razeri_plugin_contract/

replication.rs

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

//! Replication rendering traits with ANSI defaults.
//!
//! [`ReplicationRenderer`] extends [`SqlRenderer`] with methods for
//! SID (surrogate ID) function generation, data extraction, data loading, and
//! checksum validation. Each dialect crate overrides these methods to produce
//! platform-native SQL.
//!
//! ## Format names
//!
//! Methods that accept a `format` parameter use well-known string identifiers:
//!
//! | Name | Description |
//! |------|-------------|
//! | `"csv"` | Comma-separated values |
//! | `"parquet"` | Apache Parquet columnar format |
//! | `"json_lines"` | JSON Lines (one object per line) |
//! | `"native"` | Platform-native binary format |

use crate::SqlRenderer;

// ════════════════════════════════════════════════════════════════════════
// SID Prerequisites — the statement boundary
// ════════════════════════════════════════════════════════════════════════

/// **The line a dialect writes between two independently-executable statements
/// inside [`ReplicationRenderer::sid_prerequisites`].**
///
/// A prerequisite script can be mostly prose — the `MariaDB` C source is
/// emitted as a comment block for an operator to compile — but several arms
/// emit real DDL a machine must run one statement at a time: Snowflake's
/// JavaScript UDF plus its SQL wrapper, Databricks' Python UDF, SQL Server's
/// assembly installed from its bytes. The first two cannot be recovered by
/// splitting on `;`, because **both bodies contain semicolons**: a JavaScript
/// function body is nothing but semicolons.
///
/// So the boundary is declared rather than inferred. The delimiter is a SQL
/// comment, so a script carrying it is still one paste-able file for a human
/// operator, and [`ReplicationRenderer::sid_prerequisite_statements`] is the one
/// splitter — a consumer that re-derived the boundary would be a second
/// definition of it, and a second definition is how the two drift apart. Do not
/// write your own splitter; split on this constant or call the trait method.
pub const SID_PREREQUISITE_STATEMENT_DELIMITER: &str =
    "/* ---- RAZERI SID PREREQUISITE STATEMENT ---- */";

/// **The input every generated prerequisite probe asks about.**
///
/// One word, and it is the one [`ReplicationRenderer::sid_validation_query`]
/// already puts in its `SELECT`. It lives here because three producers must
/// agree on it and they live in different crates: the dialect writing the
/// operator's instructions, the generator writing the deploy script's
/// comparison, and the validation query itself. A probe that asked about one
/// word while the instructions promised another would be a second, hand-spelled
/// variant of the same fact — the kind of duplicate that drifts unnoticed.
pub const SID_PROBE_INPUT: &str = "test";

// ════════════════════════════════════════════════════════════════════════
// SID Prerequisites — where the helper they install lives
// ════════════════════════════════════════════════════════════════════════

/// **The schema Razeri keeps its own engine-level objects in**, where an engine
/// has schemas and the dialect asks for one.
///
/// One word, and it is Razeri's rather than the design's. The hash a key
/// function calls is installed once for the whole engine, and a design has as
/// many schemas as it has domains: putting the hash in one of them would make
/// every other domain depend on that domain's schema, and domains are meant to
/// be deployable apart. So the helper gets a home of its own, named here so
/// that a dialect, a deploy script and a test all read the same word.
pub const RAZERI_HELPER_SCHEMA: &str = "razeri";

/// **Where the objects a prerequisite installs actually live**, so that
/// something calling them from another schema can name them.
///
/// Most engines need no answer. Their prerequisite installs nothing with a
/// name — a session setting, an extension registered for the whole database, a
/// server-global function — or installs the key function itself, which Razeri
/// already creates and calls in a known schema. Those say [`Ambient`] and their
/// scripts are what they were, byte for byte.
///
/// [`Schema`] is for an engine where a routine resolves an unqualified name in
/// the schema that OWNS the routine rather than in the session's. There, a hash
/// installed wherever the deploy happened to sign in is invisible to every key
/// function created in a design's own schema — measured on Oracle as
/// `ORA-00904: "RAZERI_XXH64": invalid identifier`, once, with every function
/// beneath it then `ORA-06575 … in an invalid state`. Naming the home fixes the
/// helper in one place and lets every call site spell it.
///
/// [`Ambient`]: Self::Ambient
/// [`Schema`]: Self::Schema
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SidPrerequisiteHome {
    /// **Wherever the deploy runs** — the default, and the honest answer for an
    /// engine with no schema for a function at all.
    ///
    /// `SQLite` loads a library into each connection; `MySQL` and `MariaDB`
    /// register a library for the whole server; `ClickHouse`'s `CREATE FUNCTION` takes no
    /// qualifier; `DuckDB` loads an extension into the session. None of them
    /// has a schema to put a helper in, and a call site that invented one would
    /// name an object that does not exist.
    Ambient,

