLE {relation};")
    }

    /// **Whether this engine lets a `MERGE` target carry an alias.**
    ///
    /// Most do, and the composed load addresses the target through a short
    /// `tgt`. An engine that answers `false` gets the target's own qualified
    /// name in every position the alias would have stood — dropping the
    /// qualifier instead makes the shared column names ambiguous.
    fn merge_target_accepts_alias(&self) -> bool {
        true
    }

    /// **How this engine writes the condition a `MERGE` matches rows on.**
    ///
    /// Most take the condition after the word `ON` and nothing else. One
    /// requires it in parentheses and answers `ORA-00969: missing ON keyword`
    /// without them — pointing at the condition rather than at the word, which
    /// is there.
    ///
    /// `condition` is the whole predicate, which may run over several lines.
    fn render_merge_on(&self, condition: &str) -> String {
        format!("ON {condition}")
    }

    /// **Whether this engine refuses a `WITH` clause in front of `MERGE`.**
    ///
    /// The composed load types its staging rows in one common table expression
    /// and hashes them in a second. Where a leading `WITH` parses, the two sit
    /// in front of the statement and the source is the second one by name.
    /// Where it does not, the same two move inside the `USING` subquery, which
    /// is the same query in a position the grammar accepts.
    ///
    /// Answer `true` if `WITH … MERGE INTO …` is a syntax error on your engine.
    fn merge_source_requires_inlined_ctes(&self) -> bool {
        false
    }

    /// **Whether a `WITH` clause parses in front of this engine's `INSERT`.**
    ///
    /// A load that writes rows without matching them — one slice replaced, or
    /// rows appended — types its staging rows in one common table expression
    /// and hashes them in a second, exactly as the matching load does. Most
    /// engines take those two inside the statement, after the column list and
    /// in front of the `SELECT`, which is where Razeri puts them by default.
    ///
    /// Answer `true` where the same two have to lead instead, because the
    /// engine reads a `WITH` after a table name as something else entirely:
    /// T-SQL parses it as a table hint and rejects the statement. Nothing here
    /// changes the query — only where the engine will accept it written down.
    fn insert_accepts_leading_ctes(&self) -> bool {
        false
    }

    /// **Whether this engine can be given a function by `CREATE FUNCTION`.**
    ///
    /// Razeri derives a surrogate key through a small emitted function, and an
    /// engine whose SQL surface has no `CREATE FUNCTION` at all can never
    /// receive it — the definition has to come from application code instead.
    /// Such an engine gets no emitted helper and no per-entity key function,
    /// and its dialect states the substitute in its own words.
    ///
    /// This is about the **grammar**, not about who owns the name: an engine
    /// that can define functions but takes its key function from an installed
    /// artifact still answers `true` here.
    fn supports_sql_function_ddl(&self) -> bool {
        true
    }

    /// **Whether a scalar function's body is a bare expression.**
    ///
    /// Most engines want a full `SELECT` in front of the value a function
    /// returns. Some take the expression alone and reject the `SELECT`.
    fn scalar_function_body_is_expression(&self) -> bool {
        false
    }

    /// **Whether a scalar function belongs to a schema.**
    ///
    /// On most engines a function lives in a schema and both its definition and
    /// its call sites name it. On an engine that answers `false`, functions are
    /// server-global: `CREATE FUNCTION` takes no qualifier, and a qualified
    /// call names a function that does not exist.
    fn scalar_function_names_are_schema_qualified(&self) -> bool {
        true
    }

    /// **Whether an unqualified scalar-function call resolves.**
    ///
    /// Most engines resolve a bare function name through a search path. An
    /// engine that answers `true` resolves none, so a call with no schema in
    /// front of it has to be given one — [`DialectCapabilities::default_schema`]
    /// is the
    /// schema the function was created in when the caller named none.
    ///
    /// [`DialectCapabilities::default_schema`]: crate::DialectCapabilities::default_schema
    fn scalar_function_call_requires_schema(&self) -> bool {
        false
    }

    /// **How this engine invokes a stored procedure**, as a whole statement.
    ///
    /// The composed load calls its own identity step, and a deploy script that
    /// has no procedures at all has to recognise these calls in order to inline
    /// what they would have run — so the call is spelled once, here.
    fn render_procedure_call(&self, qualified_name: &str) -> String {
        format!("CALL {qualified_name}();")
    }

    /// The schema separator for qualified names.
    ///
    /// Almost always `.`, but could differ for some platforms.
    fn schema_separator(&self) -> &'static str {
        "."
    }

    /// The statement separator for scripts.
    ///
    /// Almost all platforms: `;\n`, `SqlServer`: `;\nGO\n`
    fn statement_separator(&self) -> &'static str {
        ";\n"
    }

    // ═══════════════════════════════════════════════════════════════
    // UPDATE behavior hooks
    // ═══════════════════════════════════════════════════════════════

    /// Whether the platform supports compound assignment operators (e.g., `col += value`).
    ///
    /// `SqlServer`: `true`, others: `false`.
    fn supports_compound_assignment(&self) -> bool {
        false
    }

    /// Whether UPDATE supports FROM clause for join-based updates.
    ///
    /// `PostgreSQL`, `SqlServer`, Snowflake, `CitusDB`: `true`, others: `false`.
    fn supports_update_from(&self) -> bool {
        false
    }

    /// **Whether an `UPDATE … FROM` takes a joined source whose columns carry
    /// the target's own names**, under an unqualified `SET`:
    /// `UPDATE t SET v = source.v FROM (SELECT k, v FROM s) source WHERE
    /// t.k = source.k`.
    ///
    /// Asked only where [`supports_update_from`](Self::supports_update_from)
    /// is `true`. The default is `true`. Oracle refuses that shape as
    /// ambiguous (*"ORA-00918: column ambiguously specified"*) and `PostgreSQL`
    /// refuses the qualified `SET` that would cure it, so an engine like
    /// Oracle answers `false` and Razeri writes the correlated update, which
    /// every engine runs.
    fn update_from_takes_a_source_naming_the_targets_columns(&self) -> bool {
        true
    }

    /// **The statement that tells this engine's planner what a table now
    /// holds**, after Razeri wrote it in bulk, or `None` where the engine needs
    /// none.
    ///
    /// `table` is spelled and qualified as the statement names it. Razeri runs
    /// the answer after a staging fill and at the end of every load, inside
    /// the load's transaction, so answer only with a statement your engine
    /// takes there. The default is `None`: an engine that keeps what its
    /// planner needs as it writes. `PostgreSQL` answers `ANALYZE <table>;` —
    /// with nothing to go on, its planner joins a hundred thousand rows to a
    /// hundred thousand one pair at a time.
    fn render_refresh_statistics(&self, table: &str) -> Option<String> {
        let _ = table;
        None
    }

    /// Whether the platform uses ALTER TABLE ... UPDATE syntax (`ClickHouse` mutations).
    fn uses_alter_table_update(&self) -> bool {
        false
    }

    /// **Whether a mutation on this platform can read only the row it is
    /// mutating.**
    ///
    /// `uses_alter_table_update` above says how the mutation is SPELLED; this
    /// says what the mutation may LOOK AT, and the two are independent facts
    /// that happen to be true of the same engine today. A platform answering
    /// `true` here cannot express *assign this column from another relation*
    /// at all: neither the `SET` list nor the `WHERE` may correlate out to a
    /// second table, so no re-spelling of a correlated `UPDATE` compiles —
    /// what is available instead is `INSERT … SELECT` over a join and a
    /// mutation whose predicate is an uncorrelated `IN`.
    ///
    /// **Measured, and it is the reason the seam exists.** `ClickHouse`
    /// 24.10.2 answered razeri's emitted re-open with
    /// `Code: 47 … UNKNOWN_IDENTIFIER` while resolving the assignment's own
    /// subquery — after the statement PARSED, which is what made the
    /// distinction visible: the syntax was right and the reach was not.
    ///
    /// It sits here on [`SqlRenderer`] and **only** here, deliberately not
    /// duplicated onto `DialectCapabilities`: `SqlRenderer` is the trait
    /// razeri's generators call, so this is the definition that decides, and a
    /// second declaration of the same fact is one that can drift.
    ///
    /// Default `false`: every engine razeri reaches except `ClickHouse` lets a
    /// mutation correlate, and the ANSI correlated `UPDATE` those nine compile
    /// is unchanged by this flag existing.
    fn mutation_reads_only_its_own_rows(&self) -> bool {
        false
    }

    /// **The spelling a SQL statement inside a generated procedure body reads
    /// a declared local variable by.**
    ///
    /// A versioning load binds its clock ONCE — a `Declare` step at the top of
    /// the procedure — and every close and every open reads the variable
    /// instead of the engine's clock, so there is no daylight between the
    /// close of one version and the open of the next. The DECLARATION is the
    /// procedure renderer's business; what the load generators need is the
    /// reference spelling to embed in the raw statements they compose, and
    /// that differs by procedure language:
    ///
    /// - bare `name` — PL/pgSQL, MySQL/MariaDB, and the default;
    /// - `@name` — T-SQL;
    /// - `:name` — Snowflake Scripting's bind syntax inside SQL statements;
    /// - `$name` — the agnostic format's marker.
    ///
    /// It mirrors the variable-reference spelling of Razeri's own procedure
    /// renderers — the two must agree, and each override says so beside the
    /// spelling.
    fn render_procedure_variable_reference(&self, name: &str) -> String {
        name.to_owned()
    }

    /// **The statement that binds one query's answer to a declared local.**
    ///
    /// A procedure body asks a question — how many rows arrived, what the
    /// latest timestamp is — and keeps the answer in a variable the rest of the
    /// body reads. Every procedural language has that statement and no two
    /// spell it alike:
    ///
    /// - `SELECT COUNT(*) FROM stage INTO v_seen;` — PL/pgSQL;
    /// - `SELECT @v_seen = (SELECT COUNT(*) FROM stage);` — T-SQL;
    /// - `v_seen := (SELECT COUNT(*) FROM stage);` — Snowflake Scripting;
    /// - `EXECUTE IMMEDIATE 'SELECT COUNT(*) FROM stage' INTO v_seen;` —
    ///   Oracle's PL/SQL.
    ///
    /// `query` is the question, already carrying this dialect's variable
    /// spelling and with no trailing terminator; `variable` is the bare name of
    /// the local, so a grammar that wants the reference spelling asks
    /// [`Self::render_procedure_variable_reference`] for it and one that wants
    /// the bare name has it already. The answer is a whole statement,
    /// terminator included.
    ///
    /// The default names the query and the target in a form that belongs to no
    /// engine, because there is no spelling every engine shares. A dialect
    /// rendering its own routine grammar answers this and the body arrives
    /// ready to compile.
    fn render_procedure_query_into_variable(&self, variable: &str, query: &str) -> String {
        format!("EXECUTE {query} INTO {};", self.render_procedure_variable_reference(variable))
    }

    /// **The statement that stops a procedure body with an error of its own.**
    ///
    /// A generated load checks what it was given and refuses to go on when the
    /// answer is wrong. Every procedural language can stop that way and no two
    /// write it alike:
    ///
    /// - `RAISE EXCEPTION '%', 'the run brought no rows';` — PL/pgSQL;
    /// - `THROW 50000, 'the run brought no rows', 1;` — T-SQL;
    /// - `SIGNAL SQLSTATE '45000' SET MESSAGE_TEXT = 'the run brought no rows';`
    ///   — `MySQL` and `MariaDB`;
    /// - `RAISE_APPLICATION_ERROR(-20001, 'the run brought no rows');` —
    ///   Oracle's PL/SQL.
    ///
    /// `message` is the message **already rendered as a SQL expression in this
    /// dialect's spelling**: a literal arrives quoted and escaped, and a
    /// variable arrives named the way
    /// [`Self::render_procedure_variable_reference`] names one, because a body
    /// may raise what it read rather than what it was written with. Put it
    /// where the statement wants it and do not quote it again. `error_code` is
    /// the number the design gave, if it gave one; an engine whose statement
    /// takes a number supplies its own when this is `None`. The answer is a
    /// whole statement, terminator included.
    ///
    /// The default names the raise in a form that belongs to no engine. A
    /// dialect rendering its own routine grammar answers this and the body
    /// arrives ready to compile.
    fn render_procedure_raise(&self, message: &str, error_code: Option<i32>) -> String {
        match error_code {
            Some(code) => format!("RAISE EXCEPTION [{code}] {message};"),
            None => format!("RAISE EXCEPTION {message};"),
        }
    }

    /// **The statement that writes a message where an operator can read it.**
    ///
    /// A load says how it is going, and where that message goes is an engine's
    /// own business — a client notice, a print stream, an event table, a
    /// result set nobody reads:
    ///
    /// - `RAISE NOTICE '%', 'loaded';` — PL/pgSQL;
    /// - `PRINT 'loaded';` — T-SQL;
    /// - `SYSTEM$LOG('INFO', 'loaded');` — Snowflake Scripting;
    /// - `DBMS_OUTPUT.PUT_LINE('loaded');` — Oracle's PL/SQL, which has no
    ///   levels and ignores the one it is given.
    ///
    /// `message` arrives the same way [`Self::render_procedure_raise`] takes
    /// it: already a SQL expression in this dialect's spelling, quoted where it
    /// is a literal. `level` is the severity the design wrote; an engine whose
    /// statement carries one reads
    /// [`ProcedureLogLevel::as_str`] and spells it the way that statement
    /// wants. The answer is a whole statement, terminator included.
    ///
    /// The default names the level and the message in a form that belongs to no
    /// engine. A dialect rendering its own routine grammar answers this and the
    /// body arrives ready to compile. **An engine with no statement that
    /// writes a message answers `None`**, and the line is left out of the
    /// routine rather than written as a comment or a statement it refuses.
    fn render_procedure_log(&self, level: ProcedureLogLevel, message: &str) -> Option<String> {
        let word = match level {
            ProcedureLogLevel::Debug => "DEBUG",
            ProcedureLogLevel::Info => "NOTICE",
            ProcedureLogLevel::Warning => "WARNING",
            ProcedureLogLevel::Error => "EXCEPTION",
        };
        Some(format!("LOG {word} {message};"))
    }

    /// **The statement a generated load asks a scalar question with and throws
    /// the answer away.**
    ///
    /// A load's guard evaluates one scalar and reads nothing back: it either
    /// passes silently or stops the load on the engine's own error. What
    /// differs between engines is not the question but the STATEMENT that may
    /// carry it, and the difference is a procedure-language rule rather than a
    /// SQL one:
    ///
    /// - a bare `SELECT` is a legal script statement everywhere, and is legal
    ///   inside the procedure bodies `MySQL`, `MariaDB` and T-SQL accept, where it
    ///   simply hands the caller a result set nobody reads — so that is the
    ///   default;
    /// - **PL/pgSQL refuses it.** A `SELECT` with no destination inside a
    ///   procedure body is `query has no destination for result data`, raised
    ///   the moment the procedure is called. PL/pgSQL's own discard form is
    ///   `PERFORM`, and the Postgres family overrides this to spell it.
    ///
    /// The engines that ship no stored procedure inline the same statement into
    /// a script, where the default is the only form that runs — and every such
    /// engine takes the default, so one answer per dialect is unambiguous.
    ///
    /// `alias` names the answered column for the forms that answer one; a
    /// discard form that answers nothing ignores it.
    fn render_discarded_scalar(&self, expression: &str, alias: &str) -> String {
        format!("SELECT {expression} AS {alias};")
    }

