From 87704f2fe12adfc62e8fb2a9984fcff4e043db05 Mon Sep 17 00:00:00 2001 From: Tomas Vondra Date: Wed, 11 Nov 2026 20:47:24 +0100 Subject: [PATCH v3-master] Fix FULL JOIN constant propagation with non-strict operands Deriving input restrictions from COALESCE(left, right) = constant can discard unmatched rows when either operand produces a non-NULL value for a null-extended input. Require both operands to be provably null-preserving before deriving either equality, so that the value remains NULL for null-extended input. An expression is null-preserving if it contains only strict functions, and at least one level-zero variable (Var or PlaceHolderVar). COALESCE clauses and PlaceHolderVar expressions are handled explicitly. COALESCE is null-preserving iff all it's arguments are. So for example COALESCE(left.a, COALESCE(right.a, right.b)) = K is null-preserving, while COALESCE(left.a, COALESCE(right.a, K)) = K is not, because the nested COALESCE discards NULL values. PlaceHolderVars at level zero are automatically null-preserving. Those represent expressions calculated below the join, so the result will be nulled no matter the expression. Add regression coverage for both safe and unsafe operands, and safe nested FULL JOIN USING cases. Backpatch to all supported branches. The issue was likely introduced in 8.1, by commit cc5e80b8d1c4. Author: Tomas Vondra Reviewed-by: Ayush Tiwari Reviewed-by: Kirill Reshke Discussion: https://postgr.es/m/0074fbf7-a92c-417a-937a-6b1fc27aed82@vondra.me Backpatch-through: 15 --- src/backend/optimizer/path/equivclass.c | 108 ++++++++++++++++++++++ src/test/regress/expected/join.out | 113 ++++++++++++++++++++++++ src/test/regress/sql/join.sql | 49 ++++++++++ 3 files changed, 270 insertions(+) diff --git a/src/backend/optimizer/path/equivclass.c b/src/backend/optimizer/path/equivclass.c index 439cb651c01..b23b7fb81b2 100644 --- a/src/backend/optimizer/path/equivclass.c +++ b/src/backend/optimizer/path/equivclass.c @@ -80,6 +80,8 @@ static bool reconsider_outer_join_clause(PlannerInfo *root, bool outer_on_left); static bool reconsider_full_join_clause(PlannerInfo *root, OuterJoinClauseInfo *ojcinfo); +static bool is_full_join_expr_null_preserving(Node *node); +static bool full_join_expr_nonstrict_walker(Node *node, bool *found_nullable); static JoinDomain *find_join_domain(PlannerInfo *root, Relids relids); static Bitmapset *get_eclass_indexes_for_relids(PlannerInfo *root, Relids relids); @@ -2481,6 +2483,15 @@ reconsider_full_join_clause(PlannerInfo *root, OuterJoinClauseInfo *ojcinfo) else return false; + /* + * For an unmatched row, the other input's expression must become NULL. + * Otherwise COALESCE could supply the constant even when this input's + * expression is different, making the derived restriction invalid. + */ + if (!is_full_join_expr_null_preserving((Node *) leftvar) || + !is_full_join_expr_null_preserving((Node *) rightvar)) + return false; + foreach(lc1, root->eq_classes) { EquivalenceClass *cur_ec = (EquivalenceClass *) lfirst(lc1); @@ -2633,6 +2644,103 @@ reconsider_full_join_clause(PlannerInfo *root, OuterJoinClauseInfo *ojcinfo) return false; /* failed to make any deduction */ } +/* + * Check whether an expression becomes NULL when its FULL JOIN input is + * null-extended. The caller has verified that all referenced rels belong + * to one input. + * + * We conservatively require that the expression contains no non-strict + * constructs, and at least one level-zero Var or PlaceHolderVar. The latter + * test rules out non-NULL constants and pseudoconstants appearing as COALESCE + * arguments. + * + * A level-zero