    /// **A schema Razeri owns**, named here, which the prerequisite creates and
    /// installs into and which every call site then qualifies with.
    ///
    /// Use [`RAZERI_HELPER_SCHEMA`] unless your engine reserves that word: one
    /// home for one engine-level helper is the point, and two dialects naming
    /// two homes would be two places to look.
    ///
    /// **What answering this promises.** The prerequisite's own text creates
    /// its objects IN that schema, and makes them callable from outside it —
    /// on an engine with privileges, that means the grant travels with the
    /// install. Razeri writes the schema's own creation in front of the
    /// install, in whatever this engine's `CREATE SCHEMA` spelling is, and
    /// qualifies the hash in every key function it composes.
    Schema(&'static str),
}

impl SidPrerequisiteHome {
    /// The schema this home names, or `None` where there is none to name.
    #[must_use]
    pub const fn schema(self) -> Option<&'static str> {
        match self {
            Self::Ambient => None,
            Self::Schema(schema) => Some(schema),
        }
    }
}

// ════════════════════════════════════════════════════════════════════════
// SID Prerequisites — what KIND of step this platform's is
// ════════════════════════════════════════════════════════════════════════

/// **What a platform's SID prerequisite actually asks of whoever deploys.**
///
/// The bundle's prerequisites step has to emit a different *shape* for each of
/// these, and the deploy script a different *behaviour*: one runs SQL, one
/// verifies and refuses, one refuses outright, one says there is nothing to do.
/// Sniffing that out of the prose at each call site would put the rule in the
/// generator, the script writer and the runner — three readings of one fact,
/// which is exactly how one derivation becomes two.
///
/// The default derives it from what the dialect already says, so a dialect that
/// overrides nothing is still classified correctly and only a platform the
/// derivation cannot reach ([`Refused`](Self::Refused), [`None`](Self::None))
/// has to override.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SidPrerequisiteKind {
    /// **Nothing to install: the engine's own hash already IS the canonical
    /// number** (XXH64 seed 0), reachable from SQL razeri emits.
    ///
    /// **A positive claim, and therefore never a default.** Saying this means
    /// razeri has *measured* that the platform's builtin returns XXH64 seed 0 —
    /// not that nobody has said otherwise. A dialect that has declared nothing
    /// gets [`Undeclared`](Self::Undeclared) instead, for the same reason
    /// `DdlRenderer::representable_timestamp_ceiling`'s `None` means *no
    /// measured ceiling* rather than *holds everything*.
    ///
    /// The derivation cannot reach this variant. A dialect claiming it
    /// **overrides [`ReplicationRenderer::sid_prerequisite_kind`] and cites the
    /// measurement**, as Razeri's own `ClickHouse` dialect does.
    None,

    /// Razeri emits statements that install it, and a deploy script can run
    /// them unattended: `INSTALL`/`LOAD` (`DuckDB`), `CREATE FUNCTION`
    /// (Snowflake's JavaScript UDF, Databricks' Python one).
    ///
    /// The test is *"can a script finish this with no human on the host"*. It is
    /// **not** "is the statement SQL": `CREATE EXTENSION` is SQL and only
    /// **registers** a shared object somebody compiled first, which is why
    /// `PostgreSQL` and Citus are [`OperatorArtifact`](Self::OperatorArtifact)
    /// and not this.
    SqlInstallable,

    /// An artifact the **operator** builds and places before any SQL can
    /// reference it — `MariaDB`'s and `MySQL`'s `.so`, `PostgreSQL`'s and
    /// Citus's `pg_xxhash`. Razeri emits the source
    /// or the build recipe as prose; it cannot compile anything, and it will not
    /// pretend to.
    ///
    /// The deploy path's obligation here is the prerequisites verb: **verify,
    /// then
    /// refuse** — ask the engine for the function and stop in front of `schema`
    /// when it is not there, rather than deploying a warehouse whose keys it
    /// cannot compute.
    OperatorArtifact,