    /// **How a script with no stored procedures binds the load's clock once**
    /// — same rule as [`Self::render_procedure_variable_reference`], on the
    /// engines where a load ships as a flat statement list rather than a
    /// procedure body.
    ///
    /// The statements run FIRST and bind the engine's current moment to
    /// `name`; the reference is the expression every close and open in the
    /// same run then reads, so a version's `record_end_timestamp` and its
    /// successor's `record_start_timestamp` are one value by construction.
    /// Every spelling must be RE-RUNNABLE in one session — a second load in
    /// the same session rebinds rather than failing — because a load script is
    /// run whole and run again.
    ///
    /// The default is a one-row temporary table capturing
    /// [`Self::render_current_timestamp`], read back through a scalar
    /// subquery. Not every engine takes it: `CockroachDB` 24.3 answers
    /// *"temporary tables are only supported experimentally"*, and an engine
    /// like that overrides this with a session value of its own (`set_config`
    /// and `current_setting` there). Engines with a native variable idiom
    /// override it too (`DuckDB`'s `SET VARIABLE`, Databricks' session
    /// variables); `ClickHouse` overrides only the temporary-table spelling its
    /// own DDL requires.
    fn render_inline_clock_binding(&self, name: &str, moment: &str) -> InlineClockBinding {
        InlineClockBinding {
            statements: vec![
                format!("DROP TABLE IF EXISTS {name};"),
                format!("CREATE TEMPORARY TABLE {name} AS SELECT {moment} AS load_clock;"),
            ],
            reference: format!("(SELECT load_clock FROM {name})"),
        }
    }

    /// Whether UPDATE supports TOP(n) clause (`SqlServer`).
    fn update_supports_top_clause(&self) -> bool {
        false
    }

    /// Whether UPDATE supports table hints (e.g., `WITH (NOLOCK)`) (`SqlServer`).
    fn update_supports_table_hints(&self) -> bool {
        false
    }

    /// Whether UPDATE uses OUTPUT clause positioned before FROM (`SqlServer`).
    ///
    /// When `true`, RETURNING/OUTPUT is rendered between SET and FROM.
    /// When `false`, RETURNING is rendered at the end of the statement.
    fn update_uses_output_clause(&self) -> bool {
        false
    }

    /// Whether UPDATE supports a RETURNING clause at the end of the statement.
    ///
    /// `PostgreSQL`, `CitusDB`: `true`. Others: `false`.
    fn update_supports_returning(&self) -> bool {
        false
    }

    // ========================================================================
    // INSERT behavior hooks
    // ========================================================================

    /// Whether INSERT uses OUTPUT clause positioned before VALUES/SELECT (`SqlServer`).
    ///
    /// When `true`, RETURNING/OUTPUT is rendered between column list and source.
    /// When `false`, RETURNING is rendered at the end of the statement.
    fn insert_uses_output_clause(&self) -> bool {
        false
    }

    /// Whether INSERT supports RETURNING at the end (`PostgreSQL`, `CitusDB`).
    fn insert_supports_returning(&self) -> bool {
        false
    }

    /// Whether INSERT supports table hints (e.g., `WITH (TABLOCK)`) (`SqlServer`).
    fn insert_supports_table_hints(&self) -> bool {
        false
    }

    /// Whether INSERT OVERWRITE is supported (Snowflake, Databricks).
    fn supports_insert_overwrite(&self) -> bool {
        false
    }

    /// Whether INSERT ... ON CONFLICT is supported (`PostgreSQL`, `CitusDB`).
    fn supports_on_conflict(&self) -> bool {
        false
    }

    /// Whether OVERRIDING SYSTEM/USER VALUE clause is supported (`PostgreSQL`).
    fn supports_overriding_clause(&self) -> bool {
        false
    }

    /// The keyword for returning data from DML: "OUTPUT" (`SqlServer`) or "RETURNING" (others).
    fn returning_clause_keyword(&self) -> &'static str {
        "RETURNING"
    }

    /// Whether OUTPUT ... INTO is supported (`SqlServer`).
    fn supports_returning_into(&self) -> bool {
        false
    }

    /// Whether multi-table INSERT (INSERT ALL) is supported (Snowflake).
    fn supports_multi_table_insert(&self) -> bool {
        false
    }

    // ========================================================================
    // Function/procedure parameter behavior hooks
    // ========================================================================

    /// Render the security mode clause for function/procedure definitions.
    ///
    /// Standard SQL: `SECURITY INVOKER`/`SECURITY DEFINER`.
    /// T-SQL: `EXECUTE AS CALLER`/`EXECUTE AS OWNER`.
    fn render_security_mode(&self, is_definer: bool) -> &'static str {
        if is_definer { "SECURITY DEFINER" } else { "SECURITY INVOKER" }
    }

    /// Render a function parameter name.
    ///
    /// Standard SQL uses the bare name. T-SQL prefixes with `@`.
    fn render_function_parameter_name(&self, name: &str) -> String {
        name.to_string()
    }

    /// Render explicit nullability for a function parameter, if the platform requires it.
    ///
    /// Most platforms omit nullability in parameter lists. T-SQL native
    /// compilation requires explicit `NULL`/`NOT NULL`.
    fn render_function_parameter_nullable(&self, _nullable: bool) -> Option<&'static str> {
        None
    }

    /// Render the default-value clause for a function parameter.
    ///
    /// Standard SQL: `DEFAULT value`. T-SQL: `= value`.
    fn render_function_parameter_default(&self, value: &str) -> String {
        format!("DEFAULT {value}")
    }

    /// Render a column name inside a table-valued function definition.
    ///
    /// Standard SQL uses the bare name. T-SQL uses bracket quoting `[name]`.
    fn render_function_column_name(&self, name: &str) -> String {
        name.to_string()
    }

    // ========================================================================
    // Partition & generated column rendering hooks
    // ========================================================================

    /// Render a time partition expression for a column and granularity.
    ///
    /// Granularity values: `"hourly"`, `"daily"`, `"weekly"`, `"monthly"`,
    /// `"quarterly"`, `"yearly"`.
    ///
    /// Default: `date_trunc('unit', column)` (ANSI / `PostgreSQL` style),
    /// mapping granularity names to `date_trunc` units.
    fn render_time_partition_expr(&self, column: &str, granularity: &str) -> String {
        let unit = match granularity {
            "hourly" => "hour",
            "daily" => "day",
            "weekly" => "week",
            "monthly" => "month",
            "quarterly" => "quarter",
            "yearly" => "year",
            other => other,
        };
        format!("date_trunc('{unit}', {column})")
    }

    /// Render a DDL partition clause wrapping the expression list.
    ///
    /// Default: `PARTITION BY (expr)`. Snowflake: `CLUSTER BY (expr)`,
    /// Databricks: `PARTITIONED BY (expr)`, `ClickHouse`: `PARTITION BY expr` (no parens).
    fn render_partition_clause(&self, expr: &str) -> String {
        format!("PARTITION BY ({expr})")
    }

    /// Render a generated column declaration.
    ///
    /// Default: `name GENERATED ALWAYS AS (expr) STORED` (`PostgreSQL` style).
    fn render_generated_column(&self, name: &str, expr: &str) -> String {
        format!("{name} GENERATED ALWAYS AS ({expr}) STORED")
    }

    // ========================================================================
    // Window function capability hooks
    // ========================================================================

    /// Whether the platform supports the EXCLUDE clause in window frame specifications.
    ///
    /// `PostgreSQL`, Snowflake, Databricks: `true`. Others: `false`.
    fn supports_window_frame_exclude(&self) -> bool {
        false
    }

    /// Whether the platform supports the GROUPS frame mode in window functions.
    ///
    /// `PostgreSQL`, Databricks: `true`. Others: `false`.
    fn supports_window_frame_groups(&self) -> bool {
        false
    }

    /// Whether the platform supports named windows (WINDOW clause).
    ///
    /// Most platforms: `true`. `ClickHouse`, `DuckDB`, `MariaDB`: `false`.
    fn supports_named_windows(&self) -> bool {
        true
    }

    // ========================================================================
    // DDL table-scope capability hooks
    // ========================================================================

    /// Whether the platform uses a `#` prefix for session-scoped temporary table names.
    ///
    /// SQL Server: `true`. Others: `false`.
    fn uses_hash_temp_table_prefix(&self) -> bool {
        false
    }

    /// Whether the platform supports `DECLARE @name TABLE (...)` table variables.
    ///
    /// SQL Server: `true`. Others: `false`.
    fn supports_table_variables(&self) -> bool {
        false
    }

    /// The full CREATE keyword for session-scoped temporary tables.
    ///
    /// Default: `"CREATE TEMPORARY"`.
    /// SQL Server: `"CREATE"` (uses `#name` instead of TEMPORARY keyword).
    /// Databricks: `"CREATE OR REPLACE TEMPORARY"`.
    fn render_create_temp_table_keyword(&self) -> &'static str {
        "CREATE TEMPORARY"
    }

    /// Whether the platform supports transient tables (e.g. `CREATE TRANSIENT TABLE`).
    ///
    /// Snowflake: `true`. Others: `false`.
    fn supports_transient_tables(&self) -> bool {
        false
    }

    /// Whether the platform supports `ON COMMIT DROP` for transaction-scoped temp tables.
    ///
    /// `PostgreSQL`, `CitusDB`: `true`. Others: `false`.
    fn supports_on_commit_drop(&self) -> bool {
        false
    }

    /// **The clause written after a table's column list**, or `None` on an
    /// engine whose `CREATE TABLE` is complete at the closing parenthesis.
    ///
    /// Most engines take `CREATE TABLE t (…)` as it stands. Some refuse it
    /// without a clause the column list cannot carry — `ClickHouse` answers
    /// `ORDER BY or PRIMARY KEY clause is missing` — so its dialect answers the
    /// table's engine and sorting key here: `ENGINE = MergeTree()` and
    /// `ORDER BY (…)`, `ReplacingMergeTree(<version column>)` on a versioned
    /// table, and `ENGINE = Memory` on a temporary one. An engine with a
    /// mandatory clause of its own spells it here too.
    ///
    /// Razeri writes the answer on its own line, directly after the closing
    /// parenthesis of every `CREATE TABLE` it renders, and before the storage
    /// plan's clause from
    /// [`render_table_storage_clause`](crate::StorageRenderer::render_table_storage_clause).
    /// **What you answer here, the storage clause must not repeat**, and the
    /// reverse: two engine clauses in one statement is `Code: 62 … Syntax
    /// error` on `ClickHouse`. A sorting key belongs here rather than there,
    /// because it usually has to begin with the table's primary key, and only
    /// this method is handed it.
    ///
    /// Answer without a terminator; more than one line is fine.
    ///
    /// ```
    /// use razeri_plugin_contract::{SqlRenderer, TableSuffixRequest};
    ///
    /// struct Columnar;
    ///
    /// impl SqlRenderer for Columnar {
    ///     # fn render_data_type(&self, _: &razeri_plugin_contract::DataType) -> String { "TEXT".into() }
    ///     # fn quote_identifier(&self, name: &str) -> String { format!("\"{name}\"") }
    ///     # fn qualify_name(&self, _: Option<&str>, name: &str) -> String { self.quote_identifier(name) }
    ///     # fn render_current_timestamp(&self) -> String { "CURRENT_TIMESTAMP".into() }
    ///     fn render_table_suffix(&self, table: &TableSuffixRequest<'_>) -> Option<String> {
    ///         if table.temporary {
    ///             return Some("USING MEMORY".to_owned());
    ///         }
    ///         let key: Vec<String> =
    ///             table.primary_key.iter().map(|c| self.quote_identifier(c)).collect();
    ///         Some(format!("USING COLUMNSTORE\nSORT KEY ({})", key.join(", ")))
    ///     }
    /// }
    ///
    /// let key = ["id".to_owned()];
    /// let table = TableSuffixRequest::new().with_primary_key(&key);
    /// assert_eq!(
    ///     Columnar.render_table_suffix(&table).as_deref(),
    ///     Some("USING COLUMNSTORE\nSORT KEY (\"id\")"),
    /// );
    /// ```
    fn render_table_suffix(&self, table: &TableSuffixRequest<'_>) -> Option<String> {
        let _ = table;
        None
    }

    /// Whether this platform's table sorting key refuses nullable columns.
    ///
    /// `ClickHouse`: `true` — a `MergeTree` sorting key containing a
    /// `Nullable(…)` column answers `Code: 44 … Sorting key contains nullable
    /// columns, but merge tree setting allow_nullable_key is disabled`, measured
    /// on 24.10.2 against the control (the same key over a non-nullable column
    /// of the same type is accepted). Others: `false`.
    ///
    /// The materializer asks this before it lets a design's clustering columns
    /// into a sorting key, so the answer is a refusal the author can act on
    /// rather than a deployment failure they cannot.
    fn sorting_key_refuses_nullable_columns(&self) -> bool {
        false
    }

    /// Whether the platform requires a primary key on every table.
    ///
    /// `ClickHouse` (MergeTree): `true`. Others: `false`.
    fn requires_primary_key_in_create_table(&self) -> bool {
        false
    }

    /// Create a boxed [`FormulaRenderer`](crate::FormulaRenderer) for this dialect.
    ///
    /// Real dialect implementations override this to return `Box::new(Self)`.
    /// The default returns an ANSI dialect with standard function names.
    fn to_formula_renderer(&self) -> Box<dyn crate::FormulaRenderer> {
        Box::new(crate::AnsiDialect)
    }
}

/// Level 2 — Expression rendering with ANSI SQL defaults.
pub trait ExpressionRenderer: SqlRenderer {
    /// **Render a conversion of `expr` to `target`** — the expression the
    /// generated load's typed projection writes for every column.
    ///
    /// The default is `CAST(expr AS …)` over
    /// [`render_cast_type`](SqlRenderer::render_cast_type), the cast vocabulary
    /// rather than the column vocabulary: a column may be declared `STRING(36)`
    /// where a cast may name only `STRING`, and `TINYINT` where a cast must say
    /// `SMALLINT`. Override it where a conversion is not a `CAST` at all —
    /// `GoogleSQL` has no cast from `STRING` to `JSON` ("Invalid cast from
    /// STRING to JSON") and converts with `PARSE_JSON(expr)` instead.
    fn render_cast(&self, expr: &str, target: &DataType) -> String {
        format!("CAST({} AS {})", expr, self.render_cast_type(target))
    }

