inillucent_sql/bind/

cte.rs

//! Common table expressions: what a `WITH` binds, and how a recursive one is
//! filled.
//!
//! Invariant: **a CTE is bound once per reference and never bound inside
//! itself.** Two references to one CTE are two independent scans with their
//! own FROM-term numbers, which is why a binding holds an AST id rather than a
//! bound block; and a definition already being bound is a cycle, which is
//! answered rather than followed.
//!
//! ## Why this is its own module
//!
//! `bind.rs` was at its recorded ceiling and task-1913 added ninety-nine lines
//! to it, so the ratchet in `policy.rs` asked for an extraction rather than a
//! raised number. This is one question - what a name in a `WITH` stands for -
//! and the ten items here were the only ones asking it. Nothing moved changed
//! in the move.

use super::{subquery_table, unsupported, Binder, BoundSource, RecursiveBody, SourceRows};
use crate::ast::{self, CompoundOp, JoinKind, SelectId};
use crate::catalog_view::TableInfo;
use crate::diagnostic::{ParseError, ParseErrorKind};
use crate::lexer::Span;

/// The first number a derived table inside a correlated subquery keeps its rows
/// under, which is past any number a common table expression can have.
pub const FIRST_ANONYMOUS_SHARED: usize = 1 << 20;

/// One common table expression visible to a block.
///
/// The definition is kept as an AST id rather than a bound block because two
/// references to the same CTE are two independent scans: each gets its own
/// FROM-term numbers and its own materialisation. Binding once and cloning
/// would give both references the same source ids, and the second scan would
/// then read the first one's cursors.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct CteBinding {
    /// The folded name a FROM term matches against.
    pub folded: Vec<u8>,
    /// The name as written, which the expansion is aliased to.
    pub name: Vec<u8>,
    /// The explicit column list, when the `WITH` wrote one.
    pub columns: Vec<Vec<u8>>,
    /// The query the name stands for.
    pub select: SelectId,
    /// Whether the `WITH` said `RECURSIVE`.
    pub recursive: bool,
    /// `Some(true)` for `MATERIALIZED`, `Some(false)` for `NOT MATERIALIZED`.
    pub materialized: Option<bool>,
}

/// One recursive CTE whose definition is being bound.
#[derive(Clone, Debug)]
pub(super) struct RecursiveTarget {
    /// The CTE's folded name.
    pub(super) folded: Vec<u8>,
    /// The statement-wide number of the FROM term that will hold its store.
    id: usize,
    /// The columns a reference to it exposes, taken from the seed arm.
    table: TableInfo,
    /// Whether any arm bound so far referred to it.
    referenced: bool,
}

impl Binder<'_> {
    /// Pushes the CTEs of a `WITH` prefix, returning whether it pushed any.
    pub(crate) fn push_ctes(&mut self, with: &ast::With) -> Result<bool, ParseError> {
        if with.ctes.is_empty() {
            return Ok(false);
        }
        let mut bindings = Vec::with_capacity(with.ctes.len());
        for cte in &with.ctes {
            // **A name may be defined once in one `WITH`.** SQLite refuses the
            // second definition while it parses, in these words; an inner
            // `WITH` is a different clause and may reuse the name.
            let folded = self.ast.folded(cte.name);
            if bindings
                .iter()
                .any(|held: &CteBinding| held.folded.as_slice() == folded)
            {
                return Err(super::refused(
                    format!(
                        "duplicate WITH table name: {}",
                        String::from_utf8_lossy(self.ast.text(cte.name))
                    ),
                    crate::lexer::Span::default(),
                ));
            }
            bindings.push(CteBinding {
                folded: self.ast.folded(cte.name).to_vec(),
                name: self.ast.text(cte.name).to_vec(),
                columns: cte
                    .columns
                    .iter()
                    .map(|name| self.ast.text(*name).to_vec())
                    .collect(),
                select: cte.select,
                recursive: with.recursive,
                materialized: cte.materialized,
            });
        }
        self.ctes.push(bindings);
        Ok(true)
    }

