razeri_plugin_contract/

capabilities.rs

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

//! Boolean capability flags with conservative defaults.
//!
//! [`DialectCapabilities`] tells the rendering layer what syntax features
//! the platform supports. Defaults are conservative (`false` / ANSI-safe),
//! so unknown platforms produce safe SQL.

use crate::data_type::DataType;
use crate::engine_version::EngineVersion;
use crate::evolution::EvolutionAnswers;
use crate::merge_shape::MergeShape;

/// **What one engine holds in a text column**: the longest `varchar(n)` it
/// takes, how that length is counted, where the number came from, and what an
/// unbounded `text` column cannot be there.
///
/// Answered by [`DialectCapabilities::text_limits`]. A length past the
/// longest is refused by name before anything is rendered, as a decimal wider
/// than the engine holds is.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[non_exhaustive]
pub struct TextLimits {
    /// The longest `varchar(n)` a column takes, where the engine has one.
    pub longest_varchar: Option<u32>,
    /// How [`Self::longest_varchar`] is counted.
    pub counted_in: VarcharUnit,
    /// Where the number came from.
    pub source: LimitSource,
    /// What the engine refuses an unbounded `text` column as, where it
    /// refuses it as a key — `a key or UNIQUE`, `a primary key`.
    pub text_is_not: Option<&'static str>,
}

impl TextLimits {
    /// A longest `varchar`, counted in `counted_in`, from `source`.
    #[must_use]
    pub const fn new(longest_varchar: u32, counted_in: VarcharUnit, source: LimitSource) -> Self {
        Self { longest_varchar: Some(longest_varchar), counted_in, source, text_is_not: None }
    }

    /// The same limits, for an engine that refuses an unbounded `text` as
    /// `what` — `a key or UNIQUE`.
    #[must_use]
    pub const fn and_text_is_not(self, what: &'static str) -> Self {
        Self { text_is_not: Some(what), ..self }
    }

    /// Why `varchar(length)` does not fit, in a few plain words, or `None`
    /// when it does.
    #[must_use]
    pub fn misfit(&self, length: u32) -> Option<String> {
        let longest = self.longest_varchar?;
        (length > longest).then(|| {
            format!(
                "its varchar holds at most {longest} {}, and this asks for {length}",
                self.counted_in.word()
            )
        })
    }
}

/// **How an engine counts a `varchar`'s declared length.**
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[non_exhaustive]
pub enum VarcharUnit {
    /// In characters, whatever each takes to store.
    Characters,
    /// In bytes: a character outside ASCII takes more than one.
    Bytes,
}

impl VarcharUnit {
    /// The unit as a sentence says it: `characters`, `bytes`.
    #[must_use]
    pub const fn word(self) -> &'static str {
        match self {
            Self::Characters => "characters",
            Self::Bytes => "bytes",
        }
    }
}

/// **Where a stated limit came from.**
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[non_exhaustive]
pub enum LimitSource {
    /// Measured on the engine: one length past it refused at `CREATE TABLE`.
    MeasuredOnTheEngine,
    /// Read from the engine's documentation; no engine was run.
    TheEnginesDocumentation,
}

/// **One stated limit**: the most an engine holds, and where the number came
/// from.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Limit {
    /// The most the engine holds: digits of a second, or a length.
    pub most: u32,
    /// Where the number came from.
    pub source: LimitSource,
}

impl Limit {
    /// The most measured on the engine: that many created, one more refused.
    #[must_use]
    pub const fn measured(most: u32) -> Self {
        Self { most, source: LimitSource::MeasuredOnTheEngine }
    }

    /// The most the engine's documentation states; no engine was run.
    #[must_use]
    pub const fn documented(most: u32) -> Self {
        Self { most, source: LimitSource::TheEnginesDocumentation }
    }
}

/// **What one engine's columns hold, past its decimals and its varchars** —
/// the answer to [`DialectCapabilities::type_limits`].
///
/// `None` in a field means *nothing stated*: the engine stores the type as
/// text that holds any value, or the number has not been recorded. Either way
/// nothing is refused for it. Start from [`TypeLimits::NONE`] and fill in
/// what your engine has.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct TypeLimits {
    /// The most digits of a second this dialect's spelling of a `timestamp`,
    /// `timestamptz` or `datetime` keeps.
    pub timestamp_digits: Option<Limit>,
    /// The most digits of a second this dialect's spelling of a `time` keeps.
    pub time_digits: Option<Limit>,
    /// The longest `char(n)`, counted in [`Self::char_counted_in`].
    pub longest_char: Option<Limit>,
    /// How [`Self::longest_char`] is counted.
    pub char_counted_in: VarcharUnit,
    /// The longest `binary(n)`, in bytes.
    pub longest_binary: Option<Limit>,
    /// The longest `varbinary(n)`, in bytes.
    pub longest_varbinary: Option<Limit>,
}

impl TypeLimits {
    /// Nothing stated: every field `None`, a `char` counted in characters.
    pub const NONE: Self = Self {
        timestamp_digits: None,
        time_digits: None,
        longest_char: None,
        char_counted_in: VarcharUnit::Characters,
        longest_binary: None,
        longest_varbinary: None,
    };