    /// **This engine's canonical text for a value**: the same characters on
    /// every engine, whatever the session is set to.
    ///
    /// `expr` holds a value of `value_type`. The answer is its text as
    /// [`CanonicalText::of`](crate::CanonicalText::of) defines it for that
    /// type — `2026-01-05` for a date, `2026-01-05 12:34:56.12` for a
    /// timestamp, `2026-01-05 12:34:56.12+00:00` for a zoned moment, `10.00`
    /// for a `DECIMAL(12, 2)` ten, `10` for a floating ten, `true` for a truth
    /// value — and `NULL` where `expr` is `NULL`. `text` is the text type the value would otherwise be cast
    /// to, and every type written by the engine's own cast is cast to it.
    ///
    /// A load that refuses its rows names their values in this text, and a
    /// reconciliation writes a decimal measurement in it before comparing it
    /// with the source's by string equality. So an engine that writes a date
    /// by a format its session sets, drops a decimal's trailing zeros, writes
    /// every digit of a timestamp's precision, writes a zoned moment in the
    /// session's zone, writes a floating number to more or fewer digits, or
    /// holds a truth value as a number answers here with an explicit format. The default is the plain cast to `text`, which is the
    /// canonical text on an engine that does none of those things.
    fn render_value_text(&self, expr: &str, value_type: &DataType, text: &DataType) -> String {
        let _ = value_type;
        self.render_cast(expr, text)
    }

    /// **The functions [`Self::render_value_text`] calls, and the statements
    /// that create them**, or `None` where it calls none.
    ///
    /// An engine with no `printf` computes a floating number's text from its
    /// own arithmetic ([`computed_float_text`](crate::computed_float_text)),
    /// and that arithmetic is several thousand characters long. Written into
    /// every place a value is named, it makes a statement hard to read and a
    /// line some clients misread. So such an engine writes it once, as
    /// [`FLOAT_TEXT_FUNCTION`](crate::FLOAT_TEXT_FUNCTION) and
    /// [`SINGLE_FLOAT_TEXT_FUNCTION`](crate::SINGLE_FLOAT_TEXT_FUNCTION), and
    /// its value text calls them by those names.
    ///
    /// A generated script creates them before anything that calls them, and
    /// only where something does. The default is `None`: the engine writes
    /// every value's text inline.
    fn value_text_functions(&self) -> Option<crate::ValueTextFunctions> {
        None
    }

    /// **An expression that stops the statement with `sentence`**, where
    /// `sentence` is not `NULL`, and is `NULL` where it is.
    ///
    /// A generated load refuses rows it cannot write honestly by building a
    /// sentence about them and asking the engine for this expression. The
    /// engine's error then carries the sentence to whoever reads it. The
    /// default converts the sentence to a whole number, which most engines
    /// refuse and quote back: *Could not convert string '…' to INT64*. An
    /// engine that answers that conversion without an error spells a refusal
    /// of its own here, or says through
    /// [`DialectCapabilities::refusal_stops_the_statement`](crate::DialectCapabilities::refusal_stops_the_statement)
    /// that its refusal only warns.
    fn render_refusal(&self, sentence: &str) -> String {
        self.render_cast(sentence, &DataType::bigint())
    }

    /// **A statement that reads `relation`, changes nothing, and stops with
    /// `sentence` where a row meets `condition`** — or `None`, the default,
    /// where the engine has none, and a load's refusal stays a statement that
    /// would write and stops on [`Self::render_refusal`].
    ///
    /// `condition` is true of a row holding a value the load refuses, and
    /// `sentence` is a text expression over that row: the words the engine's
    /// error should carry, and nothing around them. A statement that only
    /// reads is one an engine may run on several workers, which a statement
    /// that writes is not; the refusal ahead of a load reads every staged row,
    /// so on a large file that is most of what it costs. And an error raised
    /// with its own message carries the sentence alone, where a failed
    /// conversion wraps it in the engine's words about a type the column is
    /// not.
    fn render_read_only_refusal(
        &self,
        relation: &str,
        condition: &str,
        sentence: &str,
    ) -> Option<String> {
        let _ = (relation, condition, sentence);
        None
    }

    /// **Whether a number written as text fits a decimal column, asked in
    /// this engine's own words** — or `None`, the default, where Razeri asks
    /// it portably: the digits stripped one `REPLACE` at a time to see what is
    /// left, and nine `LIKE` patterns, one for each digit but `0`, wherever a
    /// digit's place is asked.
    ///
    /// `text` is a text expression, the value as the file wrote it with its
    /// surrounding spaces and the marks a plain number may carry already
    /// taken out. The column is `decimal(precision, scale)`. The answer is
    /// three conditions over `text`; see [`DecimalFit`] for what each is true
    /// of. A load refuses a value too large for the column and one with more
    /// places than it keeps, in sentences of their own, rather than letting
    /// the engine cap, round or refuse it in its own words. Each condition is
    /// asked of every staged value, so an engine whose `LIKE` takes a
    /// character class, or which matches a regular expression, answers here
    /// in one or two tests where the portable form takes thirty.
    ///
    /// A try-conversion (`TRY_CONVERT`, `TRY_CAST`, `SAFE_CAST`) is no
    /// answer: it rounds a value with more places than the type keeps, so it
    /// cannot say that it has them.
    fn render_decimal_fit(&self, text: &str, precision: u8, scale: u8) -> Option<DecimalFit> {
        let _ = (text, precision, scale);
        None
    }

    /// **How many characters `expr` holds**, counted the way a person counts
    /// them: one for each character, whatever the bytes that store it, and
    /// every space inside or at either end of the value included.
    ///
    /// A reconciliation totals this over a text column and compares the total
    /// with the one razeri counted in the source file, so a value that gained
    /// a carriage return, lost its last letters or was padded on the way in
    /// shows up as a different number. `expr` is a text value; `NULL` counts
    /// as `NULL`, which a total leaves out.
    ///
    /// The default is the standard `CHAR_LENGTH`. An engine without it
    /// answers with its own function, and an engine whose usual length leaves
    /// out trailing spaces, counts bytes, or counts a character outside the
    /// basic plane as two answers with one that does none of those.
    fn render_text_length(&self, expr: &str) -> String {
        format!("CHAR_LENGTH({expr})")
    }

    /// **The unit [`render_text_length`](Self::render_text_length) counts a
    /// value of this type in**, held where `held` says.
    ///
    /// A reconciliation compares the total with one counted from the source
    /// file, and counts the file in the same unit, so the two agree and the
    /// engine is asked for nothing it does not do quickly. The default is
    /// [`TextLengthUnit::Characters`]. An engine whose length of a large
    /// object counts UTF-16 code units — a character outside the basic plane
    /// counting two — answers [`TextLengthUnit::Utf16CodeUnits`] for the types
    /// it holds as one.
    fn text_length_unit(&self, data_type: &DataType, held: HeldAs) -> TextLengthUnit {
        let _ = (data_type, held);
        TextLengthUnit::Characters
    }

    /// **The expression a load compares in place of `value`**, a column of
    /// this type held where `held` says, or `None` where this engine cannot
    /// compare such a value at all.
    ///
    /// A load looks for a row that arrives twice: it collapses the rows it
    /// reads with `SELECT DISTINCT` and counts each column's distinct values,
    /// so every column it reads is compared. The default answers `value`
    /// itself, which is right wherever the engine compares the type
    /// directly. An engine that holds the type as a large object it cannot
    /// group or compare answers `None`, and Razeri refuses such a column by
    /// name when it writes the load, saying to give it a length; or it answers
    /// an expression that stands for the value wherever two values are
    /// compared — the same text for the same value, and different text for
    /// different values.
    fn render_comparable(&self, value: &str, data_type: &DataType, held: HeldAs) -> Option<String> {
        let _ = (data_type, held);
        Some(value.to_owned())
    }

    /// **A test that is true where a value of a date or timestamp column is
    /// not written as an ISO 8601 date**, on an engine that holds such a
    /// column as text it does not check — or `None`, the default, where the
    /// engine's date type checks every value written into it.
    ///
    /// On such an engine a fill written by hand can put `17/09/2026` in a
    /// load's day column and nothing refuses it: the load matches no row, the
    /// day loads empty, and every count agrees with itself. The reconciliation
    /// counts the values this test finds, and expects none. `value` is the
    /// column as the statement names it; `data_type` is the type the design
    /// declares, a date or a timestamp.
    fn render_not_an_iso_day(&self, value: &str, data_type: &DataType) -> Option<String> {
        let _ = (value, data_type);
        None
    }

    /// **`text`, or a missing value where it is exactly one of `words`**: how
    /// a load reads a source field that holds a word meaning "no value", such
    /// as `NULL`, `n/a` or `?`.
    ///
    /// A source file names its own words for a missing value, and one design
    /// has to load the same warehouse on every engine, so a word matches
    /// letter for letter and in the same case, whatever the engine's usual
    /// comparison. `text` is a text value, already trimmed; `words` are the
    /// words as the file writes them, and the empty word is among them.
    ///
    /// The default writes each word as a standard string literal, its quotes
    /// doubled, and compares with the standard `IN`. An engine whose usual
    /// comparison ignores case or accents answers with a comparison that does
    /// not, and one whose string literals treat a backslash as an escape
    /// writes each word so that a backslash in it stays a backslash.
    fn render_missing_value(&self, text: &str, words: &[&str]) -> String {
        let listed = words
            .iter()
            .map(|word| format!("'{}'", word.replace('\'', "''")))
            .collect::<Vec<_>>()
            .join(", ");
        format!("CASE WHEN {text} IN ({listed}) THEN NULL ELSE {text} END")
    }

    /// **Read a value the source spells its own way** into `target`, which is
    /// a date or a timestamp.
    ///
    /// An import arrives with `1/1/2015` in one column and
    /// `2015-01-01T00:00:00Z` in the next, and no engine reads either of them
    /// the same way as the engine beside it. So the design states the
    /// spelling, razeri reads it once into a [`DateFormat`], and this
    /// translates that list into whatever this engine's own pattern language
    /// is — `MM/DD/YYYY`, `MM/dd/yyyy`, a style number, or a format string
    /// this engine calls its own.
    ///
    /// **A value that is not spelled that way has to come back as NULL.**
    /// razeri reads the NULL as *this row did not parse* and writes the row
    /// into the table it sets rows aside in, naming the column and the format
    /// it was told to expect. An expression that stops the load instead leaves
    /// a warehouse half-loaded over one bad row, and one that answers a
    /// different day than the value says is worse than either. An engine whose
    /// parse reads a value it cannot read — rolling the forty-fifth of a month
    /// forward rather than answering nothing — writes its own guard beside the
    /// parse; [`DateFormat::shape_pattern`] and
    /// [`DateFormat::capture_pattern`] come from the same directive list, so
    /// the guard and the parse cannot hold two opinions about one spelling.
    ///
    /// **A directive this engine cannot say is a refusal, not a guess.**
    /// [`DateFormat::translate`] hands back the first directive this engine
    /// answered nothing for, and answering `None` here is how that reaches
    /// the author — by name, at the door, rather than as a wrong day in the
    /// warehouse.
    ///
    /// `None` — the default — is this engine stating nothing about how it
    /// reads a spelled value. razeri then refuses the column and names the
    /// engine.
    fn render_parse_datetime(
        &self,
        _expr: &str,
        _format: &DateFormat,
        _target: &DataType,
    ) -> Option<String> {
        None
    }

    /// **Read a value the source writes its own way** into `target`, which is
    /// a number.
    ///
    /// A file writes a price `$2,899.99 `. The symbol, the thousands mark and
    /// the space around it are not part of the number, and no two engines
    /// agree about what to do with them — one converts the text to nothing,
    /// one to `2`, one raises. So the design states the picture, razeri reads
    /// it once into a [`NumberFormat`], and this renders the reading in
    /// whatever this engine's own words for *take this out* and *convert this*
    /// are.
    ///
    /// **A value that is not written that way has to come back as NULL.**
    /// razeri reads the NULL as *this row did not parse* and writes the row
    /// into the table it sets rows aside in, naming the column and the picture
    /// it was told to expect. An expression that stops the load instead leaves
    /// a warehouse half-loaded over one bad row, and one that answers `0` for
    /// text that was never a number is worse than either. An engine whose
    /// conversion raises, or answers a number for text that is not one, writes
    /// a guard in front of it; [`NumberFormat::shape_pattern`] is built from
    /// the same picture the marks come from, so the guard and the reading
    /// cannot hold two opinions about one spelling.
    ///
    /// `None` — the default — is this engine stating nothing about how it
    /// reads a written amount. razeri then refuses the column and names the
    /// engine. `None` is also the right answer for a target that is not a
    /// number: a picture says how an amount is written, and there is nothing
    /// else for it to be about.
    ///
    /// [`NumberFormat`]: crate::number_format::NumberFormat
    /// [`NumberFormat::shape_pattern`]: crate::number_format::NumberFormat::shape_pattern
    fn render_parse_number(
        &self,
        _expr: &str,
        _format: &NumberFormat,
        _target: &DataType,
    ) -> Option<String> {
        None
    }

    /// Render string concatenation.
    fn render_concat(&self, parts: &[String]) -> String {
        format!("CONCAT({})", parts.join(", "))
    }

    /// Render a COALESCE expression.
    fn render_coalesce(&self, parts: &[String]) -> String {
        format!("COALESCE({})", parts.join(", "))
    }

    /// Render a string aggregation function.
    fn render_string_agg(&self, expr: &str, separator: &str) -> String {
        format!("STRING_AGG({expr}, '{separator}')")
    }
}

/// A foreign key's referential action — SQL's own five words, no synonyms.
///
/// Lives here (rather than in the table model) so a dialect can answer
/// [`DdlRenderer::supports_foreign_key_action`] about a specific action
/// without depending on the modeling crate.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum ReferentialAction {
    /// Also delete or update the referencing rows
    Cascade,
    /// Set the referencing columns to NULL
    SetNull,
    /// Reject the delete or update; some engines defer the check
    Restrict,
    /// Reject the delete or update (the SQL default)
    NoAction,
    /// Set the referencing columns to their default values
    SetDefault,
}

impl ReferentialAction {
    /// Parses the SQL words, case-insensitively; underscores read as spaces
    /// so `set_null` and `SET NULL` both land.
    ///
    /// # Errors
    /// Returns a plain message listing the valid words when the input is not
    /// one of them.
    pub fn from_sql_word(word: &str) -> Result<Self, String> {
        match word.trim().replace('_', " ").to_uppercase().as_str() {
            "CASCADE" => Ok(Self::Cascade),
            "SET NULL" => Ok(Self::SetNull),
            "RESTRICT" => Ok(Self::Restrict),
            "NO ACTION" => Ok(Self::NoAction),
            "SET DEFAULT" => Ok(Self::SetDefault),
            _ => Err(format!(
                "'{word}' is not a referential action. Use one of: CASCADE, SET NULL, \
                 RESTRICT, NO ACTION, SET DEFAULT."
            )),
        }
    }
}

impl std::fmt::Display for ReferentialAction {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Cascade => write!(f, "CASCADE"),
            Self::SetNull => write!(f, "SET NULL"),
            Self::Restrict => write!(f, "RESTRICT"),
            Self::NoAction => write!(f, "NO ACTION"),
            Self::SetDefault => write!(f, "SET DEFAULT"),
        }
    }
}