    /// Drops the innermost level of CTE bindings.
    pub(crate) fn pop_ctes(&mut self) {
        self.ctes.pop();
    }

    /// Returns the innermost CTE a folded name matches.
    pub(super) fn find_cte(&self, folded: &[u8]) -> Option<CteBinding> {
        for level in self.ctes.iter().rev() {
            if let Some(found) = level.iter().find(|cte| cte.folded == folded) {
                return Some(found.clone());
            }
        }
        None
    }

    /// Reports whether the statement refers to a name in more than one FROM term.
    ///
    /// Counted over every FROM term the statement was parsed into, so an inner
    /// `WITH` that reuses the name is counted too. That only ever shares a CTE
    /// that did not need to be shared.
    ///
    /// @param folded - the folded name
    pub(super) fn name_is_used_twice(&self, folded: &[u8]) -> bool {
        let mut uses = 0usize;
        for at in 0..self.ast.from_term_count() {
            let Some(term) = self.ast.from_term(ast::FromTermId(at as u32)) else {
                continue;
            };
            if let ast::FromSource::Table {
                database: None,
                name,
                ..
            } = &term.source
            {
                if self.ast.folded(*name) == folded {
                    uses = uses.saturating_add(1);
                }
            }
        }
        uses > 1
    }

    /// Marks the block a CTE reference was just bound to as one that shares its
    /// evaluation with the other references, when that can be seen.
    ///
    /// SQLite evaluates a CTE used more than once a single time unless it is
    /// `NOT MATERIALIZED`. The difference can only be seen when the body is not
    /// a function of its tables, so only a body that calls `random()` or a
    /// function like it is marked, and one that reads a column of an enclosing
    /// query is left alone because it has a different answer for every row.
    ///
    /// @param cte - the definition
    pub(super) fn share_last_source(&mut self, cte: &CteBinding) {
        if cte.materialized == Some(false) || !self.name_is_used_twice(&cte.folded) {
            return;
        }
        let key = match self.shared_ctes.iter().position(|(arena, select)| {
            *arena == self.ast as *const _ as usize && *select == cte.select
        }) {
            Some(key) => key,
            None => {
                self.shared_ctes
                    .push((self.ast as *const _ as usize, cte.select));
                self.shared_ctes.len().saturating_sub(1)
            }
        };
        let Some(source) = self.sources.last_mut() else {
            return;
        };
        let SourceRows::Subquery(block) = &mut source.rows else {
            return;
        };
        if !block.correlations.is_empty() {
            return;
        }
        let mut volatile = false;
        let mut probe = (**block).clone();
        crate::rewrite::rewrite_select(&mut probe, &mut |expr: &mut super::BoundExpr| {
            if crate::plan::calls_a_volatile_function(expr) {
                volatile = true;
            }
        });
        if volatile {
            block.shared = Some(key);
        }
    }

    /// Makes the derived tables of a correlated subquery that read nothing of
    /// the enclosing query keep their rows for the whole statement.
    ///
    /// **SQLite materialises such a derived table once** (`OP_Once`), however
    /// many outer rows the subquery runs for. Read again for every row, a
    /// `SELECT ... FROM (SELECT sum(v) OVER () ...)` inside an `UPDATE`'s `SET`
    /// saw the rows the statement had already changed.
    ///
    /// @param block - a subquery that reads a column of an enclosing query
    pub(super) fn share_uncorrelated_sources(&mut self, block: &mut super::BoundSelect) {
        for source in &mut block.sources {
            let SourceRows::Subquery(inner) = &mut source.rows else {
                continue;
            };
            if inner.correlations.is_empty() {
                if inner.shared.is_none() {
                    inner.shared = Some(FIRST_ANONYMOUS_SHARED + self.shared_anonymous);
                    self.shared_anonymous = self.shared_anonymous.saturating_add(1);
                }
            } else {
                self.share_uncorrelated_sources(inner);
            }
        }
        for (_, arm) in &mut block.compounds {
            self.share_uncorrelated_sources(arm);
        }
    }