    /// The platform cannot be made to compute razeri's key from SQL **at all**,
    /// and no artifact an operator drops in changes that.
    ///
    /// Redshift is the case: its `prerequisites.sql` says why. A platform with
    /// no `CREATE FUNCTION` whose client can load a compiled library is not
    /// this case — `SQLite` ships its key function as a library the operator
    /// compiles, an [`OperatorArtifact`](Self::OperatorArtifact).
    ///
    /// The answer is a **named refusal carried in the bundle**, checked before
    /// any data moves. Never a silent fall-back expression: a number that looks
    /// like a key and is not razeri's writes keys no other deployment can
    /// reproduce.
    Refused,

    /// **Nothing to install here, because razeri's own key function travels in
    /// `schema.sql` instead** — one definition in every namespace whose
    /// objects call it, created beside those objects, so a namespace deploys
    /// on its own.
    ///
    /// `BigQuery` is the case and the reason is name resolution, not taste: a
    /// bare `generate_sid('test')` is *"Function not found: `generate_sid`; Did
    /// you mean `generate_uuid`?"* there — no search path, no session schema, no
    /// default dataset inside a procedure body — so every call site must name
    /// the dataset the function was created in, and a prerequisites script has
    /// no dataset to create it in. The definition therefore rides `schema.sql`,
    /// where the domain's namespace is known.
    ///
    /// It is **not** [`None`](Self::None). That variant is the positive claim
    /// *"the engine's own hash IS the canonical number"*, which is false here:
    /// `BigQuery` has no XXH64 in any spelling and the key is a razeri-owned
    /// JavaScript UDF. What the two share is only the deploy behaviour —
    /// nothing to run before `schema.sql` — and sharing a behaviour is not
    /// sharing a claim.
    EmittedInSchema,

    /// **Razeri has no measured answer for this platform** — the honest default,
    /// and the one a dialect gets by saying nothing.
    ///
    /// # Why this variant exists
    ///
    /// A derivation that answered [`None`](Self::None) here would turn *"this
    /// dialect declared nothing"* into the positive claim *"its own hash is
    /// razeri's number"* — a claim that has been false on real engines. Silence
    /// and safety must never read the same, so silence gets its own variant.
    ///
    /// # What it means to a deploy path
    ///
    /// The same as [`OperatorArtifact`](Self::OperatorArtifact): **verify, then
    /// refuse.** Safe-by-default is the only defensible reading of "razeri does
    /// not know" — the contract allows an emitted UDF or a named refusal and
    /// explicitly not a local approximation, and proceeding on an unmeasured
    /// platform is neither.
    ///
    /// A dialect that genuinely needs no prerequisite says so by overriding
    /// [`ReplicationRenderer::sid_prerequisite_kind`] to
    /// [`None`](Self::None), with its measurement cited.
    Undeclared,
}

/// **The executable statements inside a prerequisite script**, split on
/// [`SID_PREREQUISITE_STATEMENT_DELIMITER`].
///
/// The body of [`ReplicationRenderer::sid_prerequisite_statements`], lifted out
/// so a caller holding a script that is not `self.sid_prerequisites()` — the
/// bundle, when it swaps in
/// [`ReplicationRenderer::sid_pure_sql_fallback`] — splits it the same way
/// rather than growing a second reading of the boundary.
///
/// Leading comment and blank lines are dropped from each segment and one
/// trailing terminator is stripped; see the trait method for why both.
#[must_use]
pub fn statements_in_prerequisite_script(script: &str) -> Vec<String> {
    if !script.contains(SID_PREREQUISITE_STATEMENT_DELIMITER) {
        return Vec::new();
    }
    script
        .split(SID_PREREQUISITE_STATEMENT_DELIMITER)
        .filter_map(|segment| {
            // The comment lines in front of the statement, block or line, and
            // the blank lines among them, are the operator's, not the engine's.
            let lines: Vec<&str> = segment.lines().collect();
            let leading = lines
                .iter()
                .zip(crate::comment::comment_line_flags(segment))
                .take_while(|(line, is_note)| *is_note || line.trim().is_empty())
                .count();
            let statement = lines[leading..].join("\n");
            // One trailing terminator, not a run of them: `$$;` ends a body
            // whose own last character may be a `;` the statement needs.
            let statement = statement.trim_end().strip_suffix(';').map_or_else(
                || statement.trim_end().to_owned(),
                |trimmed| trimmed.trim_end().to_owned(),
            );
            (!statement.is_empty()).then_some(statement)
        })
        .collect()
}