/// Level 3 — DDL rendering with reasonable defaults.
pub trait DdlRenderer: SqlRenderer {
    /// Prefix for CREATE TABLE statements.
    fn render_create_table_prefix(&self) -> &'static str {
        "CREATE TABLE"
    }

    /// **Whether a column's `DEFAULT` is written before its `NOT NULL`.**
    ///
    /// The default, `false`, writes `NOT NULL DEFAULT …`, which is what every
    /// engine that does not override this has always been given. An engine
    /// that refuses that order and takes only `DEFAULT … NOT NULL` answers
    /// `true`.
    fn column_default_precedes_not_null(&self) -> bool {
        false
    }

    /// Whether this engine carries the given foreign-key referential action
    /// (`ON DELETE …` / `ON UPDATE …`) in `CREATE TABLE` DDL.
    ///
    /// The ANSI default says yes to all five words. A dialect answers `false`
    /// for an action its engine rejects or silently never enforces; the
    /// renderer then keeps the author's declaration visible as a plain
    /// comment instead of emitting a clause the engine would refuse — a
    /// declared action never vanishes without a trace.
    fn supports_foreign_key_action(&self, action: ReferentialAction) -> bool {
        let _ = action;
        true
    }

    /// Prefix for CREATE VIEW statements.
    fn render_create_view_prefix(&self) -> &'static str {
        "CREATE OR REPLACE VIEW"
    }

    /// **Whether `CREATE OR REPLACE VIEW` replaces a view that other objects
    /// read.**
    ///
    /// The default, `true`, is what most engines do, and Razeri writes each
    /// view's statement as it stands. Answer `false` for an engine that
    /// refuses to replace a view while a later view, function or procedure
    /// depends on it, and Razeri asks
    /// [`render_view_replacement`](Self::render_view_replacement) for each
    /// view instead, naming the objects that read it. See
    /// [`view_replacement`](crate::view_replacement).
    fn replaces_a_view_that_is_read(&self) -> bool {
        true
    }

    /// **The statements that make a view other objects read**, on an engine
    /// that answers `false` to
    /// [`replaces_a_view_that_is_read`](Self::replaces_a_view_that_is_read).
    ///
    /// The default drops each reader, the last made first, and then writes
    /// the view's own statement. The file makes every reader again when it
    /// reaches it. A view is dropped with
    /// [`render_drop_view`](Self::render_drop_view), a function with
    /// [`render_drop_function`](Self::render_drop_function), and a procedure
    /// with `DROP PROCEDURE IF EXISTS`; a routine's parameter types are
    /// written where
    /// [`supports_drop_function_arg_types`](Self::supports_drop_function_arg_types)
    /// says the engine takes them. Each statement but the last ends with
    /// [`statement_separator`](SqlRenderer::statement_separator); the caller
    /// ends the last.
    ///
    /// Override it for an engine that replaces such a view some other way.
    fn render_view_replacement(&self, replacement: &crate::ViewReplacement<'_>) -> String {
        let separator = self.statement_separator();
        let mut statements = String::new();
        for reader in replacement.readers {
            let types: Vec<&str> = if self.supports_drop_function_arg_types() {
                reader.parameter_types.iter().map(String::as_str).collect()
            } else {
                Vec::new()
            };
            let drop = match reader.kind {
                crate::ReaderKind::View => {
                    self.render_drop_view(reader.object(), true).map_or_else(
                        || format!("{} {}", self.render_drop_view_prefix(true), reader.relation),
                        |written| written.text().to_owned(),
                    )
                }
                crate::ReaderKind::Function => {
                    self.render_drop_function(&reader.relation, &types, true)
                }
                crate::ReaderKind::Procedure => {
                    let keyword = self.render_drop_object_keyword("procedure");
                    if types.is_empty() {
                        format!("DROP {keyword} IF EXISTS {}", reader.relation)
                    } else {
                        format!(
                            "DROP {keyword} IF EXISTS {}({})",
                            reader.relation,
                            types.join(", ")
                        )
                    }
                }
            };
            statements.push_str(drop.trim_end().trim_end_matches(';'));
            statements.push_str(separator);
        }
        statements.push_str(replacement.create);
        statements
    }

    /// Render a column definition (name, type, nullability).
    fn render_column_definition(&self, name: &str, dt: &DataType, nullable: bool) -> String {
        let quoted = self.quote_identifier(name);
        let (type_str, null_str) = if nullable {
            (self.render_nullable_data_type(dt), "")
        } else {
            (self.render_data_type(dt), " NOT NULL")
        };
        format!("{quoted} {type_str}{null_str}")
    }

    /// **The statement this engine takes for one change to a table that
    /// already holds rows**, or `None` where it has none.
    ///
    /// Razeri classifies every difference between two deploys and hands the
    /// ones that lose nothing to this method, one at a time. What comes back
    /// goes into the file an operator runs; `None` means the change is named in
    /// the plan with this engine's name beside it, so the reader learns which
    /// engine has no statement rather than that no engine does.
    ///
    /// # The default, and why it is the only one
    ///
    /// `ALTER TABLE … ADD COLUMN … <type>` is the spelling most engines take,
    /// so a dialect that answers nothing still writes the commonest evolution
    /// there is: a column that was not there yesterday. Some engines write
    /// `ADD` without `COLUMN` and refuse this one — Firebird answers *"Token
    /// unknown"* at `COLUMN` — so run it on yours, and override this method
    /// where it is refused.
    /// The column is declared without `NOT NULL`, which is the only form that
    /// can be added to a table that already holds rows.
    ///
    /// Every other change answers `None` by default. A widening and a record
    /// that gained a field are each spelled three or four ways across the
    /// engines Razeri reaches, and a default picked from one of them writes SQL
    /// the others reject — a statement handed to an operator who then finds out
    /// their engine will not take it. A dialect that has measured its engine
    /// says so here, and the ones that have not say nothing.
    ///
    /// The statement comes back without a terminator; the caller separates
    /// statements the way this engine separates them.
    fn render_column_evolution(&self, change: &ColumnEvolution<'_>) -> Option<String> {
        match change {
            ColumnEvolution::AddColumn { schema, table, column, data_type } => {
                let target = self.qualify_name(*schema, table);
                let name = self.quote_identifier(column);
                let rendered = self.render_nullable_data_type(data_type);
                Some(format!("ALTER TABLE {target} ADD COLUMN {name} {rendered}"))
            }
            ColumnEvolution::WidenColumn { .. }
            | ColumnEvolution::AddNestedField { .. }
            | ColumnEvolution::RequireColumn { .. } => None,
        }
    }

    /// Render the surrogate identity column — a plain, ungenerated `NOT NULL`
    /// column of the type it is given, on **every** dialect.
    ///
    /// **razeri's surrogate keys are logical and DETERMINATE, never
    /// engine-generated.** The identity value is a hash
    /// derived from the entity's identity grain, WRITTEN BY THE LOAD
    /// (`materialize_core`, `sid_expression`), so the same design loaded with the
    /// same data converges to the same key on `Postgres`, `Snowflake`, `MariaDB`,
    /// `ClickHouse` — every target. A database-assigned `AUTO_INCREMENT` /
    /// `IDENTITY(1,1)` / `GENERATED ALWAYS AS IDENTITY` would be sequential,
    /// load-order-dependent and **non-portable** — a different key per database
    /// for the same row — which is the one thing a portable key may not be. So no
    /// dialect emits a generator here; every one renders a plain column and the
    /// load fills it.
    ///
    /// `SQLite` narrows the type to `INTEGER` and `ClickHouse`/`DuckDB` carry
    /// their own overrides only to document the same plain shape; no override
    /// re-introduces a generator.
    ///
    /// # The type is the engine's own word for it
    ///
    /// The word `BIGINT` is not written here, because not every engine has it:
    /// Oracle refuses it outright — `ORA-00902: invalid datatype` — so the
    /// table holding the column could not be created and nothing downstream
    /// of it could load. The shape is the decision here, a plain ungenerated
    /// column; how this engine spells a 64-bit integer is the dialect's answer
    /// to [`render_data_type`](SqlRenderer::render_data_type), asked for
    /// rather than assumed. An engine whose word for it *is* `BIGINT` gets a
    /// `BIGINT` column.
    fn render_identity_column(&self, name: &str, data_type: &DataType) -> String {
        let quoted = self.quote_identifier(name);
        let rendered = self.render_data_type(data_type);
        format!("{quoted} {rendered} NOT NULL")
    }

    /// Whether this platform's engine actually **assigns** an identity column's
    /// value, rather than merely tolerating the declaration.
    ///
    /// **This is a dialect fact, not a field fact, and the difference is a
    /// deploy-time failure.** `Field::identity` says razeri intends the engine
    /// to generate the value; whether it does depends entirely on the target.
    /// `render_identity_column`'s own table above records that `DuckDB`,
    /// `SQLite` and `ClickHouse` have **no native identity** — there the column
    /// comes out as a plain `NOT NULL` and **the writer must supply it.**
    ///
    /// The default is `false` because the default `render_identity_column`
    /// emits no generation clause: a dialect that overrides the rendering to
    /// generate values must override this too, and
    /// `the_identity_capability_matches_what_each_dialect_renders` holds the two
    /// in step so they cannot drift.
    ///
    /// Read by the deploy bundle's write contract: a column
    /// the engine does not generate is a column a hand-written `INSERT` owes.
    fn generates_identity_values(&self) -> bool {
        false
    }

    /// **The latest instant this platform can hold EXACTLY in the column it
    /// renders for `data_type`** — the ceiling the refuse-by-name rule compares
    /// against, or `None` for *no declared ceiling*.
    ///
    /// Spelled `YYYY-MM-DD HH:MM:SS`, which is how razeri writes a timestamp
    /// literal (`razeri_materialize::convert::OPEN_INTERVAL_SENTINEL`), so the
    /// comparison is a string comparison over a format that sorts
    /// chronologically. A ceiling and a value in the same spelling cannot be
    /// compared wrongly by a parser neither of them goes through.
    ///
    /// # What the refusal contract asks of this
    ///
    /// > A value razeri cannot store exactly on a target platform is a
    /// > deploy-time refusal naming the value, the column, the platform and the
    /// > ceiling — never a truncation, never a saturated write.
    ///
    /// The *refusal* lives in `razeri_materialize::bundle`, which is the one
    /// place it is built — the function is `refuse_unrepresentable_values`, and
    /// it is reached from `generate_bundle` before a single file is rendered.
    /// This is only where a platform declares the number.
    ///
    /// # A stated variance from the decision's original wording
    ///
    /// The refusal was decided as a *deploy-time* act, homed in the
    /// prerequisites step. Razeri refuses **earlier**: at bundle
    /// generation, by declining to emit the bundle at all. That is strictly
    /// ahead of deploy time and cannot be skipped by an operator who never runs
    /// the script — a deliberate strengthening, recorded rather
    /// than glossed. The consequence worth knowing: a *hand-assembled* bundle,
    /// or one from an older razeri, is not re-checked at deploy time by
    /// anything. **The default is `None`, which
    /// means "razeri has no measured ceiling for this platform and type" and not
    /// "this platform holds everything"** — an unmeasured platform gets no
    /// refusal rather than an invented one.
    ///
    /// # The ceiling is a property of the TYPE, not of the platform
    ///
    /// That is the half a platform-flat capability would get wrong, and it is
    /// measured rather than assumed (reading the renderers):
    ///
    /// | platform | `Timestamp(p)` renders | ceiling | `TimestampTz(p)` renders | ceiling |
    /// |---|---|---|---|---|
    /// | `MySQL`, `MariaDB` | `DATETIME(p)` | `None` — the end of the four-digit calendar is not a constraint | `TIMESTAMP(p)` | **`2038-01-19 03:14:07`** |
    /// | `ClickHouse` | `DateTime64(p)` | `2299-12-31 23:59:59` at p ≤ 6, **`2262-04-11 23:47:16`** at p ≥ 7 | `DateTime64(p,'UTC')` | the same |
    /// | everything else | — | not measured → `None` | — | `None` |
    ///
    /// On `MySQL` the two razeri type variants land on *different engine types*
    /// and get different answers: `Timestamp` renders as the
    /// non-converting `DATETIME` — the type whose semantics match the declared
    /// non-tz timestamp — which runs to the end of the calendar and so
    /// declares no ceiling at all, while `TimestampTz` keeps `TIMESTAMP` and its
    /// 32-bit epoch boundary. A check keyed on the platform alone would either
    /// miss the real ceiling or refuse a column that is fine. On `ClickHouse`
    /// the ceiling moves with the **declared precision** of one type:
    /// `DateTime64(9)` is int64 nanoseconds from the epoch and stops at
    /// 2262-04-11, while `DateTime64(6)` reaches 2299.
    ///
    /// Parameters:
    /// - `data_type`: the column's declared type, as the design carries it —
    ///   not the rendered string, because the precision is what moves the answer
    ///   and re-parsing it back out of `DateTime64(9)` would be a second reading
    ///   of what the dialect already knows.
    fn representable_timestamp_ceiling(&self, data_type: &DataType) -> Option<&'static str> {
        let _ = data_type;
        None
    }

    /// Whether the platform requires every auto-increment / identity column to
    /// be the leading column of an index (PK or KEY).
    ///
    /// `MySQL` / `MariaDB`: `true` (otherwise error 1075). Others: `false`.
    ///
    /// When this is `true` and an identity column is not already the leading
    /// column of the primary key, the DDL renderer must emit an additional
    /// `KEY (identity_col)` clause on the table.
    fn auto_increment_requires_key(&self) -> bool {
        false
    }

    /// Prefix for DROP TABLE statements.
    fn render_drop_table_prefix(&self, if_exists: bool) -> String {
        if if_exists { "DROP TABLE IF EXISTS".to_string() } else { "DROP TABLE".to_string() }
    }

    /// Prefix for DROP VIEW statements.
    fn render_drop_view_prefix(&self, if_exists: bool) -> String {
        if if_exists { "DROP VIEW IF EXISTS".to_string() } else { "DROP VIEW".to_string() }
    }

    /// Prefix for CREATE SCHEMA / CREATE NAMESPACE.
    fn render_create_schema_prefix(&self, if_not_exists: bool) -> String {
        if if_not_exists {
            "CREATE SCHEMA IF NOT EXISTS".to_string()
        } else {
            "CREATE SCHEMA".to_string()
        }
    }

    /// Render a complete CREATE SCHEMA statement.
    ///
    /// Dialects that need fundamentally different syntax (e.g. T-SQL wrapping
    /// in `IF SCHEMA_ID(...) IS NULL EXEC(...)`) should override this method.
    /// The default concatenates [`Self::render_create_schema_prefix`] with the name.
    fn render_create_schema_statement(&self, name: &str, if_not_exists: bool) -> String {
        let prefix = self.render_create_schema_prefix(if_not_exists);
        format!("{prefix} {name}")
    }