    /// Reports whether a CTE's own query names it in a FROM clause.
    ///
    /// **What makes a CTE recursive is the self-reference, not the keyword.**
    /// SQLite accepts `WITH c AS (SELECT 1 UNION ALL SELECT ... FROM c)` with
    /// no `RECURSIVE` written and answers it; this binder read only the
    /// keyword, so the same query bound `c`'s definition inside `c`'s
    /// definition until the process ran out of stack (task-1913).
    ///
    /// An inner `WITH` that binds the same name shadows the outer one, so
    /// nothing under it can be the recursion - which is why this stops there
    /// rather than reporting every mention of the name.
    ///
    /// @param select - the CTE's query
    /// @param folded - the CTE's folded name
    pub(super) fn select_names_itself(&self, select: ast::SelectId, folded: &[u8]) -> bool {
        let Some(query) = self.ast.select(select) else {
            return false;
        };
        if query
            .with
            .ctes
            .iter()
            .any(|inner| self.ast.folded(inner.name) == folded)
        {
            return false;
        }
        if self.core_names_cte(query.first, folded) {
            return true;
        }
        query
            .compounds
            .iter()
            .any(|(_, arm)| self.core_names_cte(*arm, folded))
    }

    /// Reports whether one arm of a compound names a CTE in its FROM clause.
    ///
    /// @param core - the arm
    /// @param folded - the CTE's folded name
    pub(super) fn core_names_cte(&self, core: ast::SelectCoreId, folded: &[u8]) -> bool {
        let Some(arm) = self.ast.core(core) else {
            return false;
        };
        let ast::SelectBody::Select { from, .. } = &arm.body else {
            return false;
        };
        self.terms_name_cte(from, folded)
    }

    /// Reports whether any FROM term names a CTE.
    ///
    /// @param terms - the FROM terms
    /// @param folded - the CTE's folded name
    pub(super) fn terms_name_cte(&self, terms: &[ast::FromTermId], folded: &[u8]) -> bool {
        terms.iter().any(|id| match self.ast.from_term(*id) {
            Some(term) => match &term.source {
                ast::FromSource::Table { database, name, .. } => {
                    database.is_none() && self.ast.folded(*name) == folded
                }
                ast::FromSource::Subquery(select) => self.select_names_itself(*select, folded),
                ast::FromSource::Join(inner) => self.terms_name_cte(inner, folded),
            },
            None => false,
        })
    }

    /// Registers a reference to the recursive CTE currently being bound.
    pub(super) fn push_recursive_self(
        &mut self,
        position: usize,
        alias: Option<ast::NameId>,
        join: JoinKind,
    ) -> Result<(), ParseError> {
        let Some(target) = self.recursing.get_mut(position) else {
            return Err(unsupported("unknown recursive reference", Span::default()));
        };
        target.referenced = true;
        let cte = target.id;
        let table = target.table.clone();
        let alias = match alias {
            Some(alias) => self.ast.text(alias).to_vec(),
            None => table.name.clone(),
        };
        let id = self.sources.len();
        self.sources.push(BoundSource {
            index_hint: crate::bind::IndexChoice::Any,
            id,
            rows: SourceRows::RecursiveSelf { cte },
            table: std::rc::Rc::new(table),
            alias,
            join,
            constraint: None,
            suppressed: Vec::new(),
            index_exprs: Vec::new(),
            written_schema: None,
        });
        if let Some(scope) = self.scopes.last_mut() {
            scope.push(id);
        }
        Ok(())
    }