/// **What shape a prerequisite script is**, read off the script itself.
///
/// The body of [`ReplicationRenderer::sid_prerequisite_kind`]'s default, lifted
/// out for the same reason as [`statements_in_prerequisite_script`]: a caller
/// classifying a script the dialect did not return from `sid_prerequisites()`
/// must reach the same three answers by the same rule. A second reading of the
/// table in the trait method's own doc comment is how a script comes to be
/// classified two ways.
///
/// The two answers this cannot reach — [`SidPrerequisiteKind::None`] and
/// [`SidPrerequisiteKind::Refused`] — are positive claims about an engine, and
/// a dialect makes them by overriding, never by writing a script this reads.
#[must_use]
pub fn kind_of_prerequisite_script(script: &str) -> SidPrerequisiteKind {
    if script.trim().is_empty() {
        SidPrerequisiteKind::Undeclared
    } else if statements_in_prerequisite_script(script).is_empty() {
        SidPrerequisiteKind::OperatorArtifact
    } else {
        SidPrerequisiteKind::SqlInstallable
    }
}

// ════════════════════════════════════════════════════════════════════════
// The external artifact that IS `generate_sid`
// ════════════════════════════════════════════════════════════════════════

/// **The identity of the artifact the bundle's prerequisites step installs as
/// `generate_sid`** — where one exists.
///
/// Returned by [`ReplicationRenderer::sid_external_artifact`], and the only
/// thing a dialect still has to say about the key function's *body*: on every
/// platform where razeri owns the name the body is
/// Razeri's one surrogate-key derivation and reads out of one table, so
/// there is nothing per-dialect left to declare.
///
/// This is an **identity, never a body**: a dialect names the artifact, and the
/// derivation lives inside the artifact. A dialect carries no SQL spelling of
/// the key function's body — a second spelling of the derivation is one that
/// drifts. `generate_sid`'s parameter has **one** name, `input_value`.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct SidExternalArtifact {
    /// Language name, e.g. `"tsql"`, `"C"`, `"javascript"`, `"python"`.
    pub language: String,
    /// Path or name of the external library, assembly, or emitted definition.
    pub object_file: String,
    /// Fully-qualified symbol within the library.
    pub link_symbol: String,
    /// Descriptive comment for the function — the **dialect's**, because *why
    /// the platform's built-in was refused* is a platform fact.
    pub comment: String,
}

/// **What loads a compiled key-function library**, which decides what the
/// operator does with it once it is built.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SidLibraryLoader {
    /// The client loads it into each connection it opens, as `SQLite`'s
    /// `.load` does. Nothing is installed on a server.
    Client,
    /// The server loads it from its own plugin directory, and one statement
    /// registers it for every session, as `CREATE FUNCTION … SONAME` does on
    /// `MySQL` and `MariaDB`.
    Server,
}

/// **A library's source, shipped in the deploy bundle for the operator to
/// compile** — returned by [`ReplicationRenderer::sid_library_source`].
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct SidLibrarySource {
    /// The name the bundle writes the source under, e.g. `razeri_sid.c`.
    pub file_name: String,
    /// The source itself.
    pub source: String,
    /// The compiled library's name without its suffix, e.g. `razeri_sid`. The
    /// build writes it with the suffix the operating system gives a shared
    /// library (`.so`, `.dylib`, `.dll`).
    pub library: String,
    /// The function called when the library is loaded.
    pub entry_point: String,
    /// The one line that builds it, for the platform most operators have.
    pub build: String,
    /// What loads it: the client, into each connection, or the server, once.
    pub loaded_by: SidLibraryLoader,
}

// ════════════════════════════════════════════════════════════════════════
// ReplicationRenderer Trait
// ════════════════════════════════════════════════════════════════════════

/// Rendering methods for replication operations: SID generation, data
/// extraction, data loading, and checksum validation.
///
/// Defaults produce ANSI-safe SQL so extension dialects (e.g., `SQLite`) work
/// without implementing every method.
pub trait ReplicationRenderer: SqlRenderer {
    // -- The session, before anything runs in it ----------------------