    /// Render a table comment (platform-specific syntax).
    ///
    /// Like [`Self::render_column_comment`], nothing in the tree calls this and
    /// no dialect overrides it — **and unlike its sibling it has not been
    /// measured.** Read that method's table before wiring either: the ANSI
    /// spelling is a hard syntax error on five of the ten platforms it was
    /// measured on at the column level, and this form was not probed.
    fn render_table_comment(&self, table: &str, comment: &str) -> Option<String> {
        Some(format!("COMMENT ON TABLE {table} IS '{comment}'"))
    }

    /// Render a column comment — **not wired to anything, and measured**.
    ///
    /// # Nothing calls this
    ///
    /// No caller in the tree, no dialect override, and, until measured, no
    /// execution against a live engine — the exact state the column `DEFAULT`
    /// clause was in when it turned out to be refused outright on four platforms
    /// of ten and silently wrong on a fifth. Moving the open
    /// interval's meaning here was proposed **on the condition that it be measured first**,
    /// because *"a comment that fails to compile is a worse outcome than the
    /// clause it replaced"*.
    ///
    /// # It was measured, on ten platforms, and this ANSI form is right on five
    ///
    /// Executed against six containers, in-process `DuckDB` and
    /// `SQLite`, and both live cloud accounts:
    ///
    /// | platform | this `COMMENT ON COLUMN` | inline `COMMENT` in `CREATE TABLE` | what it does take |
    /// |---|---|---|---|
    /// | `PostgreSQL`, Citus | ✅ | ❌ syntax error | this, as a separate statement |
    /// | `DuckDB` | ✅ | ❌ syntax error | this, as a separate statement |
    /// | Snowflake, Databricks | ✅ | ✅ | either |
    /// | `ClickHouse` | ❌ `SYNTAX_ERROR` (code 62) | ✅ | inline, or `ALTER TABLE … COMMENT COLUMN c '…'` |
    /// | `MySQL`, `MariaDB` | ❌ `ERROR 1064` | ✅ | inline, or `ALTER TABLE … MODIFY <whole column definition> COMMENT '…'` |
    /// | SQL Server | ❌ `Msg 156, Incorrect syntax near the keyword 'ON'` | ❌ `Msg 102` | `EXEC sp_addextendedproperty …` — a stored procedure, not DDL |
    /// | `SQLite` | ❌ syntax error | ❌ syntax error | **nothing at all** |
    ///
    /// Four mechanisms across the ten platforms measured, and two of them carry
    /// a hazard the `DEFAULT` clause never had:
    ///
    /// - `MySQL`'s and `MariaDB`'s only `ALTER` route **redefines the column**.
    ///   Measured: a `MODIFY` that did not restate `NOT NULL` left
    ///   `is_nullable = YES` behind it. A documentation mechanism that can
    ///   silently de-constrain a column is strictly worse than the clause it
    ///   replaces.
    /// - SQL Server's `sp_addextendedproperty` **is not idempotent**:
    ///   `Msg 15233 — Property 'MS_Description' already exists`. Razeri emits
    ///   `CREATE TABLE IF NOT EXISTS` so a deploy bundle can be re-applied; a
    ///   comment mechanism that fails on the second run takes that away.
    ///
    /// # So the escape hatch was taken
    ///
    /// Q1's answer included its own fall-back — *"if the measurement comes back
    /// ugly, take the simpler answer: no comment, and the bundle README carries
    /// the contract"* — and the measurement is ugly. **Razeri emits no column
    /// comment.** The open interval's meaning ships instead in the deploy
    /// bundle's generated *write contract*
    /// (`razeri_materialize::bundle::write_contract`, Q5), which is prose an
    /// operator reads, is right on every platform because no engine parses
    /// it, and is owed regardless of how this measurement came out.
    ///
    /// The hook stays, unchanged and unwired, for the same reason
    /// `render_column_default` stays: a dialect that genuinely wants to emit one
    /// has somewhere to say so. **A dialect that overrides this owes a
    /// measurement first** — the ANSI spelling below is a hard syntax error on
    /// five of the ten platforms it was measured on.
    fn render_column_comment(&self, table: &str, column: &str, comment: &str) -> Option<String> {
        Some(format!("COMMENT ON COLUMN {table}.{column} IS '{comment}'"))
    }

    /// Whether to render IF NOT EXISTS on CREATE TABLE.
    fn supports_if_not_exists(&self) -> bool {
        true
    }

    /// Render a column's `DEFAULT` clause — **keyword included** — or decline it.
    ///
    /// `None` means *this dialect renders no default clause for this value*,
    /// and Razeri's column renderer omits the whole clause. The ANSI default
    /// returns `None` for every value, which
    /// is the write-contract rule expressed where the renderer can see it:
    /// **razeri emits no
    /// column `DEFAULT` clause in rendered DDL, on any platform, for any value —
    /// a value the generated load owns is written by the load, never by the
    /// schema.**
    ///
    /// *Read that qualifier deliberately.* The sentence is about the DDL an engine
    /// executes. The `.raz.sql` **agnostic interchange** form —
    /// `Table::to_ddl()`, which `Table::from_ddl()` parses back — still
    /// serializes `Field::default_value` as `DEFAULT <value>`, because that is
    /// where a design's declared default survives a save and a load.
    ///
    /// **That exception is a serializer, not a platform, and the word "agnostic"
    /// names both.** `to_ddl()` renders through a free function and consults no
    /// dialect; you reach it by calling it, never by choosing a target.
    /// `SchemaObject::render_ddl` short-circuits to it only for a renderer whose
    /// [`SqlRenderer::is_agnostic_dialect`] answers `true` — which defaults to
    /// `false` here and is overridden by exactly one type in the tree,
    /// `AnsiDialect`, which no registration hands out. The platform the registry
    /// publishes under the name `agnostic` is `AgnosticDialect`; it does **not**
    /// override the method, so it declines the clause like every other target.
    /// The rule therefore holds on every registered platform, `agnostic`
    /// included, on every rendering path, and a regression test pins it.
    ///
    /// # Why the keyword lives in here
    ///
    /// The answer carries the keyword so that the hook can **decline a clause**
    /// as well as rewrite a value. Were the caller to add `DEFAULT` in front of
    /// whatever came back, a dialect returning `""` would emit a bare dangling
    /// `DEFAULT`. The same holds for statement boundaries: *the emission
    /// declares the boundary; a consumer never infers it.*
    ///
    /// # Why the hook survives the decision
    ///
    /// `Field::default_value` stays on the struct, so the design
    /// still knows what a column's intended default is, and a future dialect that
    /// genuinely wants to emit one has somewhere to say so. The distinction being
    /// bought is between **what razeri knows** and **what razeri emits**.
    ///
    /// A dialect that overrides this owes the whole clause, keyword and all —
    /// `Some(format!("DEFAULT {value}"))` — because there is nowhere else it is
    /// added.
    fn render_column_default(&self, value: &str) -> Option<String> {
        let _ = value;
        None
    }

    /// **Spell an author's OWN declared default in this engine's words.**
    ///
    /// This is the other sense of "default", and the seam where a dialect
    /// translates it. [`render_column_default`](Self::render_column_default)
    /// governs the tables Razeri's generated load owns, and it declines
    /// everywhere by design. A `tile: base` table is the exception: the author
    /// owns every column, no generated statement writes into it, and the
    /// declared default is an engine constraint they asked for — so it is
    /// rendered, on every platform.
    ///
    /// Rendered **verbatim**, it is a portability hole with the design's own
    /// words in it. A design declaring `default: CURRENT_TIMESTAMP` is asking
    /// for *the current moment*, not for those eighteen characters; on `BigQuery` those
    /// characters name the zone-aware type, the column Razeri builds is
    /// `DATETIME`, and the `CREATE TABLE` is refused outright — *"Column
    /// default expression has type TIMESTAMP which cannot be assigned to column
    /// type DATETIME"*. The design is right, and this is where it is
    /// translated.
    ///
    /// The default returns the value unchanged, so a dialect that says nothing
    /// renders the author's words as written.
    ///
    /// # A dialect overriding this owes a measurement, and owes restraint
    ///
    /// Translate an expression whose *meaning* your engine spells differently.
    /// Do not rewrite an author's literal, do not widen a type, and do not
    /// silently drop a default you cannot express — an author who declared one
    /// and got none has a column that fills with NULL and a document that says
    /// otherwise. Where the engine has no equivalent at all, leave the value
    /// alone and let the engine refuse it: a loud failure belongs to the value
    /// the author wrote, and a quiet substitution belongs to nobody.
    ///
    /// Parameters:
    /// - `value`: the default exactly as the design declares it.
    /// - `column_type`: the type of the column it is being assigned to —
    ///   load-bearing, because the same expression is legal against one type
    ///   and refused against another on the same engine.
    fn render_declared_column_default(&self, value: &str, column_type: &DataType) -> String {
        let _ = column_type;
        value.to_owned()
    }

    // ═══════════════════════════════════════════════════════════════
    // DDL Extensions
    // ═══════════════════════════════════════════════════════════════

    /// Render a BEGIN transaction statement.
    ///
    /// `PostgreSQL`: `BEGIN`, `SqlServer`: `BEGIN TRANSACTION`, `ClickHouse`: empty (no DDL transactions)
    fn render_begin_transaction(&self) -> &'static str {
        "BEGIN"
    }

    /// Render a COMMIT transaction statement.
    fn render_commit(&self) -> &'static str {
        "COMMIT"
    }

    /// Whether the platform supports DDL transactions.
    fn supports_ddl_transactions(&self) -> bool {
        true
    }

    /// Render a DROP FUNCTION statement with optional parameter types for overloaded functions.
    fn render_drop_function(&self, name: &str, param_types: &[&str], if_exists: bool) -> String {
        let exists = if if_exists { " IF EXISTS" } else { "" };
        if param_types.is_empty() {
            format!("DROP FUNCTION{exists} {name}")
        } else {
            format!("DROP FUNCTION{exists} {name}({})", param_types.join(", "))
        }
    }

    /// Render the drop keyword for a SQL object type.
    fn render_drop_object_keyword(&self, obj_type: &str) -> String {
        obj_type.to_uppercase()
    }

    /// Render the CREATE VIEW keyword/prefix with `secure/global_temp` modifiers.
    fn render_create_view_keyword(&self, secure: bool, global_temp: bool) -> String {
        let mut parts = vec!["CREATE OR REPLACE"];
        if secure {
            parts.push("SECURE");
        }
        if global_temp {
            parts.push("GLOBAL TEMPORARY");
        }
        parts.push("VIEW");
        parts.join(" ")
    }

    /// Whether `ALTER TABLE … DROP COLUMN IF EXISTS` is part of the grammar.
    ///
    /// The default [`render_drop_column`](Self::render_drop_column) reads it:
    /// where it is `false`, a drop asked to run only where the column stands
    /// is one the default cannot write.
    fn supports_alter_drop_if_exists(&self) -> bool {
        true
    }

    /// **The statement that drops one column of a table that holds rows.**
    ///
    /// `table` is spelled as the statement writes it, and `column` too.
    /// `only_where_it_stands` asks for a drop that does nothing where the
    /// column is already gone, so a file of drops that stopped part way runs
    /// again whole: Razeri asks for it on an engine that commits each
    /// statement on its own. Answer `None` where this engine cannot say
    /// that, and Razeri writes the plain drop and tells the operator to run
    /// the file again from the statement that stopped.
    ///
    /// The default writes `ALTER TABLE … DROP COLUMN …`, with `IF EXISTS`
    /// where [`supports_alter_drop_if_exists`](Self::supports_alter_drop_if_exists)
    /// says the engine takes it. An engine with neither `IF EXISTS` nor the
    /// `COLUMN` word answers in its own words: a guard over its catalogue, a
    /// prepared statement chosen by `information_schema`, or a block.
    fn render_drop_column(
        &self,
        table: WrittenObject<'_>,
        column: &str,
        only_where_it_stands: bool,
    ) -> Option<Written> {
        let relation = table.relation;
        if !only_where_it_stands {
            return Some(Written::Statement(format!(
                "ALTER TABLE {relation} DROP COLUMN {column}"
            )));
        }
        self.supports_alter_drop_if_exists().then(|| {
            Written::Statement(format!("ALTER TABLE {relation} DROP COLUMN IF EXISTS {column}"))
        })
    }

    /// **The statement that drops one of Razeri's views**, spelled as the
    /// statement writes it.
    ///
    /// `only_where_it_stands` asks for a drop that does nothing where the
    /// view is already gone. The default is
    /// [`render_drop_view_prefix`](Self::render_drop_view_prefix) and the
    /// view; an engine with no `DROP VIEW IF EXISTS` answers with a guard of
    /// its own, or `None` where it has none.
    fn render_drop_view(
        &self,
        view: WrittenObject<'_>,
        only_where_it_stands: bool,
    ) -> Option<Written> {
        Some(Written::Statement(format!(
            "{} {}",
            self.render_drop_view_prefix(only_where_it_stands),
            view.relation
        )))
    }

    /// **A `CREATE TABLE` that runs only where the table is not there yet**,
    /// on an engine whose grammar has no `CREATE TABLE IF NOT EXISTS`.
    ///
    /// A deploy runs again over the warehouse it made, so each table it makes
    /// has to pass over one that already stands. Where
    /// [`supports_if_not_exists`](Self::supports_if_not_exists) is `true` the
    /// statement says so itself and this is never asked. Where it is `false`,
    /// `create` is the whole plain statement and `table` its name as written:
    /// answer with the statement behind a question of your engine's
    /// catalogue, or `None` — the default — where you have none, and a second
    /// deploy is refused at the first table by the engine, with nothing
    /// changed.
    fn render_create_table_if_absent(
        &self,
        table: WrittenObject<'_>,
        create: &str,
    ) -> Option<Written> {
        let _ = (table, create);
        None
    }

    /// **The statement that copies a table, every row it holds, into a new
    /// table beside it.**
    ///
    /// Razeri asks for it before a change destroys data, where the operator
    /// asks to keep a copy first (`razeri evolve --rescue`): `source` is the
    /// table, `copy` the new table's name, both spelled as the statement
    /// writes them. The copy takes the source's columns and types as the
    /// engine reads them; it needs no key, no index and no constraint.
    ///
    /// The default is `CREATE TABLE … AS SELECT * FROM …`, with
    /// [`render_create_table_prefix`](Self::render_create_table_prefix) and,
    /// on its own line before `AS`, the clause
    /// [`render_table_suffix`](crate::SqlRenderer::render_table_suffix)
    /// answers for a plain table with no key — so an engine that needs an
    /// engine clause on every table has it here too. An engine with no
    /// `CREATE TABLE … AS` answers in its own words: SQL Server's
    /// `SELECT * INTO … FROM …`, or SAP HANA's `AS (…) WITH DATA`.
    fn render_copy_table(&self, source: WrittenObject<'_>, copy: WrittenObject<'_>) -> Written {
        let suffix = self
            .render_table_suffix(&TableSuffixRequest::new())
            .map(|clause| format!("\n{clause}"))
            .unwrap_or_default();
        Written::Statement(format!(
            "{} {}{suffix}\nAS SELECT * FROM {}",
            self.render_create_table_prefix(),
            copy.relation,
            source.relation
        ))
    }