    /// Binds a `WITH RECURSIVE` CTE reference.
    ///
    /// The seed arm is bound first, alone, because until it is bound nothing
    /// knows what columns the CTE has - and the step arm cannot be bound until
    /// a reference to the CTE has columns to resolve against. A CTE declared
    /// `RECURSIVE` that turns out not to reference itself is an ordinary
    /// compound, and is rebuilt as one rather than run through a queue that
    /// would never be fed.
    pub(super) fn bind_recursive_cte(
        &mut self,
        cte: &CteBinding,
        alias: Vec<u8>,
        join: JoinKind,
        span: Span,
    ) -> Result<(), ParseError> {
        let Some(select) = self.ast.select(cte.select) else {
            return Err(unsupported("missing select", span));
        };
        if select.compounds.is_empty() {
            return self.bind_subquery_term(
                cte.select,
                Some(alias),
                cte.columns.clone(),
                join,
                span,
            );
        }
        let arms: Vec<(CompoundOp, ast::SelectCoreId)> = select.compounds.clone();
        let order_by = select.order_by.clone();
        let limit = select.limit;
        let offset = select.offset;
        let first = select.first;

        let id = self.sources.len();
        // The store's FROM-term number is reserved before anything is bound, so
        // that a self-reference inside the step arm can name the store it will
        // read without the two being bound in an impossible order.
        self.sources.push(BoundSource {
            index_hint: crate::bind::IndexChoice::Any,
            id,
            rows: SourceRows::Table,
            table: std::rc::Rc::new(TableInfo::subquery(alias.clone(), 0, Vec::new())),
            alias: alias.clone(),
            join,
            constraint: None,
            suppressed: Vec::new(),
            index_exprs: Vec::new(),
            written_schema: None,
        });

        let seed = self.bind_isolated_arm(first)?;
        let table = subquery_table(&alias, &cte.columns, &seed);
        if !cte.columns.is_empty() && cte.columns.len() != seed.columns.len() {
            return Err(super::refusal::named_column_count(
                &alias,
                seed.columns.len(),
                cte.columns.len(),
                span,
            ));
        }
        self.recursing.push(RecursiveTarget {
            folded: cte.folded.clone(),
            id,
            table: table.clone(),
            referenced: false,
        });
        let mut seeds = vec![(CompoundOp::UnionAll, seed)];
        let mut steps = Vec::new();
        let mut outcome = Ok(());
        for (op, arm) in &arms {
            if !matches!(op, CompoundOp::Union | CompoundOp::UnionAll) {
                outcome = Err(ParseError::new(
                    ParseErrorKind::Unsupported("recursive query does not use UNION or UNION ALL"),
                    span,
                ));
                break;
            }
            if let Some(target) = self.recursing.last_mut() {
                target.referenced = false;
            }
            let bound = match self.bind_isolated_arm(*arm) {
                Ok(bound) => bound,
                Err(reason) => {
                    outcome = Err(reason);
                    break;
                }
            };
            let referenced = self
                .recursing
                .last()
                .is_some_and(|target| target.referenced);
            if referenced {
                steps.push((*op, bound));
            } else {
                seeds.push((*op, bound));
            }
        }
        self.recursing.pop();
        outcome?;
        // The ORDER BY, LIMIT and OFFSET belong to the whole recursive query:
        // SQLite orders its queue by them and stops the recursion at the limit,
        // so they are kept on the body rather than on the seed arm.
        let seed_columns = seeds
            .first()
            .map_or_else(Vec::new, |(_, seed)| seed.columns.clone());
        // An ORDER BY name may come from any arm, as in every other compound.
        let other_arms: Vec<(ast::CompoundOp, crate::bind::BoundSelect)> =
            seeds.iter().skip(1).chain(steps.iter()).cloned().collect();
        let order_by = self.bind_compound_order_by(&order_by, &seed_columns, &other_arms)?;
        let limit = limit.map(|expr| self.bind_expr(expr)).transpose()?;
        let offset = offset.map(|expr| self.bind_expr(expr)).transpose()?;
        let mut source = BoundSource {
            index_hint: crate::bind::IndexChoice::Any,
            id,
            rows: SourceRows::Recursive(Box::new(RecursiveBody {
                seeds,
                steps,
                order_by,
                limit,
                offset,
            })),
            table: std::rc::Rc::new(table),
            alias,
            join,
            constraint: None,
            suppressed: Vec::new(),
            index_exprs: Vec::new(),
            written_schema: None,
        };
        if let SourceRows::Recursive(body) = &mut source.rows {
            if body.steps.is_empty() {
                // Declared recursive, never refers to itself: an ordinary
                // compound wearing the keyword.
                let mut arms = core::mem::take(&mut body.seeds);
                if arms.is_empty() {
                    return Err(unsupported("missing select core", span));
                }
                let mut head = arms.remove(0).1;
                head.compounds = arms;
                head.order_by = core::mem::take(&mut body.order_by);
                head.limit = body.limit.take();
                head.offset = body.offset.take();
                source.rows = SourceRows::Subquery(Box::new(head));
            }
        }
        if let Some(slot) = self.sources.get_mut(id) {
            *slot = source;
        }
        if let Some(scope) = self.scopes.last_mut() {
            scope.push(id);
        }
        Ok(())
    }
}