    /// **Statements every generated script runs before its first real
    /// statement**, in this order, in the same session.
    ///
    /// Some engines decide how a literal is read from *session settings* rather
    /// than from the statement. Where that is true, a cast that is ordinary
    /// everywhere else fails on the first row of the first load until the
    /// session has been told how to read it — a date literal parsed by a
    /// session-wide format is the measured case. There is nowhere else to put
    /// such a statement: it belongs to the connection, not to any object.
    ///
    /// # What belongs here
    ///
    /// Settings that change **how the engine reads what follows** and are lost
    /// when the session ends: how literals parse, how identifiers fold, which
    /// time zone a timestamp is stamped in, how strings compare.
    ///
    /// # What does not
    ///
    /// Anything that **creates something**. A function, an extension, an
    /// assembly, a table — none of those is a session setting, and putting one
    /// here means running it again on every connection. Those are
    /// [`Self::sid_prerequisites`] where they install the key function, and
    /// ordinary DDL otherwise.
    ///
    /// Each string is one statement without its terminator; the script that
    /// emits them adds whatever this engine ends a statement with. Every
    /// statement must be safe to run again — a script that runs twice runs
    /// these twice.
    ///
    /// The default is empty, and an engine that reads nothing off the session
    /// leaves it that way: nothing is emitted, and its scripts are byte for
    /// byte what they were.
    fn session_prerequisites(&self) -> Vec<String> {
        Vec::new()
    }

    /// **The session's settings for one design**, which may need fewer than
    /// [`Self::session_prerequisites`] names.
    ///
    /// A setting that only a key needs — loading the extension that computes
    /// it — is wasted on a design that computes no key, and on a machine that
    /// never installed the extension it is a stop. `computes_key` says whether
    /// the design's schema calls the key function; the prerequisites step
    /// installs the key's function under the same condition, so the two agree.
    ///
    /// The default hands back [`Self::session_prerequisites`] whole, which is
    /// right for an engine whose settings never depend on the key.
    fn session_prerequisites_for(&self, computes_key: bool) -> Vec<String> {
        let _ = computes_key;
        self.session_prerequisites()
    }

    // -- SID (Surrogate ID) Generation --------------------------------

    /// **Does an external artifact already carry the name `generate_sid` on
    /// this platform, and what is it?**
    ///
    /// `None` — the default, and the honest answer for most engines — means
    /// razeri itself defines the key function here: the body is Razeri's one
    /// surrogate-key derivation with the platform's own hash inlined, composed
    /// from the platform name alone, so there is nothing for a dialect to
    /// spell.
    ///
    /// `Some(…)` names what the bundle's prerequisites step installs: SQL
    /// Server's CLR assembly, MySQL/MariaDB's C UDF, Snowflake's emitted
    /// JavaScript UDF, Databricks' emitted Python UDF. The derivation is inside
    /// the artifact rather than in any SQL razeri emits, so what a dialect
    /// states is the artifact's *identity*, matched against the very
    /// prerequisites text that installs it. **This is an identity, not a
    /// body** — see [`SidExternalArtifact`].
    ///
    /// A platform that can be given no function at all answers `None` here too
    /// and says so through [`Self::sid_prerequisite_kind`]
    /// ([`SidPrerequisiteKind::Refused`]). Razeri's conformance checks require
    /// that refusal to be said out loud: where no artifact is declared and
    /// razeri emits no helper either, a dialect that has simply declared
    /// nothing is reported, which is the distinction
    /// [`SidPrerequisiteKind::Undeclared`] exists to keep.
    fn sid_external_artifact(&self) -> Option<SidExternalArtifact> {
        None
    }

    /// **The source of a library the operator compiles**, where the key
    /// function arrives that way — shipped in the deploy bundle as a file of
    /// its own, so the build is one line.
    ///
    /// `SQLite` answers it: its SQL has no `CREATE FUNCTION`, and its client
    /// loads a compiled extension into each connection it opens. `MySQL` and
    /// `MariaDB` answer it too: their server loads a compiled function from
    /// its plugin directory. The default is `None`: nothing ships as a
    /// separate source file.
    fn sid_library_source(&self) -> Option<SidLibrarySource> {
        None
    }

    /// Returns prerequisite SQL/instructions for SID deployment.
    ///
    /// For platforms that require extensions or external libraries (`PostgreSQL`
    /// `pg_xxhash`, SQL Server CLR, `MariaDB` C UDF), this returns the deployment
    /// script. Default: empty (no prerequisites needed).
    fn sid_prerequisites(&self) -> String {
        String::new()
    }