    /// **The clause a table's storage settings are written in**, after its
    /// column list — SQL Server's `WITH (DATA_COMPRESSION = PAGE)` — or `None`
    /// where none of `settings` is this engine's, which is the default.
    ///
    /// See [`EngineSetting`]: spell the settings written for your engine and
    /// pass over the rest.
    fn render_table_storage_options(&self, settings: &[EngineSetting<'_>]) -> Option<String> {
        let _ = settings;
        None
    }

    // ═══════════════════════════════════════════════════════════════
    // Drop Behavior Hooks
    // ═══════════════════════════════════════════════════════════════

    /// Whether DROP INDEX uses ON syntax: `DROP INDEX idx ON table` (`SqlServer`).
    fn drop_index_uses_on_syntax(&self) -> bool {
        false
    }

    /// **Whether this engine checks the constraints it accepts.**
    ///
    /// The default is [`ConstraintEnforcement::Enforced`] — eight of Razeri's
    /// registered platforms, and the safe assumption for a dialect that has not
    /// said, because *"this engine guarantees nothing"* is the unusual claim.
    ///
    /// Answer [`ConstraintEnforcement::DeclaredOnly`] where a `PRIMARY KEY`,
    /// `FOREIGN KEY` or `UNIQUE` is accepted and never checked. Two engines are
    /// that, in two different ways, and the type carries both:
    ///
    /// - `BigQuery` has the words. `NOT ENFORCED` is the only legal spelling,
    ///   so the keyword goes on the constraint line and the engine makes the
    ///   declaration honest by itself.
    /// - `Redshift` has no words. It takes all three constraints, checks none,
    ///   and renders DDL a reader cannot tell from `PostgreSQL`'s — so the note
    ///   is written above the constraint list instead, and it is the only place
    ///   that DDL says what it means.
    ///
    /// [`key_constraint_suffix`](Self::key_constraint_suffix) reads this, so
    /// stating enforcement states the suffix too.
    fn constraint_enforcement(&self) -> ConstraintEnforcement {
        ConstraintEnforcement::Enforced
    }

    /// **The words a key constraint must carry to be accepted at all.**
    ///
    /// Appended to a table-level `PRIMARY KEY (…)` and to a
    /// `FOREIGN KEY (…) REFERENCES …`. Empty on every engine that enforces its
    /// keys, and empty on a declaring engine that has no keyword for it.
    ///
    /// The default reads
    /// [`constraint_enforcement`](Self::constraint_enforcement), so the two are
    /// one fact rather than two that must be kept in step. Override this only
    /// for a keyword a key constraint needs for some *other* reason than
    /// enforcement — there is no such engine in Razeri's set today.
    fn key_constraint_suffix(&self) -> &'static str {
        self.constraint_enforcement().keyword()
    }

    /// **Whether a `PRIMARY KEY` is part of the grammar.**
    ///
    /// `false` on an engine that has no primary key at all — one that refuses
    /// the declaration, not one that accepts it and checks nothing (that is
    /// [`Self::constraint_enforcement`]). The key is left out of
    /// the `CREATE TABLE` and named in a comment beside it, and the loads still
    /// match rows on its columns, as they do everywhere. Default `true`.
    fn supports_primary_key_constraint(&self) -> bool {
        true
    }

    /// **Whether a `FOREIGN KEY … REFERENCES` is part of the grammar.**
    ///
    /// `false` on an engine that refuses the declaration; it is left out and
    /// named in a comment beside the table. Default `true`.
    fn supports_foreign_key_constraint(&self) -> bool {
        true
    }

    /// Whether a table-level `UNIQUE (…)` constraint is part of the grammar.
    ///
    /// `false` on an engine that has no such constraint at all, where the
    /// declaration is left out of the `CREATE TABLE` and named in a comment
    /// beside it rather than emitted and refused.
    fn supports_unique_constraint(&self) -> bool {
        true
    }

    /// Whether a table-level `CHECK (…)` constraint is part of the grammar.
    ///
    /// `false` on an engine that has none, with the same treatment as
    /// [`supports_unique_constraint`](Self::supports_unique_constraint): named
    /// in a comment, never written as SQL the engine will reject.
    fn supports_check_constraint(&self) -> bool {
        true
    }

    /// Whether the platform supports CASCADE/RESTRICT in DROP statements.
    ///
    /// True for `PostgreSQL`, `CitusDB`, `DuckDB`, Snowflake, Agnostic.
    /// False for `SqlServer`, `ClickHouse`, Databricks, `MySQL`, `MariaDB`.
    fn supports_cascade_drop(&self) -> bool {
        true
    }

    /// Whether `COMMENT ON PROCEDURE` is part of the grammar.
    ///
    /// `true` almost everywhere, and **false on `Redshift`**, where
    /// `COMMENT ON FUNCTION`, `COMMENT ON VIEW` and `COMMENT ON COLUMN` all
    /// exist and the procedure form alone does not: *"syntax error at or near
    /// `PROCEDURE`"*, measured on a live workgroup. `CockroachDB` 24.3 refuses
    /// it the same way. A description is worth having and is not worth a
    /// statement that stops the script, so a declining engine simply carries
    /// no description on its procedures. Pass this answer to
    /// [`render_procedure_as`](crate::routine_render::render_procedure_as)
    /// when you spell a procedure through it, and it leaves the description
    /// out too.
    fn supports_procedure_comment(&self) -> bool {
        true
    }

    /// Whether the platform supports function argument types in DROP FUNCTION.
    ///
    /// False for `ClickHouse` (no overloaded functions).
    fn supports_drop_function_arg_types(&self) -> bool {
        true
    }

    /// Whether the platform supports SYNC option for DROP (`ClickHouse`).
    fn supports_sync_drop(&self) -> bool {
        false
    }

    /// Whether the platform supports FORCE option for DROP (Databricks).
    fn supports_force_drop(&self) -> bool {
        false
    }

    /// Whether the platform supports CASCADE in ALTER TABLE DROP.
    ///
    /// Only `PostgreSQL`, `CitusDB`, `DuckDB` support CASCADE in ALTER TABLE DROP.
    fn supports_alter_drop_cascade(&self) -> bool {
        false
    }
}

/// **Three questions a load asks of a number written as text**, each a
/// condition over the value, for a column declared `decimal(precision,
/// scale)` ([`ExpressionRenderer::render_decimal_fit`]).
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct DecimalFit {
    /// True of a value written as a plain number: digits, at most one point,
    /// and at most one sign, ahead of them. It may be true of a value with no
    /// digits at all, such as the empty text or a lone point.
    pub plain: String,
    /// True where a digit other than `0` stands more places before the point
    /// than the column leaves room for, `precision - scale`, counted from the
    /// point (or from the end, where there is none). Asked of any text; false
    /// of every plain number that fits.
    pub past_room: String,
    /// True where a digit other than `0` stands more places after the point
    /// than `scale`. Asked of any text; false of every plain number that fits.
    pub past_scale: String,
}

impl DecimalFit {
    /// **The three questions as regular expressions**, each pattern handed
    /// to `matches` to be written as the engine matches one somewhere in the
    /// value (`text ~ 'pattern'`, `regexp_matches(text, 'pattern')`): a plain
    /// number is an optional sign, digits and at most one point; a digit
    /// other than `0` followed by the column's room in digits, before any
    /// point, is too large; one past the scale after the point is a place the
    /// column does not keep. The patterns hold no backslash and no quote, so
    /// they read the same under every string-literal setting.
    pub fn by_regular_expression(
        precision: u8,
        scale: u8,
        matches: impl Fn(&str) -> String,
    ) -> Self {
        let room = precision.saturating_sub(scale);
        Self {
            plain: matches("^[+-]?[0-9]*[.]?[0-9]*$"),
            past_room: matches(&format!("^[^.]*[1-9][0-9]{{{room}}}")),
            past_scale: matches(&format!("[.][0-9]{{{scale}}}[0-9]*[1-9]")),
        }
    }
}

/// **A table a procedure writes once and reads by name**
/// ([`ProcedureRenderer::render_procedure_table`]).
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ProcedureTable {
    /// The statements that write it, each ended for a procedure's body.
    pub statements: Vec<String>,
    /// The name the statements after them read it by.
    pub reference: String,
}

/// **One row, written once** — the pieces of an INSERT that must do nothing
/// when a row already carries the same key.
///
/// The key value is not a field of its own: it is whichever `values` entry sits
/// at the same index as the key column in `columns`. Two fields that must agree
/// are two fields free to disagree, and the one they would disagree about here
/// is the one that decides whether a second write is a duplicate.
#[derive(Debug, Clone, Copy)]
pub struct InsertIfAbsent<'a> {
    /// The target table, already spelled the way the caller wants it written —
    /// schema-qualified or bare.
    pub table: &'a str,
    /// The columns being written, in the order `values` supplies them.
    pub columns: &'a [&'a str],
    /// One value expression per column.
    pub values: &'a [String],
    /// The columns whose equality means *this row is already here*.
    pub key_columns: &'a [&'a str],
}

impl InsertIfAbsent<'_> {
    /// The value expression written into `column`, if it is being written.
    #[must_use]
    pub fn value_of(&self, column: &str) -> Option<&str> {
        let at = self.columns.iter().position(|name| *name == column)?;
        self.values.get(at).map(String::as_str)
    }

    /// `key = value AND …` — the test for *already here*, in terms of the
    /// values this INSERT would have written.
    ///
    /// `qualifier` prefixes the column side, which the guarded form needs so
    /// the probe's own row is named unambiguously.
    ///
    /// # Panics
    ///
    /// When a key column is not among the columns being written. A key the
    /// INSERT does not supply cannot be tested for, and silently dropping it
    /// from the predicate would make the guard match everything.
    #[must_use]
    pub fn key_predicate(&self, qualifier: &str) -> String {
        self.key_columns
            .iter()
            .map(|column| {
                let value = self
                    .value_of(column)
                    .unwrap_or_else(|| panic!("{column} is a key column and must be written"));
                format!("{qualifier}{column} = {value}")
            })
            .collect::<Vec<_>>()
            .join(" AND ")
    }

    /// **`ON CONFLICT (key) DO NOTHING`** — for the platforms that let the
    /// engine decide, which is every platform that has the clause.
    ///
    /// The engine tests the key as it writes, so two sessions registering one
    /// file at the same moment settle on one row rather than on whichever of
    /// them looked first.
    #[must_use]
    pub fn on_conflict_do_nothing(&self) -> String {
        format!(
            "{}\n{}\nON CONFLICT ({}) DO NOTHING;",
            self.target(),
            self.values_clause(),
            self.key_columns.join(", ")
        )
    }

    /// **`ON DUPLICATE KEY UPDATE key = key`** — the `MySQL` family's no-op.
    ///
    /// `INSERT IGNORE` would be shorter and it is not the same request: it
    /// downgrades *every* error on the statement to a warning — a truncated
    /// value, a bad date — so a row that should have been refused arrives
    /// mangled and silent. Assigning the key to itself touches only the
    /// collision this is about, and leaves everything else to complain
    /// normally.
    #[must_use]
    pub fn on_duplicate_key_no_op(&self) -> String {
        let held = self
            .key_columns
            .iter()
            .map(|column| format!("{column} = {column}"))
            .collect::<Vec<_>>()
            .join(", ");
        format!("{}\n{}\nON DUPLICATE KEY UPDATE {held};", self.target(), self.values_clause())
    }

    /// **The guarded form over an explicit one-row source** — the same
    /// `INSERT … SELECT … WHERE NOT EXISTS` the portable default writes, with a
    /// `FROM` clause under it.
    ///
    /// `GoogleSQL` refuses a `WHERE` on a query that selects from nothing —
    /// *"Query without FROM clause cannot have a WHERE clause"*, measured on the
    /// live service on every generated `register_file` procedure — while
    /// `PostgreSQL` and its cousins accept the bare form. `(SELECT 1)` is a
    /// one-row table that changes no result: the guard still decides whether
    /// the row is written, and the values are still the caller's.
    ///
    /// It stays a separate form rather than becoming the default because the
    /// engines that need no `FROM` read better without one, and a clause every
    /// platform carries for one platform's sake is how a rendering stops saying
    /// what it means.
    #[must_use]
    pub fn guarded_over_one_row(&self) -> String {
        format!(
            "{}\nSELECT {}\nFROM (SELECT 1)\nWHERE NOT EXISTS (SELECT 1 FROM {} WHERE {});",
            self.target(),
            self.values.join(", "),
            self.table,
            self.key_predicate("")
        )
    }

    /// `INSERT INTO t (a, b)` — shared by every form below.
    fn target(&self) -> String {
        format!("INSERT INTO {} ({})", self.table, self.columns.join(", "))
    }

    /// `VALUES (…)`.
    fn values_clause(&self) -> String {
        format!("VALUES ({})", self.values.join(", "))
    }
}

/// Level 4 — DML rendering with reasonable defaults.
pub trait DmlRenderer: SqlRenderer {
    /// Prefix for INSERT statements.
    fn render_insert_prefix(&self) -> &'static str {
        "INSERT INTO"
    }

    /// **Write one row, and do nothing if its key is already there.**
    ///
    /// Every platform can express this and they do not agree on how, so the
    /// spelling belongs to the dialect and not to the caller. The default is
    /// the portable one — an `INSERT … SELECT` guarded by `WHERE NOT EXISTS` —
    /// which needs no upsert syntax at all and is therefore the right answer
    /// for a platform nobody has written an arm for. Platforms with a native
    /// idiom override it: `ON CONFLICT … DO NOTHING` where that exists,
    /// `ON DUPLICATE KEY UPDATE` on the `MySQL` family.
    ///
    /// **No dialect is left without an idempotent form.** The guarded form is
    /// weaker than the native ones under concurrency — two sessions can both
    /// find the row absent and both insert — which is why a platform that can
    /// let the engine decide should, and why this one leans on the key's own
    /// uniqueness constraint to catch the race rather than pretending it cannot
    /// happen. On an engine that enforces no key at all the guard is the whole
    /// of what there is, and it is worth saying so rather than implying more.
    fn render_insert_if_absent(&self, insert: &InsertIfAbsent<'_>) -> String {
        format!(
            "{}\nSELECT {}\nWHERE NOT EXISTS (SELECT 1 FROM {} WHERE {});",
            insert.target(),
            insert.values.join(", "),
            insert.table,
            insert.key_predicate("")
        )
    }

    /// Prefix for MERGE statements.
    fn render_merge_prefix(&self) -> &'static str {
        "MERGE INTO"
    }