    /// **Why the engine cannot hold `data_type`**, or `None` when it can.
    ///
    /// Only the type itself is asked, not a type nested inside it: a caller
    /// walks an array's or a record's members itself.
    #[must_use]
    pub fn misfit(&self, data_type: &DataType) -> Option<Misfit> {
        match data_type {
            DataType::Timestamp(moment) => {
                self.too_many_digits(moment.precision, |most| format!("timestamp({most})"))
            }
            // A design spells a zoned timestamp and a datetime only at six
            // digits, which every engine that states a limit keeps.
            DataType::TimestampTz(moment) => {
                self.too_many_digits(moment.precision, |_| "timestamptz".to_owned())
            }
            DataType::DateTime(moment) => {
                self.too_many_digits(moment.precision, |_| "datetime".to_owned())
            }
            DataType::Time(time) => {
                let limit = self.time_digits?;
                // A bare `time` keeps the engine's own, which it holds.
                let asked = u32::from(time.precision?);
                (asked > limit.most).then(|| Misfit {
                    why: if limit.most == 0 {
                        format!(
                            "its times of day keep whole seconds only, and this asks for {asked} \
                             digits of a second"
                        )
                    } else {
                        format!(
                            "its times of day keep at most {} digits of a second, and this asks \
                             for {asked}",
                            limit.most
                        )
                    },
                    kind: "time",
                    fits: if limit.most == 0 {
                        "time".to_owned()
                    } else {
                        format!("time({})", limit.most)
                    },
                })
            }
            DataType::Char(text) => {
                let limit = self.longest_char?;
                (text.length > limit.most).then(|| Misfit {
                    why: format!(
                        "its char holds at most {} {}, and this asks for {}",
                        limit.most,
                        self.char_counted_in.word(),
                        text.length
                    ),
                    kind: "char",
                    fits: format!("char({})", limit.most),
                })
            }
            DataType::Binary(bytes) => {
                let limit = self.longest_binary?;
                (bytes.length > limit.most).then(|| Misfit {
                    why: format!(
                        "its binary holds at most {} bytes, and this asks for {}",
                        limit.most, bytes.length
                    ),
                    kind: "binary",
                    fits: format!("binary({})", limit.most),
                })
            }
            DataType::VarBinary(bytes) => {
                let limit = self.longest_varbinary?;
                let asked = bytes.max_length?;
                (asked > limit.most).then(|| Misfit {
                    why: format!(
                        "its varbinary holds at most {} bytes, and this asks for {asked}",
                        limit.most
                    ),
                    kind: "varbinary",
                    fits: format!("varbinary({})", limit.most),
                })
            }
            _ => None,
        }
    }

    /// A timestamp's digits of a second against [`Self::timestamp_digits`].
    fn too_many_digits(&self, asked: u8, fits: impl Fn(u32) -> String) -> Option<Misfit> {
        let limit = self.timestamp_digits?;
        (u32::from(asked) > limit.most).then(|| Misfit {
            why: format!(
                "its timestamps keep at most {} digits of a second, and this asks for {asked}",
                limit.most
            ),
            kind: "timestamp",
            fits: fits(limit.most),
        })
    }
}

/// **Why a column type does not fit an engine**, in the words a refusal uses.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Misfit {
    /// What the engine holds and what was asked, in a few plain words:
    /// `its timestamps keep at most 6 digits of a second, and this asks for 12`.
    pub why: String,
    /// The kind of type, as a refusal names it: `timestamp`, `char`.
    pub kind: &'static str,
    /// The nearest type the engine holds, as a design declares it:
    /// `timestamp(6)`, `char(255)`.
    pub fits: String,
}

/// **The widest exact decimal one engine's column holds.**
///
/// Every engine that records one refuses a precision of zero, so the fewest
/// digits is always one and is not a field.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct DecimalLimits {
    /// The most digits a column may declare.
    pub max_precision: u16,
    /// The most digits after the decimal point.
    pub max_scale: u16,
    /// Whether the digits after the point must be no more than the digits in
    /// all. Some engines let a column hold only a fraction smaller than a
    /// thousandth, with more scale than precision; most refuse it.
    pub scale_within_precision: bool,
    /// The most digits before the decimal point, where the engine bounds
    /// them apart from the digits in all; `None` where the digits in all are
    /// the only bound. `BigQuery` holds 76 digits, but never more than 38 of
    /// them before the point.
    pub max_integer_digits: Option<u16>,
}

impl DecimalLimits {
    /// Why `decimal(precision, scale)` does not fit, in a few plain words, or
    /// `None` when it does.
    ///
    /// Parameters:
    /// - `precision`: the declared digits in all;
    /// - `scale`: the declared digits after the point.
    #[must_use]
    pub fn misfit(&self, precision: u16, scale: u16) -> Option<String> {
        let most = self.max_precision;
        if precision == 0 || precision > most {
            return Some(format!(
                "its decimals hold 1 to {most} digits, and this asks for {precision}"
            ));
        }
        if scale > self.max_scale {
            return Some(format!(
                "its decimals hold at most {} digits after the point, and this asks for {scale}",
                self.max_scale
            ));
        }
        if self.scale_within_precision && scale > precision {
            return Some(format!(
                "its decimals hold no more digits after the point than in all, and this asks for \
                 {scale} of {precision}"
            ));
        }
        let before_point = precision.saturating_sub(scale);
        if let Some(most) = self.max_integer_digits
            && before_point > most
        {
            return Some(format!(
                "its decimals hold at most {most} digits before the point, and this asks for \
                 {before_point}"
            ));
        }
        None
    }

    /// The nearest `decimal(precision, scale)` these limits hold, for a
    /// refusal to suggest in place of the one declared.
    ///
    /// Parameters:
    /// - `precision`: the declared digits in all;
    /// - `scale`: the declared digits after the point.
    #[must_use]
    pub fn nearest(&self, precision: u16, scale: u16) -> (u16, u16) {
        let mut scale = scale.min(self.max_scale);
        let mut precision = precision.clamp(1, self.max_precision);
        if self.scale_within_precision {
            scale = scale.min(precision);
        }
        if let Some(most) = self.max_integer_digits {
            precision = precision.min(scale + most).max(1);
        }
        if self.scale_within_precision {
            scale = scale.min(precision);
        }
        (precision, scale)
    }
}

