From 148ad41d7fc531ad55fbc7e79a7e5c15a587377d Mon Sep 17 00:00:00 2001 From: Antonin Houska Date: Fri, 21 Aug 2026 10:21:20 +0200 Subject: [PATCH] Attempt to reword a comment. In particular, the phrase "so every output row satisfies them" seems problematic because an output row of a semijoin is NULL-extended, so the output row usually does not satisfy the join clauses. --- src/backend/optimizer/path/uniquekeys.c | 11 ++++++++--- 1 file changed, 8 insertions(+), 3 deletions(-) diff --git a/src/backend/optimizer/path/uniquekeys.c b/src/backend/optimizer/path/uniquekeys.c index f9d9c37f447..34b2507d6b7 100644 --- a/src/backend/optimizer/path/uniquekeys.c +++ b/src/backend/optimizer/path/uniquekeys.c @@ -538,9 +538,14 @@ populate_joinrel_uniquekeys(PlannerInfo *root, RelOptInfo *joinrel, if (jointype == JOIN_SEMI || jointype == JOIN_ANTI) { /* - * A semijoin emits an LHS row only when some RHS row satisfied the - * join clauses, so every output row satisfies them. An antijoin - * emits exactly the rows that no RHS row matched. + * A semijoin emits a row only when the LHS satisfied the join clauses + * (for at least one RHS row). Given that all the current unique keys + * originate from the LHS, it's possible that rows having NULL in any + * unique key attribute are filtered out by the join clauses. + * + * (An antijoin emits exactly the rows that do not satisfy the join + * clauses, so rows having NULL in the unique key are not filtered + * out.) */ if (jointype == JOIN_SEMI) strengthen_uniquekeys_for_join(root, joinrel, restrictlist); -- 2.52.0