impl<'a> Binder<'a> {
    /// Binds a FROM term that names a common table expression.
    ///
    /// @param cte - the expression the name stands for
    /// @param folded - the folded name, to tell a recursive reference from a plain one
    /// @param alias - the alias written on the term, if any
    /// @param join - how the term joins the ones before it
    /// @param span - where the term is, for an error
    pub(super) fn bind_cte_term(
        &mut self,
        cte: CteBinding,
        folded: &[u8],
        alias: Option<ast::NameId>,
        join: JoinKind,
        span: Span,
    ) -> Result<(), ParseError> {
        let alias = match alias {
            Some(alias) => self.ast.text(alias).to_vec(),
            None => cte.name.clone(),
        };
        // A definition already being bound cannot be bound again: that
        // is a cycle, and following it does not end.
        if self.binding_ctes.contains(&cte.select) {
            // SQLite names the expression and points at nothing.
            let _ = span;
            return Err(ParseError::new(
                ParseErrorKind::Refused(format!(
                    "circular reference: {}",
                    String::from_utf8_lossy(&cte.name)
                )),
                Span::default(),
            ));
        }
        self.binding_ctes.push(cte.select);
        // **`RECURSIVE` is a keyword SQLite does not require.** A CTE
        // whose FROM names itself *is* the recursion, written or not,
        // and reading the keyword as the only evidence sent this
        // binder round the same definition until the stack ran out.
        let outcome = if cte.recursive || self.select_names_itself(cte.select, folded) {
            self.bind_recursive_cte(&cte, alias, join, span)
        } else {
            let bound =
                self.bind_subquery_term(cte.select, Some(alias), cte.columns.clone(), join, span);
            if bound.is_ok() {
                self.share_last_source(&cte);
            }
            bound
        };
        self.binding_ctes.pop();
        outcome
    }

    /// Finds the table a FROM term names, falling back to a table valued
    /// function when the view's own database does not hold the name.
    ///
    /// A name that is not a table of the view's database may still be a table
    /// valued function such as `json_each`, which belongs to no schema.
    ///
    /// @param database - the schema written on the term, if any
    /// @param database_name - the schema to look in, folded
    /// @param folded - the table name, folded
    pub(super) fn find_term_table(
        &self,
        database: Option<ast::NameId>,
        database_name: Option<Vec<u8>>,
        folded: &[u8],
    ) -> (Option<&'a TableInfo>, Option<Vec<u8>>) {
        let found = self.catalog.find_table(database_name.as_deref(), folded);
        if found.is_none() && database.is_none() && database_name.is_some() {
            let eponymous = self
                .catalog
                .find_table(None, folded)
                .filter(|table| table.kind == crate::catalog_view::TableKind::Virtual);
            if eponymous.is_some() {
                return (eponymous, None);
            }
        }
        (found, database_name)
    }
}