/// Level 6 — Boolean capability flags with conservative defaults.
///
/// These flags tell the rendering layer what syntax features the platform supports.
/// Defaults are `false` / conservative, so unknown platforms produce safe ANSI SQL.
pub trait DialectCapabilities {
    // ═══════════════════════════════════════════════════════════════
    // DML Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// **How this engine performs the upsert Razeri's generated load is.**
    ///
    /// See [`MergeShape`]. The default is a refusal, because the honest answer
    /// for an engine nobody has measured is that Razeri does not know how to
    /// load it — not that it spells its upsert the way `MySQL` does.
    fn merge_shape(&self) -> MergeShape {
        MergeShape::None
    }

    /// **How much of the deployed schema a reconciliation can compare with
    /// the design here.**
    ///
    /// Razeri's schema check reads the engine's catalogue in one query: the
    /// ANSI `information_schema.columns`, a `FULL OUTER JOIN` against the
    /// design's own columns, and a table value constructor with a derived
    /// column list, `(VALUES ('a'), ('b')) AS designed (column_name)`. See
    /// [`CatalogueReach`]. The default is
    /// [`CatalogueReach::Nothing`] with a sentence saying Razeri has not
    /// written this engine's catalogue query: an engine nobody has measured
    /// gets no check, and a warning that says why, rather than a query that
    /// fails.
    fn catalogue_reach(&self) -> CatalogueReach {
        CatalogueReach::Nothing(
            "razeri has not written this platform's catalog query, and guessing at one would \
             emit SQL that fails rather than a verdict",
        )
    }

    /// **What this engine keeps when a table that already holds rows changes
    /// shape.**
    ///
    /// See [`EvolutionAnswers`]. The default answers nothing, which Razeri
    /// reads as *nobody has measured this engine* — so a change Razeri cannot
    /// otherwise prove keeps every value is reported as one that does not, and
    /// an operator gets a warning they can read instead of a surprise.
    fn evolution_answers(&self) -> EvolutionAnswers {
        EvolutionAnswers::none()
    }

    /// Whether the platform supports INSERT ... ON CONFLICT (PostgreSQL-style upsert).
    fn supports_on_conflict(&self) -> bool {
        false
    }

    /// Whether the platform supports INSERT OVERWRITE.
    fn supports_insert_overwrite(&self) -> bool {
        false
    }

    /// Whether the platform supports UPDATE ... FROM.
    fn supports_update_from(&self) -> bool {
        false
    }

    /// Whether the platform supports RETURNING / OUTPUT clauses.
    fn supports_returning(&self) -> bool {
        false
    }

    // ═══════════════════════════════════════════════════════════════
    // The server's version
    // ═══════════════════════════════════════════════════════════════

    /// **How to ask a running server its version**, or `None` where this
    /// engine has no version a user chooses, or nobody has written the
    /// question.
    ///
    /// The query answers one row whose first word is the version in the
    /// spelling this dialect declares (`17`, `8.4`, `23ai`); anything after it
    /// is the server's own words, which a refusal prints. A generated deploy
    /// script sends it after login and before the schema, and refuses a server
    /// older than the version the bundle was rendered for, naming both. The
    /// default is `None`, and a script that has no question asks none.
    fn server_version_query(&self) -> Option<&str> {
        None
    }

    /// **The version in one answer to [`Self::server_version_query`]**, or
    /// `None` when the answer holds none.
    ///
    /// Parameters:
    /// - `answer`: the one row the query answered, as text.
    ///
    /// The default reads the first word, which is what a deploy script reads
    /// too; override it only to read more of the answer than that.
    fn parse_server_version(&self, answer: &str) -> Option<EngineVersion> {
        answer.split_whitespace().next().and_then(|word| EngineVersion::parse(word).ok())
    }

    // ═══════════════════════════════════════════════════════════════
    // Query Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// Whether the platform supports Common Table Expressions (WITH).
    fn supports_cte(&self) -> bool {
        true
    }

    /// Whether the platform supports recursive CTEs.
    fn supports_recursive_cte(&self) -> bool {
        true
    }

    /// Whether the platform supports window functions.
    fn supports_window_functions(&self) -> bool {
        true
    }

    /// Whether the platform supports named windows (WINDOW clause).
    fn supports_named_windows(&self) -> bool {
        true
    }

    /// Whether the platform supports QUALIFY clause.
    fn supports_qualify(&self) -> bool {
        false
    }

    /// Whether the platform supports GROUPS frame type in window functions.
    fn supports_groups_frame(&self) -> bool {
        false
    }

    /// Whether the platform supports NULLS FIRST/LAST in ORDER BY.
    fn supports_nulls_ordering(&self) -> bool {
        true
    }

    // ═══════════════════════════════════════════════════════════════
    // DDL Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// Whether the platform supports IDENTITY columns.
    fn supports_identity(&self) -> bool {
        false
    }

    /// Whether the platform supports sequences (CREATE SEQUENCE).
    fn supports_sequences(&self) -> bool {
        false
    }

    /// Whether the platform supports schema namespacing.
    fn supports_schemas(&self) -> bool {
        true
    }

    /// **Can a routine created in one schema be called from a routine created
    /// in another, with nothing else done?**
    ///
    /// True nearly everywhere, and true by default: a schema on most engines is
    /// a namespace, and naming an object in another namespace is all that
    /// reaching it takes.
    ///
    /// Answer `false` where a schema is also an owner and reaching across one
    /// costs a privilege granted per object. On Oracle a schema IS a user; a
    /// routine runs with its owner's privileges, so a function created in
    /// `acme_core` calling one created in `acme_raw` compiles with errors —
    /// `ORA-60535: invalid identifier`, measured — however exactly it spells
    /// the name.
    ///
    /// What Razeri does with the answer is put a copy of the key function in
    /// **every namespace that calls it** instead of one per domain, so each
    /// namespace stands on its own and no deploy owes a grant. It is one
    /// derivation either way: the copies are the same builder over the same
    /// vocabulary, not two spellings free to drift. The engine-level hash
    /// beneath them keeps its single home — see
    /// [`ReplicationRenderer::sid_prerequisite_home`](crate::ReplicationRenderer::sid_prerequisite_home)
    /// — because that one is Razeri's own object and installs its own grant.
    fn routine_is_callable_across_schemas(&self) -> bool {
        true
    }

