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sql: bound INTERSECT plan size by ordering its inputs #37897

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Merged
ggevay merged 1 commit into from
Jul 27, 2026
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sql: bound INTERSECT plan size by ordering its inputs #37897

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12 changes: 12 additions & 0 deletions src/sql/src/plan/hir.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1763,6 +1763,18 @@ impl HirRelationExpr {
}
}

/// The number of relation nodes in this expression.
///
/// Relations reached through scalar subqueries are included. The scalar
/// nodes themselves are not, so a large predicate over a small input still
/// counts as small. This is a structural size for comparing two
/// expressions against each other, not a cost estimate.
pub fn relation_node_count(&self) -> usize {
let mut count = 0;
self.visit_post(&mut |_| count += 1);
count
}

/// If self is a constant, return the value and the type, otherwise `None`.
pub fn as_const(&self) -> Option<(&Vec<Row>, &SqlRelationType)> {
match self {
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15 changes: 14 additions & 1 deletion src/sql/src/plan/query.rs
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Expand Up @@ -1969,8 +1969,21 @@ fn plan_set_expr(
}
SetOperator::Except => Hir::except(all, lhs, rhs),
SetOperator::Intersect => {
// Planning below duplicates whichever input ends up on the left, so a
// left-deep chain of INTERSECTs doubles the plan at every level, i.e. would be
// exponential. INTERSECT is commutative, so put the cheaper input on the left.
// A subtree is then only duplicated when it is the smaller of the two, so plan
// size obeys T(a + b) <= 2*T(a) + T(b) for input sizes a <= b. The worst case
// is balanced trees, where this solves to O(n^log2(3)) instead of O(2^n).
let (lhs, rhs) = if lhs.relation_node_count() > rhs.relation_node_count() {
(rhs, lhs)
} else {
(lhs, rhs)
};
// TODO: Let's not duplicate the left-hand expression into TWO dataflows!
// Though we believe that render() does The Right Thing (TM)
// The optimizer de-duplicates at some point, but it would be good to already
// not duplicate here.
//
// Also note that we do *not* need another threshold() at the end of the method chain
// because the right-hand side of the outer union only produces existing records,
// i.e., the record counts for differential data flow definitely remain non-negative.
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10 changes: 10 additions & 0 deletions test/sqllogictest/recursion_limit.slt
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Expand Up @@ -18,3 +18,13 @@ query I
SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1 UNION SELECT 1;
----
1

# 35 chained INTERSECTs.
# Planning duplicates an INTERSECT's left input, so a left-deep chain doubles
# the plan at every level. `plan_set_expr` puts the cheaper input on the left to
# keep that polynomial. Without that, this query needs 2^35 HIR nodes and
# exhausts memory while planning.
query I
SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1 INTERSECT SELECT 1;
----
1
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