    /// Render the platform's upsert strategy keyword.
    ///
    /// Returns `None` if platform doesn't support upsert natively.
    fn render_upsert_strategy(&self) -> Option<&str> {
        None
    }

    /// Render RETURNING / OUTPUT clause for INSERT/UPDATE.
    fn render_returning_clause(&self, _columns: &[&str]) -> Option<String> {
        None
    }

    /// Render a LIMIT clause.
    fn render_limit_clause(&self, n: usize) -> String {
        format!("LIMIT {n}")
    }

    /// Render a temporary table prefix or keyword.
    fn render_temp_table_prefix(&self) -> &'static str {
        "TEMPORARY"
    }

    /// **One row holding an array becomes one row per element.**
    ///
    /// Razeri hands over the relation it is reading, the array column, and the
    /// element fields the statement needs; the engine answers with the `FROM`
    /// item that produces the rows and the expressions that read them. The
    /// three parts come back together because on at least one engine they
    /// depend on each other: its field expressions name an alias its own
    /// `FROM` item introduced.
    ///
    /// **The ordinal is one for the first element, on every engine.** Engines
    /// disagree about where an array starts — some count from one, some from
    /// zero, one answers in text — and the element's surrogate key is derived
    /// from its position. An engine answering in its own base would give one
    /// element two different keys depending on where it was stored, so the
    /// normalization belongs here, where the engine that counts from zero pays
    /// for it with one addition.
    ///
    /// `None` — the default — is this engine stating no flatten. Razeri then
    /// refuses the declaration and names the engine, rather than guessing an
    /// idiom or emitting a statement that does not run.
    fn render_flatten(&self, _request: &FlattenRequest<'_>) -> Option<Flatten> {
        None
    }
}

/// Level 5 — Stored procedure / function rendering with defaults.
pub trait ProcedureRenderer: SqlRenderer {
    /// Prefix for CREATE PROCEDURE.
    fn render_create_procedure_prefix(&self) -> &'static str {
        "CREATE OR REPLACE PROCEDURE"
    }

    /// Prefix for CREATE FUNCTION.
    fn render_create_function_prefix(&self) -> &'static str {
        "CREATE OR REPLACE FUNCTION"
    }

    /// **Rows a procedure writes once and reads by name**: the statements,
    /// inside a procedure's body, that keep `query`'s rows in a table only
    /// this call sees, and the name the statements after them read it by —
    /// or `None`, the default, where the engine has no such table and each
    /// statement reads `query` itself.
    ///
    /// A core table's load converts its staged text into the design's types
    /// once, here, and its refusals and its write read the converted rows by
    /// name: one pass over the staged rows rather than one for each
    /// statement. `name` is a plain identifier, unique within the procedure.
    /// The table must not outlive the call, and two calls running at once
    /// must not see each other's.
    fn render_procedure_table(&self, name: &str, query: &str) -> Option<ProcedureTable> {
        let _ = (name, query);
        None
    }

    /// Render a variable declaration inside a procedure body.
    fn render_variable_declaration(&self, name: &str, dt: &DataType) -> String {
        format!("DECLARE {} {};", name, self.render_data_type(dt))
    }

    /// Render BEGIN/END block delimiters.
    fn render_begin_end(&self, body: &str) -> String {
        format!("BEGIN\n{body}\nEND;")
    }

    /// Whether procedures use a language clause (e.g., Snowflake JavaScript).
    fn requires_language_clause(&self) -> bool {
        false
    }

    /// **A statement that calls each of `calls` once for every row of
    /// `query`**, or `None` where this engine has none to write, the default.
    ///
    /// `query` answers one column; each row's value is held in `variable`, of
    /// the type `variable_type` spells in this engine, and each call names
    /// `variable` where it takes the value. A generated fill asks this to make
    /// a staging partition for every day a file holds before it stages a row
    /// of any, on an engine that keeps nowhere for a row whose day has no
    /// partition. Where the answer is `None`, the fill is refused by name,
    /// with the calls to make by hand.
    fn render_call_for_each_row(
        &self,
        variable: &str,
        variable_type: &str,
        query: &str,
        calls: &[String],
    ) -> Option<String> {
        let _ = (variable, variable_type, query, calls);
        None
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::DialectCapabilities;
    use crate::sid_spelling::SidHelperLanguage;

    /// A test dialect that implements only the required `SqlRenderer` methods.
    /// All other traits use defaults.
    struct TestDialect;

    impl SqlRenderer for TestDialect {
        fn render_data_type(&self, dt: &DataType) -> String {
            match dt {
                DataType::Boolean(_) => "BOOLEAN".to_string(),
                DataType::Integer(_) => "INTEGER".to_string(),
                _ => "TEXT".to_string(),
            }
        }
        fn quote_identifier(&self, name: &str) -> String {
            format!("\"{name}\"")
        }
        fn qualify_name(&self, schema: Option<&str>, name: &str) -> String {
            match schema {
                Some(s) => format!("{}.{}", self.quote_identifier(s), self.quote_identifier(name)),
                None => self.quote_identifier(name),
            }
        }
        fn render_current_timestamp(&self) -> String {
            "CURRENT_TIMESTAMP".to_string()
        }
    }

    impl ExpressionRenderer for TestDialect {}
    impl DdlRenderer for TestDialect {}
    impl DmlRenderer for TestDialect {}
    impl ProcedureRenderer for TestDialect {}
    impl DialectCapabilities for TestDialect {}

    /// **An alias the engine reserves is quoted, and one it leaves free is
    /// not** — by the standard's list until the dialect says otherwise.
    #[test]
    fn an_alias_is_quoted_only_where_the_engine_reserves_the_word() {
        let d = TestDialect;
        assert_eq!(d.render_alias("matched"), "matched");
        assert_eq!(d.render_alias("standing"), "standing");
        assert_eq!(d.render_alias("year"), "\"year\"");
        assert_eq!(d.render_alias("VALUE"), "\"VALUE\"");
    }

    /// **A view nothing reads is made by its own statement, and a view that
    /// is read is made after its readers are dropped, the last made first.**
    #[test]
    fn a_view_that_is_read_is_made_after_its_readers_are_dropped() {
        use crate::{ReaderKind, ViewReader, ViewReplacement, WrittenObject};
        let d = TestDialect;
        let create = "CREATE OR REPLACE VIEW core.\"customers_identity_current\" AS SELECT 1";
        let view = WrittenObject::new(
            Some("core"),
            "\"customers_identity_current\"",
            "core.\"customers_identity_current\"",
        );
        assert_eq!(d.render_view_replacement(&ViewReplacement::new(view, create, &[])), create);
        let readers = [
            ViewReader::new(
                ReaderKind::Procedure,
                Some("core".to_owned()),
                "\"customers_load\"".to_owned(),
                "core.\"customers_load\"".to_owned(),
                Vec::new(),
            ),
            ViewReader::new(
                ReaderKind::Function,
                Some("core".to_owned()),
                "\"customers_as_of\"".to_owned(),
                "core.\"customers_as_of\"".to_owned(),
                vec!["TIMESTAMP".to_owned()],
            ),
            ViewReader::new(
                ReaderKind::View,
                Some("core".to_owned()),
                "\"customers_current\"".to_owned(),
                "core.\"customers_current\"".to_owned(),
                Vec::new(),
            ),
        ];
        assert_eq!(
            d.render_view_replacement(&ViewReplacement::new(view, create, &readers)),
            format!(
                "DROP PROCEDURE IF EXISTS core.\"customers_load\";\n\
                 DROP FUNCTION IF EXISTS core.\"customers_as_of\"(TIMESTAMP);\n\
                 DROP VIEW IF EXISTS core.\"customers_current\";\n\
                 {create}"
            )
        );
        assert!(d.replaces_a_view_that_is_read(), "most engines replace in place");
    }

    /// **No engine is handed the statement that requires a column until its
    /// dialect writes one.**
    #[test]
    fn requiring_a_column_is_no_statement_until_a_dialect_writes_one() {
        let data_type = DataType::boolean();
        let change = ColumnEvolution::RequireColumn {
            schema: Some("core"),
            table: "customers",
            column: "is_active",
            data_type: &data_type,
        };
        assert_eq!(change.column(), "is_active");
        assert!(TestDialect.render_column_evolution(&change).is_none());
    }

    /// The same dialect with the two file-and-key facts stated. It exists to
    /// prove that a dialect Razeri never compiled against reaches the behaviour
    /// by *declaring* it — which is the whole property, and is not visible from
    /// a dialect that ships in this tree.
    struct StatedDialect;

    impl SqlRenderer for StatedDialect {
        fn render_data_type(&self, _dt: &DataType) -> String {
            "TEXT".to_string()
        }
        fn quote_identifier(&self, name: &str) -> String {
            format!("\"{name}\"")
        }
        fn qualify_name(&self, _schema: Option<&str>, name: &str) -> String {
            self.quote_identifier(name)
        }
        fn render_current_timestamp(&self) -> String {
            "CURRENT_TIMESTAMP".to_string()
        }
        fn sid_helper_body(&self) -> Option<SidHelperBody> {
            Some(SidHelperBody {
                language: SidHelperLanguage::Named("ecmascript"),
                body: "return 1;",
            })
        }
        fn delimited_file_relation(&self, path_literal: &str) -> Option<String> {
            Some(format!("SCAN({path_literal})"))
        }
    }

    /// **A dialect that says nothing writes a value by its plain cast**, and
    /// renders exactly what it rendered before the question existed.
    #[test]
    fn a_dialect_that_states_no_value_text_casts_the_value() {
        let d: &dyn ExpressionRenderer = &TestDialect;
        for value_type in [DataType::date(), DataType::timestamp(), DataType::decimal(12, 2)] {
            assert_eq!(
                d.render_value_text("x", &value_type, &DataType::varchar(64)),
                d.render_cast("x", &DataType::varchar(64)),
            );
        }
        assert!(d.value_text_functions().is_none(), "a plain cast calls no function");
    }

    /// **A dialect that says nothing refuses by converting the sentence to a
    /// number**, which is what every engine that refuses the conversion
    /// needs.
    #[test]
    fn a_dialect_that_states_no_refusal_converts_the_sentence() {
        let d: &dyn ExpressionRenderer = &TestDialect;
        assert_eq!(d.render_refusal("s"), d.render_cast("s", &DataType::bigint()));
    }

    /// **A dialect that says nothing counts characters with the standard
    /// function.**
    #[test]
    fn a_dialect_that_states_no_text_length_counts_with_char_length() {
        let d: &dyn ExpressionRenderer = &TestDialect;
        assert_eq!(d.render_text_length("name"), "CHAR_LENGTH(name)");
    }

    /// **A dialect that says nothing reads a missing word with the standard
    /// `IN`**, each word a standard literal with its quotes doubled.
    #[test]
    fn a_dialect_that_states_no_missing_value_reading_compares_with_in() {
        let d: &dyn ExpressionRenderer = &TestDialect;
        assert_eq!(
            d.render_missing_value("TRIM(x)", &["", "NULL", "it's"]),
            "CASE WHEN TRIM(x) IN ('', 'NULL', 'it''s') THEN NULL ELSE TRIM(x) END"
        );
    }

    /// **Silence is the default, and silence means Razeri composes.**
    ///
    /// Both facts are opt-in: a dialect that says nothing gets the SQL
    /// derivation for its key and no generated staging fill at all. Neither
    /// default may become a guess — a composed body on an engine with no hash
    /// keys a warehouse to a number nothing reproduces, and an invented file
    /// relation fails the load on its first statement.
    #[test]
    fn a_dialect_that_states_neither_fact_is_asked_for_nothing() {
        let d: &dyn SqlRenderer = &TestDialect;
        assert_eq!(d.sid_helper_body(), None);
        assert_eq!(d.delimited_file_relation("'/data/x.csv'"), None);
    }

    /// **A dialect that says nothing adds nothing after the column list.** The
    /// default is what every engine that takes a bare `CREATE TABLE t (…)`
    /// needs, whatever the table is.
    #[test]
    fn a_dialect_that_spells_no_table_suffix_is_asked_for_none() {
        let d: &dyn SqlRenderer = &TestDialect;
        let key = ["id".to_owned()];
        let cluster = ["region".to_owned()];
        for table in [
            TableSuffixRequest::new(),
            TableSuffixRequest::new().with_temporary(true),
            TableSuffixRequest::new()
                .with_primary_key(&key)
                .with_clustering_columns(&cluster)
                .with_version_column(Some("_loaded_at")),
        ] {
            assert_eq!(d.render_table_suffix(&table), None, "{table:?}");
        }
    }

    /// And a dialect that states them is heard through the trait object, which
    /// is the only way Razeri ever holds one.
    #[test]
    fn a_dialect_that_states_them_is_heard_through_the_trait_object() {
        let d: &dyn SqlRenderer = &StatedDialect;
        assert_eq!(
            d.sid_helper_body(),
            Some(SidHelperBody {
                language: SidHelperLanguage::Named("ecmascript"),
                body: "return 1;",
            })
        );
        assert_eq!(
            d.delimited_file_relation("'/data/x.csv'").as_deref(),
            Some("SCAN('/data/x.csv')")
        );
    }

    #[test]
    fn sql_renderer_core_methods_work() {
        let d = TestDialect;
        assert_eq!(d.render_data_type(&DataType::boolean()), "BOOLEAN");
        assert_eq!(d.quote_identifier("users"), "\"users\"");
        assert_eq!(d.qualify_name(Some("public"), "users"), "\"public\".\"users\"");
        assert_eq!(d.render_current_timestamp(), "CURRENT_TIMESTAMP");
    }

    #[test]
    fn dialect_name_returns_ansi_by_default() {
        let d = TestDialect;
        assert_eq!(d.dialect_name(), "ansi");
    }

    #[test]
    fn expression_renderer_defaults_produce_ansi_sql() {
        let d = TestDialect;
        assert_eq!(d.render_cast("age", &DataType::int()), "CAST(age AS INTEGER)");
        assert_eq!(d.render_concat(&["a".into(), "b".into()]), "CONCAT(a, b)");
        assert_eq!(d.render_coalesce(&["x".into(), "y".into()]), "COALESCE(x, y)");
    }

    #[test]
    fn ddl_renderer_defaults_produce_ansi_sql() {
        let d = TestDialect;
        assert_eq!(d.render_create_table_prefix(), "CREATE TABLE");
        assert_eq!(d.render_create_view_prefix(), "CREATE OR REPLACE VIEW");
        assert_eq!(
            d.render_column_definition("name", &DataType::boolean(), false),
            "\"name\" BOOLEAN NOT NULL"
        );
        assert_eq!(d.render_column_definition("age", &DataType::int(), true), "\"age\" INTEGER");
    }

    #[test]
    fn dml_renderer_defaults_produce_ansi_sql() {
        let d = TestDialect;
        assert_eq!(d.render_insert_prefix(), "INSERT INTO");
        assert_eq!(d.render_merge_prefix(), "MERGE INTO");
        assert_eq!(d.render_limit_clause(10), "LIMIT 10");
        assert!(d.render_upsert_strategy().is_none());
        assert!(d.render_returning_clause(&["id"]).is_none());
    }