    /// Whether the platform supports bulk load (COPY, BULK INSERT, etc.).
    fn supports_bulk_load(&self) -> bool {
        false
    }

    /// **Whether a generated load is written as a stored procedure** on this
    /// engine.
    ///
    /// Razeri asks this before it writes a load as a procedure, and asks
    /// [`SqlRenderer::procedure_form`](crate::SqlRenderer::procedure_form)
    /// how to spell one. The default is `false`, and a load is then written as
    /// plain statements. A dialect that answers `procedure_form` with
    /// `ProcedureForm::Custom` answers `true` here, or its procedures are
    /// never asked for; one that answers `ProcedureForm::Unsupported` leaves
    /// this `false`. The conformance report holds the two to each other.
    fn supports_stored_procedures(&self) -> bool {
        false
    }

    /// Whether the platform **stores functions at all** — a routine the schema
    /// creates and later calls by name, in any spelling.
    ///
    /// Nearly every engine razeri reaches has one: `CREATE FUNCTION` on
    /// `PostgreSQL`, `MySQL`, `MariaDB`, Snowflake, `SQL Server` and Databricks,
    /// a `MACRO` on `DuckDB`, a lambda `CREATE FUNCTION` on `ClickHouse`.
    ///
    /// **`SQLite` answers `false`, and it is the reason this question is asked
    /// out loud.** `SQLite` has no `CREATE FUNCTION` statement in any spelling
    /// and no `CREATE PROCEDURE` either — a function there is registered by the
    /// program that opens the database, so nothing written into a schema script
    /// can create one. Inheriting the `PostgreSQL` form wrote
    /// `CREATE OR REPLACE FUNCTION … LANGUAGE SQL RETURN …` into `schema.sql`,
    /// which `SQLite` answers with `near "CREATE": syntax error` part way
    /// through the script, so the build refuses in front of the script instead.
    ///
    /// The default is `true`, matching [`Self::supports_table_functions`] and
    /// for the same reason: refusing is the destructive answer, so a refusal
    /// fires only where an engine is KNOWN to have no form, never because a
    /// dialect forgot to answer. That reverses the conservative-`false` default
    /// this trait uses for flags that *add* syntax — here `false` is the
    /// unusual claim, and it is made deliberately, one dialect at a time.
    fn supports_stored_functions(&self) -> bool {
        true
    }

    /// Whether the platform has a **table-valued function** — a routine a query
    /// selects FROM, returning columns rather than one value.
    ///
    /// Most engines have one in some spelling: `RETURNS TABLE(…)` on
    /// `PostgreSQL`, Snowflake and Databricks, an inline TVF on SQL Server, a
    /// table `MACRO` on `DuckDB`, a parameterized view on `ClickHouse`.
    ///
    /// **`MySQL` and `MariaDB` answer `false`, and they are the reason this
    /// question exists.** A stored function there returns exactly one value;
    /// there is no clause that returns a relation, in any spelling. Rendering
    /// `RETURNS TABLE` for them produced `ERROR 1064 … syntax error` in the
    /// middle of a schema script (measured on `MySQL` 8.4 and `MariaDB` 11.4),
    /// so the build refuses in front of the script instead.
    ///
    /// The default is `true`: a refusal fires only where an engine is KNOWN to
    /// have no form, never because a dialect forgot to answer.
    fn supports_table_functions(&self) -> bool {
        true
    }

    /// Whether the platform supports tasks / scheduled jobs.
    fn supports_tasks(&self) -> bool {
        false
    }

    /// Whether the platform supports native table partitioning.
    fn supports_partitioning(&self) -> bool {
        true
    }

    /// Whether the platform supports materialized views.
    fn supports_materialized_views(&self) -> bool {
        true
    }

    /// Whether the platform supports generated/computed columns.
    fn supports_generated_columns(&self) -> bool {
        true
    }

    /// Whether the platform supports JSONB or native JSON type.
    fn supports_json(&self) -> bool {
        true
    }

    // ═══════════════════════════════════════════════════════════════
    // Type Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// Whether the platform supports unsigned integers natively.
    fn supports_unsigned_integers(&self) -> bool {
        false
    }

    /// Whether the platform supports arrays natively.
    fn supports_arrays(&self) -> bool {
        false
    }

    /// **Whether this engine has a type of its own for a nested type, or
    /// stores it as something else.**
    ///
    /// A nested column is an array, a map or a record. Some engines have all
    /// three; some have one; some have none and hold the value as JSON or as
    /// text. Razeri never refuses the column — the design is portable and it is
    /// the storage that degrades — but it does say so once, naming the engine,
    /// the columns and what each became.
    ///
    /// The default is `false`, which is the honest answer for an engine nobody
    /// has answered for: the reader is told what the column became rather than
    /// left to assume the type survived.
    ///
    /// Parameters:
    /// - `data_type`: the nested type in question.
    fn renders_nested_type_natively(&self, data_type: &DataType) -> bool {
        let _ = data_type;
        false
    }

    /// **Whether this engine can tell two values of a nested type apart.**
    ///
    /// Change detection asks whether an arriving value differs from the
    /// standing one. For a scalar every engine answers with `<>`. For a nested
    /// value they do not agree: some compare arrays, maps and records directly,
    /// some refuse the comparison outright, and some hold the value as text and
    /// compare the text. A column this returns `false` for is left out of
    /// change detection and named in a note, rather than compared with an
    /// operator the engine will reject.
    ///
    /// The default is `false` — an unanswered engine leaves the column
    /// unwatched and says so, which is the failure that can be read.
    ///
    /// Parameters:
    /// - `data_type`: the nested type in question.
    fn compares_nested_values(&self, data_type: &DataType) -> bool {
        let _ = data_type;
        false
    }