    /// **The executable statements inside [`Self::sid_prerequisites`], in
    /// order** — razeri's own text, split on razeri's own declared boundary.
    ///
    /// This exists so a deploy path or a test harness can *install what razeri
    /// emits* rather than a transcription of it — a hand-written copy is a
    /// second implementation that drifts, and a second splitter would be a
    /// second definition of the boundary.
    ///
    /// **A dialect that does not write
    /// [`SID_PREREQUISITE_STATEMENT_DELIMITER`] declares no executable
    /// statements**, and gets an empty vector. That is the honest answer for
    /// every arm whose prerequisite is an operator's compile step (`MariaDB`'s
    /// and `MySQL`'s `.so`) and for the platforms with
    /// no prerequisite at all — it is *not* a promise that a caller can run the
    /// prose.
    ///
    /// Leading comment and blank lines are dropped from each segment, so the
    /// prose that explains a statement travels with it in the script and does
    /// not travel to the engine. Comments *inside* a statement survive: they are
    /// part of the statement the operator would paste.
    ///
    /// **The trailing terminator is dropped here, once.** A `;` is how the
    /// script reads to a human pasting the whole file, and is not part of a
    /// single statement over a REST statement API — Snowflake's and Databricks'
    /// both take exactly one. Stripping it in each consumer instead would put
    /// the same rule in the cloud arms *and* in the bundle executor, which
    /// is the shape this method exists to prevent. `sid_prerequisites()`' text
    /// is unchanged and keeps its semicolons.
    fn sid_prerequisite_statements(&self) -> Vec<String> {
        statements_in_prerequisite_script(&self.sid_prerequisites())
    }

    /// **A prerequisite script that installs the same key function without
    /// compiling anything** — where the platform has one, and `None` where it
    /// does not.
    ///
    /// The default is `None`: most platforms have exactly one prerequisite and
    /// [`Self::sid_prerequisites`] is it. A dialect answers `Some` only where
    /// **two honest routes to the same number** exist and the choice belongs to
    /// whoever deploys rather than to whoever designed — the case being a
    /// managed `PostgreSQL` service (`RDS`, Aurora, Cloud SQL), where nobody has
    /// a shell on the server host and so the compiled extension the ordinary
    /// step names can never be built.
    ///
    /// # What answering `Some` promises
    ///
    /// **The same number, byte for byte.** Not a near-enough hash and not a
    /// different algorithm: the surrogate-key rule is one hash and one
    /// derivation everywhere, so a key minted through this route and a key
    /// minted through the compiled one are the same `BIGINT` and a warehouse can
    /// be moved between them without re-keying a row. A dialect that cannot
    /// promise that answers `None` and lets the ordinary step refuse.
    ///
    /// It is a **deploy-time** choice and nothing about it reaches a design: the
    /// text swaps into `prerequisites.sql` alone, `schema.sql` is unchanged, and
    /// every call site still calls the one public name.
    fn sid_pure_sql_fallback(&self) -> Option<String> {
        None
    }

    /// **[`Self::sid_pure_sql_fallback`], installed in every schema a
    /// design writes into.**
    ///
    /// `schemas` names them, in the order the design first writes to each,
    /// spelled as the rendered SQL spells them. A deploy bundle asks this
    /// rather than the plain method, because it knows the schemas and the
    /// plain method does not.
    ///
    /// The default ignores the list and answers the plain method. That is
    /// right wherever an installed function is visible from every schema: a
    /// function on the session's search path, or one with a schema of its own
    /// that every call names.
    ///
    /// Override it where a routine resolves a bare function name in the
    /// schema that owns the routine, and a key function created by plain SQL
    /// has to live in that same schema. There one definition is not enough:
    /// the answer installs the function once in each schema on the list, and
    /// makes each schema first if it is not there yet, because this step runs
    /// before `schema.sql` does. An empty list means the session's current
    /// schema, which is what the plain method answers.
    fn sid_pure_sql_fallback_in(&self, schemas: &[&str]) -> Option<String> {
        let _ = schemas;
        self.sid_pure_sql_fallback()
    }

    /// **Which of [`SidPrerequisiteKind`]'s shapes this platform's prerequisite
    /// is.**
    ///
    /// Derived from what the dialect already says, so the classification cannot
    /// drift from the text it classifies:
    ///
    /// | [`sid_prerequisites`](Self::sid_prerequisites) | [`sid_prerequisite_statements`](Self::sid_prerequisite_statements) | kind |
    /// |---|---|---|
    /// | empty | — | [`Undeclared`](SidPrerequisiteKind::Undeclared) |
    /// | prose **and** statements | non-empty | [`SqlInstallable`](SidPrerequisiteKind::SqlInstallable) |
    /// | prose only | empty | [`OperatorArtifact`](SidPrerequisiteKind::OperatorArtifact) |
    ///
    /// # Silence is `Undeclared`, never `None`
    ///
    /// The two answers the derivation **cannot** reach are the two that are
    /// *positive claims about an engine*:
    ///
    /// - [`None`](SidPrerequisiteKind::None) says the platform's own builtin
    ///   returns the canonical number. That is a measurement, not an absence, and
    ///   a dialect claiming it overrides this method and cites the arm that
    ///   measured it;
    /// - [`Refused`](SidPrerequisiteKind::Refused) says no SQL can ever install
    ///   the function here, which a platform with a native hash and a lazy
    ///   dialect author would also spell as silence.
    ///
    /// It is the same asymmetry `DdlRenderer::representable_timestamp_ceiling`
    /// documents on its own `None`: *not measured* and *nothing to worry about*
    /// are opposite facts that read identically when a default supplies them.
    fn sid_prerequisite_kind(&self) -> SidPrerequisiteKind {
        kind_of_prerequisite_script(&self.sid_prerequisites())
    }

