From af1e2fe6dac300cddd8e80126dd1308c6f1e5ac9 Mon Sep 17 00:00:00 2001 From: Evdokimov Ilia Date: Thu, 23 Jul 2026 13:26:07 +0300 Subject: [PATCH v2 3/9] Fix false NOT NULL marking of subquery-derived UniqueKey columns convert_unique_keys_for_rel() marked a subquery output column as NOT NULL solely because it was covered by a UniqueKey, without checking whether the underlying key can still contain NULLs. A unique index does not by itself rule out multiple NULLs, so this produced wrong query results: relation_is_distinct_for() would then treat the column as NULL-safe and elide a DISTINCT step that was still needed. CREATE TABLE t (a int, b int); CREATE UNIQUE INDEX ON t(a); INSERT INTO t VALUES (NULL,1), (NULL,2), (1,3); SELECT DISTINCT a FROM (SELECT a, b FROM t OFFSET 0) s; returned 3 rows instead of 2, silently dropping the DISTINCT. Only mark the output column NOT NULL when the underlying expression is a plain Var that is itself already known not-null in the subquery's own planning context, per var_is_nullable(). --- src/backend/optimizer/path/uniquekey.c | 15 ++++++++++----- 1 file changed, 10 insertions(+), 5 deletions(-) diff --git a/src/backend/optimizer/path/uniquekey.c b/src/backend/optimizer/path/uniquekey.c index 42ba7e7c35c..3974c624799 100644 --- a/src/backend/optimizer/path/uniquekey.c +++ b/src/backend/optimizer/path/uniquekey.c @@ -1020,12 +1020,17 @@ convert_unique_keys_for_rel(PlannerInfo *root, RelOptInfo *rel, if (matched >= 0) { /* - * By generating the unique key the subquery - * guarantees that the value of the output column - * cannot be NULL. + * A unique index does not by itself guarantee the + * column is never NULL (Postgres unique indexes + * permit multiple NULLs). Only mark the output + * not-null when the underlying expression is a + * plain Var that is itself already known not-null + * in the subquery's own planning context. */ - rel->notnullattrs = bms_add_member(rel->notnullattrs, - target_var->varattno - FirstLowInvalidHeapAttributeNumber); + if (IsA(expr, Var) && + !var_is_nullable(rel->subroot, (Var *) expr, input_rel)) + rel->notnullattrs = bms_add_member(rel->notnullattrs, + target_var->varattno - FirstLowInvalidHeapAttributeNumber); } else matched = -1; -- 2.34.1