    /// **Whether this engine can collapse rows on a nested column** — group by
    /// it, or select distinct over it.
    ///
    /// A load reads its staged rows through one collapsed form so that a row
    /// stated twice, identically, is one arriving row rather than a conflict.
    /// The plain spelling of that collapse is `SELECT DISTINCT` over every
    /// column, and grouping is what `DISTINCT` is: an engine that will not
    /// group a value of a nested type will not take the column in a `DISTINCT`
    /// either.
    ///
    /// It is a separate question from [`Self::compares_nested_values`], and the
    /// two engines that answer them differently are the reason it is asked
    /// separately: `BigQuery` compares two records with `=` and refuses to group
    /// on one.
    ///
    /// Where the answer is `false` the collapse is written as a `GROUP BY` over
    /// the other columns with [`crate::SqlRenderer::render_any_value`] around
    /// this one.
    ///
    /// The default is `true`: `SELECT DISTINCT` over every column is the plain
    /// spelling, and an engine says otherwise only where somebody has found
    /// that it refuses.
    ///
    /// Asked only of a nested type — an array, a map or a record. The answer
    /// for anything else is not read.
    ///
    /// Parameters:
    /// - `data_type`: the nested type in question.
    fn groups_nested_values(&self, data_type: &DataType) -> bool {
        let _ = data_type;
        true
    }

    /// Whether the platform supports native BOOLEAN type (vs BIT).
    fn supports_boolean(&self) -> bool {
        true
    }

    // ═══════════════════════════════════════════════════════════════
    // Limits & Defaults
    // ═══════════════════════════════════════════════════════════════

    /// The default schema name for this platform.
    fn default_schema(&self) -> &'static str {
        "public"
    }

    /// Maximum identifier length for this platform.
    fn max_identifier_length(&self) -> usize {
        63
    }

    /// Maximum number of columns in a table (approximate).
    fn max_columns(&self) -> usize {
        10000
    }

    /// Maximum query parameters (approximate, 0 = unlimited).
    fn max_parameters(&self) -> usize {
        0
    }

    /// **The widest exact decimal a column on this platform holds**, where it
    /// has been measured against the engine.
    ///
    /// A type over it is refused by name rather than rendered: the engine
    /// would refuse the `CREATE TABLE` anyway, later and in its own words.
    ///
    /// The default is `None`, which means *not measured here*, not *no
    /// limit*. An engine with no limit at all says so in its own doc comment
    /// and still answers `None`.
    fn decimal_limits(&self) -> Option<DecimalLimits> {
        None
    }

    /// **What a text column on this platform holds**, where it has been
    /// measured against the engine or read from its documentation (see
    /// [`TextLimits`]).
    ///
    /// The default is `None`, which means *not recorded here*, not *no
    /// limit*. Measure the longest `varchar(n)` your engine creates — one
    /// more is refused at `CREATE TABLE` — and say how it counts it.
    fn text_limits(&self) -> Option<TextLimits> {
        None
    }

    /// **What this engine's other columns hold** — the digits of a second a
    /// timestamp and a time of day keep, and the longest `char(n)`,
    /// `binary(n)` and `varbinary(n)` (see [`TypeLimits`]).
    ///
    /// A type over a stated limit is refused by name before anything is
    /// rendered. The default is `None`: nothing is stated, and nothing is
    /// refused for this engine, which then refuses the `CREATE TABLE` later in
    /// its own words. Measure each the way the varchar is measured — the most
    /// your engine creates, one more refused — and leave `None` in a field
    /// your engine stores with no limit.
    fn type_limits(&self) -> Option<TypeLimits> {
        None
    }

    /// **Whether this engine has a type that keeps a time zone**, for a
    /// `timestamptz` column: a moment that keeps its offset, or one read in
    /// the session's zone. The default is `true`. An engine with no such type
    /// answers `false`, and a `timestamptz` column is refused by name rather
    /// than stored as a clock time two hours off the moment the file named.
    fn keeps_a_time_zone(&self) -> bool {
        true
    }

    // ═══════════════════════════════════════════════════════════════
    // Drop Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// Whether DROP supports CASCADE modifier.
    fn supports_cascade_drop(&self) -> bool {
        true
    }

    /// Whether DROP FUNCTION requires parameter types for overloaded functions.
    fn supports_function_overloading(&self) -> bool {
        false
    }

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

    /// Whether DROP supports FORCE modifier (Databricks).
    fn supports_force_drop(&self) -> bool {
        false
    }

    // ═══════════════════════════════════════════════════════════════
    // DML Extended Capabilities
    // ═══════════════════════════════════════════════════════════════

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

    /// Whether the platform supports EXCLUDE clause in window frame specs.
    fn supports_window_frame_exclude(&self) -> bool {
        false
    }

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

    /// Keyword emitted for MERGE NOT MATCHED branch.
    fn merge_not_matched_keyword(&self) -> &'static str {
        "WHEN NOT MATCHED"
    }

    /// Whether the platform supports UPDATE SET * / INSERT * in MERGE.
    fn supports_merge_star_syntax(&self) -> bool {
        false
    }

    /// Whether an UPDATE is spelled `ALTER TABLE … UPDATE` (`ClickHouse`).
    ///
    /// **There is a second declaration of the same fact** on
    /// [`crate::SqlRenderer::uses_alter_table_update`], and that one is the
    /// declaration Razeri's generators consult. A dialect that overrides one
    /// must override both and keep them agreeing; the duplication is named
    /// here so a reader finds it before an ambiguous-method error does.
    fn uses_alter_table_update(&self) -> bool {
        false
    }

    /// Whether the platform supports multi-table INSERT (Snowflake).
    fn supports_multi_table_insert(&self) -> bool {
        false
    }