    /// **Where [`Self::sid_prerequisites`] puts what it installs**, so that a
    /// key function created in some other schema can call it by name.
    ///
    /// The default is [`SidPrerequisiteHome::Ambient`]: whatever the
    /// prerequisite creates, it creates wherever the deploy runs, and every
    /// call site names it bare. That is the right answer on every engine whose
    /// prerequisite installs nothing with a schema — a session load, a
    /// server-wide library, a function whose name has no qualifier — and it
    /// leaves those dialects' scripts exactly as they were.
    ///
    /// Answer [`SidPrerequisiteHome::Schema`] where a routine on this engine
    /// resolves an unqualified name in the schema that OWNS it. There, the one
    /// hash a whole warehouse shares needs a fixed address, because every layer
    /// schema holds its own copy of the key function, each calls the one hash,
    /// and at most one of those schemas is the one the deploy signed in as.
    ///
    /// See [`SidPrerequisiteHome::Schema`] for what answering it obliges the
    /// dialect's own prerequisite text to do.
    fn sid_prerequisite_home(&self) -> SidPrerequisiteHome {
        SidPrerequisiteHome::Ambient
    }

    /// Returns a validation query to verify the SID function is operational.
    ///
    /// Default: schema-qualified `SELECT generate_sid('test')`.
    fn sid_validation_query(&self, schema: Option<&str>, null_sid: i64) -> String {
        let qualified = match schema {
            Some(s) => format!("{s}.generate_sid"),
            None => "generate_sid".to_string(),
        };
        format!("SELECT {qualified}('test') AS test_sid, {null_sid} AS null_sid;\n")
    }

    // -- Data Extraction ----------------------------------------------

    /// Renders an extract/export SQL statement for the given table.
    ///
    /// The `format` parameter is one of: `"csv"`, `"parquet"`, `"json_lines"`, `"native"`.
    ///
    /// Default: ANSI `SELECT ... FROM ... WHERE ...` (no COPY).
    fn render_data_extract(
        &self,
        qualified: &str,
        columns: &str,
        file_path: &str,
        format: &str,
        where_clause: &str,
        _batch_size: Option<usize>,
    ) -> String {
        let _ = file_path;
        let _ = format;
        if where_clause.is_empty() {
            format!("SELECT {columns} FROM {qualified};\n")
        } else {
            format!("SELECT {columns} FROM {qualified} {where_clause};\n")
        }
    }

    // -- Data Loading -------------------------------------------------

    /// Renders a load/import SQL statement for the given table.
    ///
    /// The `format` parameter is one of: `"csv"`, `"parquet"`, `"json_lines"`, `"native"`.
    ///
    /// Default: ANSI comment placeholder.
    fn render_data_load(&self, qualified: &str, file_path: &str, _format: &str) -> String {
        format!(
            "{said_0}\n",
            said_0 = crate::comment::block_comment(&format!(
                "COPY INTO {qualified} FROM '{file_path}';"
            ))
        )
    }

    // -- Checksum / Validation ----------------------------------------

    /// Renders a checksum validation query for data comparison.
    ///
    /// Default: `SELECT COUNT(*) AS cnt FROM <table>` (no checksum).
    fn render_checksum_query(&self, qualified: &str) -> String {
        format!("SELECT COUNT(*) AS cnt FROM {qualified};\n")
    }

    // -- Platform Warnings --------------------------------------------

    /// Warning about Parquet export limitations on this platform.
    ///
    /// Default: `None` (no warning needed).
    fn parquet_export_warning(&self) -> Option<&str> {
        None
    }

    /// Warning about SID function deployment requirements on this platform.
    ///
    /// Default: `None` (no special deployment needed).
    fn sid_deployment_warning(&self) -> Option<&str> {
        None
    }