    #[test]
    fn procedure_renderer_defaults_produce_ansi_sql() {
        let d = TestDialect;
        assert_eq!(d.render_create_procedure_prefix(), "CREATE OR REPLACE PROCEDURE");
        assert_eq!(
            d.render_variable_declaration("v_count", &DataType::int()),
            "DECLARE v_count INTEGER;"
        );
        assert!(!d.requires_language_clause());
    }

    #[test]
    fn trait_hierarchy_allows_selective_override() {
        struct MySqlLike;
        impl SqlRenderer for MySqlLike {
            fn render_data_type(&self, _: &DataType) -> String {
                "INT".into()
            }
            fn quote_identifier(&self, name: &str) -> String {
                format!("`{name}`")
            }
            fn qualify_name(&self, schema: Option<&str>, name: &str) -> String {
                match schema {
                    Some(s) => format!("`{s}`.`{name}`"),
                    None => format!("`{name}`"),
                }
            }
            fn render_current_timestamp(&self) -> String {
                "NOW()".into()
            }
        }
        impl ExpressionRenderer for MySqlLike {
            fn render_concat(&self, parts: &[String]) -> String {
                format!("CONCAT({})", parts.join(", "))
            }
        }
        impl DdlRenderer for MySqlLike {}
        impl DmlRenderer for MySqlLike {}
        impl ProcedureRenderer for MySqlLike {}
        impl DialectCapabilities for MySqlLike {}

        let d = MySqlLike;
        assert_eq!(d.quote_identifier("users"), "`users`");
        assert_eq!(d.render_current_timestamp(), "NOW()");
        // DDL defaults still work even though we only overrode expressions
        assert_eq!(d.render_create_table_prefix(), "CREATE TABLE");
    }

    #[test]
    fn sql_renderer_expression_extensions_produce_ansi_defaults() {
        let d = TestDialect;
        assert_sql_renderer_literal_defaults(&d);
        assert_sql_renderer_parameter_and_merge_defaults(&d);
        assert_sql_renderer_update_insert_defaults(&d);
        assert_sql_renderer_partition_and_window_defaults(&d);
    }

    /// Assert ANSI defaults for literal, date/time, and basic expression hooks.
    /// Parameters:
    /// - dialect: Renderer under test.
    fn assert_sql_renderer_literal_defaults(dialect: &TestDialect) {
        assert_eq!(dialect.render_boolean_literal(true), "TRUE");
        assert_eq!(dialect.render_boolean_literal(false), "FALSE");
        assert_eq!(dialect.render_date_literal("2024-01-15"), "DATE '2024-01-15'");
        assert_eq!(
            dialect.render_timestamp_literal("2024-01-15 12:00:00"),
            "TIMESTAMP '2024-01-15 12:00:00'"
        );
        assert_eq!(dialect.render_interval_literal("1 year"), "INTERVAL '1 year'");
        assert_eq!(dialect.render_current_date(), "CURRENT_DATE");
        assert_eq!(dialect.concat_operator(), "||");
        assert_eq!(dialect.render_nan_literal(&DataType::float()), "NULL");
        assert_eq!(dialect.render_infinity_literal(true, &DataType::float()), "NULL");
        assert_eq!(dialect.ifnull_function_name(), "COALESCE");
        assert_eq!(
            dialect.render_iif("x > 0", "pos", "neg"),
            "CASE WHEN x > 0 THEN pos ELSE neg END"
        );
        assert_eq!(
            dialect.render_date_trunc("month", "created_at"),
            "DATE_TRUNC('month', created_at)"
        );
        assert_eq!(
            dialect.render_date_extract("year", "created_at"),
            "EXTRACT(year FROM created_at)"
        );
        assert_eq!(dialect.render_try_cast("age", &DataType::int()), "TRY_CAST(age AS INTEGER)");
    }

    /// Assert ANSI defaults for parameter and merge-related extension hooks.
    /// Parameters:
    /// - dialect: Renderer under test.
    fn assert_sql_renderer_parameter_and_merge_defaults(dialect: &TestDialect) {
        assert_eq!(dialect.render_named_parameter("user_id"), ":user_id");
        assert_eq!(SqlRenderer::merge_not_matched_keyword(dialect), "WHEN NOT MATCHED");
        assert!(!SqlRenderer::supports_merge_not_matched_by_source(dialect));
        assert!(SqlRenderer::merge_supported(dialect));
        assert!(!SqlRenderer::merge_supports_schema_evolution_keyword(dialect));
        assert!(!SqlRenderer::merge_supports_table_hints(dialect));
        assert!(!SqlRenderer::merge_uses_output_clause(dialect));
        assert!(!SqlRenderer::merge_supports_returning_clause(dialect));
        assert_eq!(dialect.schema_separator(), ".");
        assert_eq!(dialect.statement_separator(), ";\n");
    }

    /// Assert ANSI defaults for update and insert behavior extension hooks.
    /// Parameters:
    /// - dialect: Renderer under test.
    fn assert_sql_renderer_update_insert_defaults(dialect: &TestDialect) {
        assert!(!SqlRenderer::supports_compound_assignment(dialect));
        assert!(!SqlRenderer::supports_update_from(dialect));
        assert!(!SqlRenderer::uses_alter_table_update(dialect));
        // How a mutation is SPELLED and what it may LOOK AT are
        // two facts, and the ANSI default is that a mutation may correlate.
        assert!(!SqlRenderer::mutation_reads_only_its_own_rows(dialect));
        assert!(!dialect.update_supports_top_clause());
        assert!(!dialect.update_supports_table_hints());
        assert!(!dialect.update_uses_output_clause());
        assert!(!dialect.update_supports_returning());
        assert!(!dialect.insert_uses_output_clause());
        assert!(!dialect.insert_supports_returning());
        assert!(!dialect.insert_supports_table_hints());
        assert!(!SqlRenderer::supports_insert_overwrite(dialect));
        assert!(!SqlRenderer::supports_on_conflict(dialect));
        assert!(!dialect.supports_overriding_clause());
        assert_eq!(dialect.returning_clause_keyword(), "RETURNING");
        assert!(!SqlRenderer::supports_returning_into(dialect));
        assert!(!SqlRenderer::supports_multi_table_insert(dialect));
    }

    /// Assert ANSI defaults for partitioning, generated columns, and window hooks.
    /// Parameters:
    /// - dialect: Renderer under test.
    fn assert_sql_renderer_partition_and_window_defaults(dialect: &TestDialect) {
        assert_eq!(
            dialect.render_time_partition_expr("event_ts", "monthly"),
            "date_trunc('month', event_ts)"
        );
        assert_eq!(dialect.render_partition_clause("event_ts"), "PARTITION BY (event_ts)");
        assert_eq!(
            dialect.render_generated_column("event_month", "DATE_TRUNC('month', event_ts)"),
            "event_month GENERATED ALWAYS AS (DATE_TRUNC('month', event_ts)) STORED"
        );
        assert!(!SqlRenderer::supports_window_frame_exclude(dialect));
        assert!(!SqlRenderer::supports_window_frame_groups(dialect));
        assert!(SqlRenderer::supports_named_windows(dialect));
    }

    #[test]
    fn ddl_renderer_extensions_produce_ansi_defaults() {
        let d = TestDialect;
        assert_ddl_renderer_transaction_and_view_defaults(&d);
        assert_ddl_renderer_drop_defaults(&d);
    }

    /// Assert ANSI DDL defaults for transaction and view keyword hooks.
    /// Parameters:
    /// - dialect: Renderer under test.
    fn assert_ddl_renderer_transaction_and_view_defaults(dialect: &TestDialect) {
        assert_eq!(dialect.render_begin_transaction(), "BEGIN");
        assert_eq!(dialect.render_commit(), "COMMIT");
        assert!(dialect.supports_ddl_transactions());
        assert_eq!(dialect.render_create_view_keyword(false, false), "CREATE OR REPLACE VIEW");
        assert_eq!(
            dialect.render_create_view_keyword(true, false),
            "CREATE OR REPLACE SECURE VIEW"
        );
        assert!(dialect.supports_alter_drop_if_exists());
    }

    /// Assert ANSI DDL defaults for drop-related behavior hooks.
    /// Parameters:
    /// - dialect: Renderer under test.
    fn assert_ddl_renderer_drop_defaults(dialect: &TestDialect) {
        assert_eq!(dialect.render_drop_object_keyword("table"), "TABLE");
        assert_eq!(
            dialect.render_drop_function("my_func", &[], true),
            "DROP FUNCTION IF EXISTS my_func"
        );
        assert!(!dialect.drop_index_uses_on_syntax());
        assert!(DdlRenderer::supports_cascade_drop(dialect));
        assert!(dialect.supports_drop_function_arg_types());
        assert!(!DdlRenderer::supports_sync_drop(dialect));
        assert!(!DdlRenderer::supports_force_drop(dialect));
        assert!(!dialect.supports_alter_drop_cascade());
    }

    /// **The one statement every engine takes**, written for a dialect that
    /// answered nothing. A plugin author gets the commonest evolution there is
    /// without writing a line for it.
    #[test]
    fn a_dialect_that_answers_nothing_still_adds_a_column() {
        let data_type = DataType::boolean();
        let written = TestDialect.render_column_evolution(&ColumnEvolution::AddColumn {
            schema: Some("telemetry_core"),
            table: "clients_daily",
            column: "is_default_browser",
            data_type: &data_type,
        });
        assert_eq!(
            written.as_deref(),
            Some(
                r#"ALTER TABLE "telemetry_core"."clients_daily" ADD COLUMN "is_default_browser" BOOLEAN"#
            )
        );
    }

    /// Everything else waits to be spelled. A default picked from one engine's
    /// grammar is a statement the others reject, handed to an operator who
    /// finds that out against their own warehouse.
    #[test]
    fn nothing_else_is_spelled_until_a_dialect_spells_it() {
        let data_type = DataType::text();
        assert!(
            TestDialect
                .render_column_evolution(&ColumnEvolution::WidenColumn {
                    schema: None,
                    table: "t",
                    column: "c",
                    data_type: &data_type,
                })
                .is_none()
        );
        assert!(
            TestDialect
                .render_column_evolution(&ColumnEvolution::AddNestedField {
                    schema: None,
                    table: "t",
                    column: "c",
                    field: "f",
                    data_type: &data_type,
                })
                .is_none()
        );
    }

    /// **The first line break decides how a file's lines end.** A header that
    /// ends `\r\n` is a Windows file; one that ends `\n`, or a file with no
    /// line break at all, reads with a newline alone.
    #[test]
    fn the_first_line_break_decides_how_the_lines_end() {
        assert_eq!(LineEnding::of(b"id,name\r\n1,Alpha\r\n"), LineEnding::CrLf);
        assert_eq!(LineEnding::of(b"id,name\r\n1,Alpha"), LineEnding::CrLf);
        assert_eq!(LineEnding::of(b"id,name\n1,Alpha\n"), LineEnding::Lf);
        assert_eq!(LineEnding::of(b"id,name"), LineEnding::Lf);
        assert_eq!(LineEnding::of(b""), LineEnding::Lf);
        assert_eq!(LineEnding::of(b"\n"), LineEnding::Lf);
        assert_eq!(LineEnding::default(), LineEnding::Lf);
        assert_eq!(LineEnding::CrLf.as_str(), "\r\n");
        assert_eq!(LineEnding::CrLf.escaped(), "\\r\\n");
        assert_eq!(LineEnding::Lf.escaped(), "\\n");
    }

    /// **A dialect that states only the path-only read reads only a plain
    /// file.** A pipe, a quote other than `"` or a missing header is answered
    /// `None`, because the path-only read cannot be told it; a plain file
    /// reads byte for byte as before.
    #[test]
    fn the_default_reads_only_a_plain_file() {
        let d: &dyn SqlRenderer = &StatedDialect;
        let plain = DelimitedFile::new("'/data/x.csv'", "'/data'", "x.csv");
        assert!(plain.is_plain());
        assert_eq!(d.delimited_file_relation_for(&plain).as_deref(), Some("SCAN('/data/x.csv')"));
        for unplain in [
            plain.with_delimiter(FileDelimiter::Pipe),
            plain.with_quote('\''),
            plain.with_header(false),
        ] {
            assert!(!unplain.is_plain());
            assert_eq!(d.delimited_file_relation_for(&unplain), None, "{unplain:?}");
        }
        // A named encoding goes where the dialect reads one, and this dialect
        // reads none.
        assert_eq!(d.delimited_file_relation_for(&plain.with_encoding(Some("windows-1252"))), None);
    }

    /// **A file is read the default way until a `with_` method says
    /// otherwise.**
    #[test]
    fn a_new_file_is_comma_delimited_quoted_headed_and_utf8() {
        let columns = vec!["a".to_owned()];
        let file = DelimitedFile::new("'/d/x.csv'", "'/d'", "x.csv");
        assert_eq!(file.delimiter, FileDelimiter::Comma);
        assert_eq!(file.quote, '"');
        assert!(file.header && file.columns.is_empty() && file.encoding.is_none());
        assert_eq!(file.line_ending, LineEnding::Lf);
        let told = file
            .with_columns(&columns)
            .with_line_ending(LineEnding::CrLf)
            .with_delimiter(FileDelimiter::Fs)
            .with_quote('\'')
            .with_header(false)
            .with_encoding(Some("shift_jis"));
        assert_eq!(told.columns, columns.as_slice());
        assert_eq!(told.line_ending, LineEnding::CrLf);
        assert_eq!(told.delimiter, FileDelimiter::Fs);
        assert_eq!((told.quote, told.header, told.encoding), ('\'', false, Some("shift_jis")));
    }

    /// **Each delimiter is its character, its byte and its word**, and the
    /// four ASCII separators are the four control characters.
    #[test]
    fn each_delimiter_is_its_character_its_byte_and_its_word() {
        let all = [
            (FileDelimiter::Comma, ',', Some("comma")),
            (FileDelimiter::Tab, '\t', Some("tab")),
            (FileDelimiter::Pipe, '|', Some("pipe")),
            (FileDelimiter::Semicolon, ';', Some("semicolon")),
            (FileDelimiter::Fs, '\u{1c}', Some("fs")),
            (FileDelimiter::Gs, '\u{1d}', Some("gs")),
            (FileDelimiter::Rs, '\u{1e}', Some("rs")),
            (FileDelimiter::Us, '\u{1f}', Some("us")),
            (FileDelimiter::Literal('~'), '~', None),
        ];
        for (delimiter, character, word) in all {
            assert_eq!(delimiter.as_char(), character);
            assert_eq!(char::from(delimiter.as_byte()), character);
            assert_eq!(delimiter.word(), word);
            assert_eq!(delimiter.is_control(), ('\u{1c}'..='\u{1f}').contains(&character));
        }
        assert_eq!(FileDelimiter::default(), FileDelimiter::Comma);
    }
}