    // ═══════════════════════════════════════════════════════════════
    // DDL / Namespace Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// The SQL keyword for the schema-level namespace.
    ///
    /// Most platforms use `SCHEMA` (`CREATE SCHEMA IF NOT EXISTS ...`).
    /// `ClickHouse` uses `DATABASE` (`CREATE DATABASE IF NOT EXISTS ...`).
    fn schema_namespace_keyword(&self) -> &'static str {
        "SCHEMA"
    }

    // ═══════════════════════════════════════════════════════════════
    // Comment Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// Whether the platform supports `#` as a single-line comment delimiter.
    ///
    /// Default: `false`. A dialect whose engine accepts `#` as a single-line
    /// comment delimiter overrides this.
    fn supports_hash_comments(&self) -> bool {
        false
    }

    // ═══════════════════════════════════════════════════════════════
    // Script Formatting
    // ═══════════════════════════════════════════════════════════════

    /// Batch separator keyword required between DDL statements in a script.
    ///
    /// SQL Server / T-SQL requires `GO` between `CREATE FUNCTION`, `CREATE VIEW`,
    /// and `CREATE PROCEDURE` statements. Other platforms return `None` and use
    /// double-newlines between statements.
    fn batch_separator(&self) -> Option<&'static str> {
        None
    }

    // ═══════════════════════════════════════════════════════════════
    // Transaction Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// Whether several statements in a script can be committed or discarded
    /// together as one unit.
    ///
    /// A generated load is a sequence of writes that only make sense finished.
    /// An SCD-2 load, for instance, closes the standing version into `_history`
    /// and then re-opens `_current` on the new attributes; a process that dies
    /// between the two leaves a closed version that `_current` still holds open,
    /// and because `_history` is keyed on (surrogate, version start), the next
    /// load writes the same key again and the engine refuses it. Every load
    /// after the crash fails until somebody deletes that row by hand.
    ///
    /// Wrapping the load in one transaction is what makes the crash a no-op
    /// instead. This flag says whether the platform offers that at all. It
    /// defaults to `true` because a transaction spanning several statements is
    /// core SQL and nearly every engine has it; a platform that does not
    /// overrides it, and the load script then says plainly that the operator
    /// owns the boundary.
    fn supports_multi_statement_transactions(&self) -> bool {
        true
    }

    /// How this platform opens a transaction.
    ///
    /// The ANSI spelling, which `MySQL` and `MariaDB` also use. `DuckDB`,
    /// `SQLite`, `SQL Server` and `Snowflake` say `BEGIN TRANSACTION`;
    /// `PostgreSQL` and `Citus` say `BEGIN`.
    fn begin_transaction_keyword(&self) -> &'static str {
        "START TRANSACTION"
    }

    /// How this platform commits one.
    ///
    /// `COMMIT` everywhere razeri reaches except `SQL Server`, which spells it
    /// `COMMIT TRANSACTION`.
    fn commit_transaction_keyword(&self) -> &'static str {
        "COMMIT"
    }

    /// **Whether this engine's begin statement leaves the commit to the
    /// client.**
    ///
    /// On most engines [`begin_transaction_keyword`](Self::begin_transaction_keyword)
    /// opens a transaction that holds every statement after it until the
    /// commit, whatever the client does. On some it only names the
    /// transaction — Oracle's and SAP HANA's `SET TRANSACTION READ WRITE` —
    /// and a driver that commits each statement on its own still does, so a
    /// load run that way keeps what it wrote when it stops part way.
    ///
    /// Razeri's own deploy scripts drive clients whose commit is off. Answer
    /// `true` where the engine needs that of the client, and the bundle tells
    /// whoever runs a load another way — a scheduler running the task list —
    /// to turn its client's commit off. The default is `false`.
    fn begin_leaves_the_commit_to_the_client(&self) -> bool {
        false
    }

    /// Whether emptying a table with `TRUNCATE` ends the open transaction.
    ///
    /// On some engines `TRUNCATE TABLE` commits everything before it and
    /// cannot be rolled back: `MySQL` and `MariaDB`, which treat it as a
    /// schema change, and Oracle, SAP HANA, `BigQuery` and Redshift. A
    /// generated load empties its staging table only as its last statement,
    /// after every write it makes, so on those engines the clear commits the
    /// load with it and each load stands on its own; a load that stops before
    /// it leaves nothing behind. Everywhere else `TRUNCATE` is an ordinary
    /// statement inside the transaction.
    fn truncate_commits_implicitly(&self) -> bool {
        false
    }

    /// **Whether a load's refusal stops the statement that asks it.**
    ///
    /// A generated load stops itself by building a sentence about the rows it
    /// refuses and asking the engine for
    /// [`ExpressionRenderer::render_refusal`](crate::ExpressionRenderer::render_refusal)
    /// of it: the engine refuses and quotes the sentence back in its error.
    /// Most engines do. `MySQL` and `MariaDB` answer the default refusal, a
    /// conversion to a number, with a warning and `0`, and Spark answers
    /// `NULL` where ANSI mode is off. There the load stops on a second
    /// statement instead, in the engine's own words and without the sentence,
    /// so a deploy script reads the refused rows back and prints them itself.
    ///
    /// The default is `true`, the answer of every engine whose refusal stops.
    fn refusal_stops_the_statement(&self) -> bool {
        true
    }

    /// **Whether this engine's file reader may read a field holding just the
    /// word `NULL` as a missing value** rather than as the four letters.
    ///
    /// A design names the words that mean "no value" in each file, and a
    /// binding that names none calls `NULL` missing, which every engine then
    /// agrees on. A file that keeps `NULL` as text cannot be read that way
    /// here, so the bundle names the file, and a reconciliation does not
    /// compare the characters of a text column holding the word. The default
    /// is `false`: the engine keeps the text, as every engine's own file
    /// reader does except `MySQL`'s and `MariaDB`'s `LOAD DATA`.
    fn delimited_file_reads_null_word_as_missing(&self) -> bool {
        false
    }