    /// General replication caveats for this platform as a target.
    ///
    /// Default: empty (no platform-specific caveats).
    fn replication_target_warnings(&self) -> Vec<String> {
        Vec::new()
    }

    /// Warning about TRUNCATE behavior on this platform.
    ///
    /// Default: `None` (TRUNCATE works normally).
    fn truncate_caveat(&self) -> Option<&str> {
        None
    }
}

#[cfg(test)]
mod tests {
    use super::{ReplicationRenderer, SID_PREREQUISITE_STATEMENT_DELIMITER};
    use crate::agnostic::AnsiDialect;

    /// A dialect whose prerequisites are whatever a test hands it.
    struct Emitting(String);

    impl crate::SqlRenderer for Emitting {
        fn dialect_name(&self) -> &'static str {
            "emitting"
        }
        fn render_data_type(&self, dt: &crate::DataType) -> String {
            AnsiDialect.render_data_type(dt)
        }
        fn quote_identifier(&self, name: &str) -> String {
            AnsiDialect.quote_identifier(name)
        }
        fn qualify_name(&self, schema: Option<&str>, name: &str) -> String {
            AnsiDialect.qualify_name(schema, name)
        }
        fn render_current_timestamp(&self) -> String {
            AnsiDialect.render_current_timestamp()
        }
    }
    impl crate::DdlRenderer for Emitting {}
    impl crate::FormulaRenderer for Emitting {}
    impl ReplicationRenderer for Emitting {
        fn sid_prerequisites(&self) -> String {
            self.0.clone()
        }
    }

    /// **A JavaScript body is nothing but semicolons**, which is exactly why the
    /// boundary is declared instead of inferred. Splitting the text below on
    /// `;` yields five fragments, none of them a statement.
    #[test]
    fn the_delimiter_recovers_statements_a_semicolon_split_would_fracture() {
        let script = format!(
            "-- prose the operator reads\n\
             {SID_PREREQUISITE_STATEMENT_DELIMITER}\n\
             -- why this one exists\n\
             CREATE FUNCTION f() AS $$\n  var a = 1; var b = 2; return a + b;\n$$;\n\
             {SID_PREREQUISITE_STATEMENT_DELIMITER}\n\
             CREATE FUNCTION g() AS $$ 1 $$;\n"
        );
        let statements = Emitting(script).sid_prerequisite_statements();
        assert_eq!(statements.len(), 2, "{statements:#?}");
        assert!(statements[0].starts_with("CREATE FUNCTION f()"), "{:?}", statements[0]);
        assert!(statements[0].contains("var a = 1; var b = 2; return a + b;"));
        assert_eq!(statements[1], "CREATE FUNCTION g() AS $$ 1 $$");
    }

    /// **The trailing terminator is dropped exactly once, and only at the end.**
    ///
    /// A REST statement API takes one statement and no terminator; a human
    /// pasting the script wants the `;`. Both are served by leaving it in
    /// `sid_prerequisites()`' text and removing it here — once, so a body whose
    /// own last character is a `;` (every JavaScript function) keeps it.
    #[test]
    fn one_trailing_terminator_is_dropped_and_the_bodys_own_semicolons_survive() {
        let script = format!(
            "{SID_PREREQUISITE_STATEMENT_DELIMITER}\n\
             CREATE FUNCTION f() AS $$\n  return 1;\n$$;\n\
             {SID_PREREQUISITE_STATEMENT_DELIMITER}\n\
             CREATE FUNCTION g() AS $$ 1 $$\n"
        );
        let statements = Emitting(script).sid_prerequisite_statements();
        assert_eq!(statements[0], "CREATE FUNCTION f() AS $$\n  return 1;\n$$");
        assert!(statements[0].contains("return 1;"), "the body's own `;` is not a terminator");
        assert_eq!(
            statements[1], "CREATE FUNCTION g() AS $$ 1 $$",
            "a statement with no terminator is unchanged"
        );
    }

    /// **No delimiter means no executable statements, and that is an answer.**
    /// The prerequisite scripts for `MySQL` and `MariaDB` are an operator's
    /// compile recipe wrapped in comments; handing a caller the prose
    /// as if it were DDL is how a deferral turns into a failure nobody can read.
    #[test]
    fn an_undelimited_script_declares_no_executable_statements() {
        let operator_recipe = "-- compile this yourself:\n/* int main() { } */\n".to_owned();
        assert!(Emitting(operator_recipe).sid_prerequisite_statements().is_empty());
        assert!(AnsiDialect.sid_prerequisite_statements().is_empty());
    }
}