    // ═══════════════════════════════════════════════════════════════
    // Storage / Partition / Compression Capabilities
    // ═══════════════════════════════════════════════════════════════

    /// Whether the platform supports a server-side partition function definable
    /// at design time (`T-SQL` `PARTITION FUNCTION`, `PostgreSQL` `PARTITION BY`,
    /// `MySQL` `PARTITION BY`, `ClickHouse` `PARTITION BY`).
    ///
    /// `Snowflake` / `DuckDB` return `false` because micro-partitioning is automatic
    /// and not user-definable.
    fn supports_partition_function(&self) -> bool {
        false
    }

    /// Whether the platform supports adding/removing partition boundaries on a
    /// live table (`T-SQL` `ALTER PARTITION FUNCTION SPLIT/MERGE RANGE`,
    /// `PostgreSQL` `ATTACH/DETACH PARTITION`, `MySQL` `REORGANIZE PARTITION`).
    fn supports_partition_split_merge(&self) -> bool {
        false
    }

    /// Whether the platform supports a columnstore index (`T-SQL` clustered/non-clustered
    /// columnstore; `ClickHouse` via `MergeTree` engine; not natively in `PostgreSQL` / `MySQL`).
    fn supports_columnstore_index(&self) -> bool {
        false
    }

    /// Whether the platform realizes a columnstore as a **table storage engine**
    /// rather than an index (`MariaDB` `ENGINE=ColumnStore`). This is deliberately
    /// distinct from [`Self::supports_columnstore_index`]: a design that asks for
    /// `index: { kind: clustered_columnstore }` is honoured on such a platform by
    /// selecting the engine, but it must NOT inherit the `T-SQL`
    /// index machinery — the composite-PK demotion to `UNIQUE NONCLUSTERED` keys
    /// off `supports_columnstore_index`, and an engine-columnstore keeps its
    /// ordinary primary key. So the two capabilities are separate axes: "supports
    /// columnstore at all" is the disjunction of them.
    fn columnstore_is_table_engine(&self) -> bool {
        false
    }

    /// Whether the platform has a clustering-key concept distinct from primary key
    /// (`Snowflake` `CLUSTER BY`, Delta Liquid Clustering / `Z-ORDER`, `ClickHouse` `ORDER BY`).
    fn supports_clustering_key(&self) -> bool {
        false
    }

    /// Whether the platform supports user-controllable table compression
    /// (`T-SQL` `DATA_COMPRESSION = PAGE/ROW/COLUMNSTORE`, `MySQL` `ROW_FORMAT=COMPRESSED`,
    /// `ClickHouse` `CODEC(ZSTD)`, Delta `delta.compressionCodec`).
    fn supports_table_compression(&self) -> bool {
        false
    }

    /// Whether the platform requires every column referenced by the partition
    /// key to also appear in the table's PRIMARY KEY (or any UNIQUE
    /// constraint).
    ///
    /// `PostgreSQL` enforces this with `SQLSTATE 0A000` ("unique constraint on
    /// partitioned table must include all partitioning columns"). `T-SQL`,
    /// `MySQL`, `ClickHouse`, `Snowflake`, and `DuckDB` do not impose this
    /// restriction.
    fn partition_requires_pk_includes_field(&self) -> bool {
        false
    }
}

/// **How much of a deployed schema Razeri's reconciliation can read back from
/// this engine's catalogue**, answered by
/// [`DialectCapabilities::catalogue_reach`].
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[non_exhaustive]
pub enum CatalogueReach {
    /// **Every check.** The engine runs the query that compares its catalogue
    /// with the design in one measurement: `information_schema.columns`, a
    /// `FULL OUTER JOIN`, and a table value constructor with a derived column
    /// list.
    Whole,
    /// **The columns it holds, and not the comparison.** The engine has
    /// `information_schema.columns` and lacks what the one-measurement
    /// comparison needs, so the reconciliation asks only whether each designed
    /// column is held. The sentence says what it lacks; a warning carries it.
    HeldColumnsOnly(&'static str),
    /// **No schema check.** The sentence says why, in a reader's words; a
    /// warning carries it.
    Nothing(&'static str),
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::merge_shape::MergeRestrictions;

    struct DefaultDialect;
    impl DialectCapabilities for DefaultDialect {}

    /// **A dialect that states no question asks none, and the first word of
    /// an answer is its version.**
    #[test]
    fn the_server_version_is_the_first_word_of_the_answer() {
        let d = DefaultDialect;
        assert_eq!(d.server_version_query(), None);
        let read = d.parse_server_version("17 17.2");
        assert_eq!(read.map(|v| v.to_string()), Some("17".to_owned()));
        assert_eq!(d.parse_server_version("  8.4 MySQL Community Server"), "8.4".parse().ok());
        assert_eq!(d.parse_server_version(""), None);
        assert_eq!(d.parse_server_version("PostgreSQL 17.2"), None);
    }

    /// Assert default DML capability flags are conservative.
    /// Parameters:
    /// - d: Dialect capabilities implementation under test.
    fn assert_default_dml_capabilities(d: &impl DialectCapabilities) {
        assert_eq!(d.merge_shape(), MergeShape::None);
        assert!(!d.supports_on_conflict());
        assert!(!d.supports_insert_overwrite());
        assert!(!d.supports_update_from());
        assert!(!d.supports_returning());
    }

    /// Assert default query capability flags and ANSI-safe behavior.
    /// Parameters:
    /// - d: Dialect capabilities implementation under test.
    fn assert_default_query_capabilities(d: &impl DialectCapabilities) {
        assert!(d.supports_cte());
        assert!(d.supports_recursive_cte());
        assert!(d.supports_window_functions());
        assert!(d.supports_named_windows());
        assert!(!d.supports_qualify());
        assert!(!d.supports_groups_frame());
        assert!(d.supports_nulls_ordering());
    }

    /// Assert default DDL capability and type flags are conservative.
    /// Parameters:
    /// - d: Dialect capabilities implementation under test.
    fn assert_default_ddl_and_type_capabilities(d: &impl DialectCapabilities) {
        assert!(!d.supports_identity());
        assert!(!d.supports_sequences());
        assert!(d.supports_schemas());
        assert!(!d.supports_bulk_load());
        assert!(!d.supports_stored_procedures());
        // The two routine-form questions default the other way on purpose:
        // answering `false` REFUSES a build, so it is never the answer an
        // unknown platform falls into.
        assert!(d.supports_stored_functions());
        assert!(d.supports_table_functions());
        assert!(!d.supports_tasks());
        assert!(d.supports_partitioning());
        assert!(d.supports_materialized_views());
        assert!(d.supports_generated_columns());
        assert!(d.supports_json());
        assert!(!d.supports_unsigned_integers());
        assert!(!d.supports_arrays());
        assert!(d.supports_boolean());
    }

    /// Assert default platform limits and storage capability flags.
    /// Parameters:
    /// - d: Dialect capabilities implementation under test.
    fn assert_default_limits_and_storage_capabilities(d: &impl DialectCapabilities) {
        assert_eq!(d.default_schema(), "public");
        assert_eq!(d.max_identifier_length(), 63);
        assert_eq!(d.max_columns(), 10000);
        assert_eq!(d.max_parameters(), 0);
        assert!(!d.supports_partition_function());
        assert!(!d.supports_partition_split_merge());
        assert!(!d.supports_columnstore_index());
        assert!(!d.supports_clustering_key());
        assert!(!d.supports_table_compression());
    }

    #[test]
    fn defaults_are_conservative() {
        let d = DefaultDialect;
        assert_default_dml_capabilities(&d);
        assert_default_query_capabilities(&d);
        assert_default_ddl_and_type_capabilities(&d);
        assert_default_limits_and_storage_capabilities(&d);
    }

    #[test]
    fn overrides_work() {
        struct PostgresLike;
        impl DialectCapabilities for PostgresLike {
            fn merge_shape(&self) -> MergeShape {
                MergeShape::Merge(MergeRestrictions::none())
            }
            fn supports_on_conflict(&self) -> bool {
                true
            }
            fn supports_returning(&self) -> bool {
                true
            }
            fn supports_identity(&self) -> bool {
                true
            }
            fn supports_sequences(&self) -> bool {
                true
            }
            fn supports_arrays(&self) -> bool {
                true
            }
            fn max_columns(&self) -> usize {
                1600
            }
        }

        let d = PostgresLike;
        assert!(d.merge_shape().merges());
        assert!(d.supports_on_conflict());
        assert!(d.supports_returning());
        assert!(d.supports_identity());
        assert!(d.supports_sequences());
        assert!(d.supports_arrays());
        assert_eq!(d.max_columns(), 1600);
        // Untouched defaults
        assert!(!d.supports_insert_overwrite());
        assert!(d.supports_cte());
    }

    #[test]
    fn new_capability_defaults_are_conservative() {
        let d = DefaultDialect;
        // Drop capabilities
        assert!(d.supports_cascade_drop()); // true = ANSI default
        assert!(!d.supports_function_overloading());
        assert!(!d.supports_sync_drop());
        assert!(!d.supports_force_drop());
        // DML extended
        assert!(!d.supports_compound_assignment());
        assert!(!d.supports_window_frame_exclude());
        assert!(!d.supports_merge_not_matched_by_source());
        assert_eq!(d.merge_not_matched_keyword(), "WHEN NOT MATCHED");
        assert!(!d.supports_merge_star_syntax());
        assert!(!d.uses_alter_table_update());
        assert!(!d.supports_multi_table_insert());
        // DDL / Namespace
        assert_eq!(d.schema_namespace_keyword(), "SCHEMA");
    }

    /// The default transaction answer is the ANSI one: a load can be wrapped,
    /// in the standard spelling, and clearing a staging table stays inside the
    /// wrap. Every platform that differs says so on its own dialect.
    #[test]
    fn the_default_load_wrap_is_the_ansi_one() {
        let d = DefaultDialect;
        assert!(d.supports_multi_statement_transactions());
        assert_eq!(d.begin_transaction_keyword(), "START TRANSACTION");
        assert_eq!(d.commit_transaction_keyword(), "COMMIT");
        assert!(!d.truncate_commits_implicitly());
        assert!(!d.begin_leaves_the_commit_to_the_client());
    }

    /// **A bound on the digits before the point is its own refusal**, and the
    /// nearest type that fits keeps the scale and gives up whole digits.
    #[test]
    fn digits_before_the_point_have_their_own_bound() {
        let bignumeric = DecimalLimits {
            max_precision: 76,
            max_scale: 38,
            scale_within_precision: true,
            max_integer_digits: Some(38),
        };
        assert_eq!(bignumeric.misfit(38, 0), None);
        assert_eq!(bignumeric.misfit(76, 38), None);
        assert_eq!(
            bignumeric.misfit(39, 0).as_deref(),
            Some("its decimals hold at most 38 digits before the point, and this asks for 39")
        );
        assert_eq!(bignumeric.nearest(39, 0), (38, 0));
        assert_eq!(bignumeric.nearest(60, 10), (48, 10));
        assert_eq!(bignumeric.nearest(99, 40), (76, 38));
        for (precision, scale) in [(39, 0), (60, 10), (99, 40), (5, 6), (0, 0)] {
            let (p, s) = bignumeric.nearest(precision, scale);
            assert_eq!(bignumeric.misfit(p, s), None, "nearest({precision}, {scale}) fits");
        }

        let unbounded = DecimalLimits { max_integer_digits: None, ..bignumeric };
        assert_eq!(unbounded.misfit(76, 0), None, "the digits in all are the only bound");
    }
}