PlaceHolderVar is evaluated below the join, so its result is + * nulled regardless of the contained expression, and we don't look inside + * it. COALESCE is not strict in any one argument, but preserves NULL if all + * its arguments do. + */ +static bool +is_full_join_expr_null_preserving(Node *node) +{ + bool found_nullable = false; + + if (full_join_expr_nonstrict_walker(node, &found_nullable)) + return false; + + return found_nullable; +} + +/* + * Walker for is_full_join_expr_null_preserving. Returns true if it finds a + * construct that might produce non-NULL output for a null-extended input, + * and sets *found_nullable if it finds an input nulled by the join. + */ +static bool +full_join_expr_nonstrict_walker(Node *node, bool *found_nullable) +{ + if (node == NULL) + return false; + + switch (nodeTag(node)) + { + case T_Var: + if (((Var *) node)->varlevelsup == 0) + *found_nullable = true; + return false; + + case T_PlaceHolderVar: + if (((PlaceHolderVar *) node)->phlevelsup == 0) + { + /* don't descend into the contained expression */ + *found_nullable = true; + return false; + } + break; + + case T_CoalesceExpr: + { + CoalesceExpr *cexpr = (CoalesceExpr *) node; + ListCell *lc; + + foreach(lc, cexpr->args) + { + if (!is_full_join_expr_null_preserving(lfirst(lc))) + return true; + } + *found_nullable = true; + return false; + } + + case T_FuncExpr: + if (!func_strict(((FuncExpr *) node)->funcid)) + return true; + break; + + case T_OpExpr: + if (!op_strict(((OpExpr *) node)->opno)) + return true; + break; + + case T_RelabelType: + case T_CoerceViaIO: + /* treated as strict, as in contain_nonstrict_functions() */ + break; + + default: + + /* + * For anything else, rely on the generic tests. Those look + * inside PlaceHolderVars, so they may give up unnecessarily. + */ + if (contain_vars_of_level(node, 0)) + *found_nullable = true; + return contain_nonstrict_functions(node); + } + + return expression_tree_walker(node, full_join_expr_nonstrict_walker, + found_nullable); +} + /* * find_join_domain * Find the highest JoinDomain enclosed within the given relid set. diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out index 526430a5ea0..b2e547af67f 100644 --- a/src/test/regress/expected/join.out +++ b/src/test/regress/expected/join.out @@ -6556,6 +6556,119 @@ select * from int8_tbl t1 left join int8_tbl t2 on t1.q2 = t2.q1, 4567890123456789 | 4567890123456789 | 4567890123456789 | 4567890123456789 | 4567890123456789 | -4567890123456789 (10 rows) +-- non-strict operands must not cause unmatched rows to be filtered out +explain (costs off) +select * from (values (null::int), (1)) a(x) + full join (values (0), (1)) b(x) on a.x = coalesce(b.x, 0) + where coalesce(a.x, coalesce(b.x, 0)) = 0 + order by a.x, b.x; + QUERY PLAN +--------------------------------------------------------------------------------------- + Sort + Sort Key: "*VALUES*".column1, "*VALUES*_1".column1 + -> Hash Full Join + Hash Cond: ("*VALUES*".column1 = COALESCE("*VALUES*_1".column1, 0)) + Filter: (COALESCE("*VALUES*".column1, COALESCE("*VALUES*_1".column1, 0)) = 0) + -> Values Scan on "*VALUES*" + -> Hash + -> Values Scan on "*VALUES*_1" +(8 rows) + +select * from (values (null::int), (1)) a(x) + full join (values (0), (1)) b(x) on a.x = coalesce(b.x, 0) + where coalesce(a.x, coalesce(b.x, 0)) = 0 + order by a.x, b.x; + x | x +---+--- + | 0 + | +(2 rows) + +explain (costs off) +select * from (values (0), (1)) a(x) + full join (values (null::int), (1)) b(x) + on (case when a.x is null then 0 else a.x end) = b.x + where coalesce(case when a.x is null then 0 else a.x end, b.x) = 0 + order by a.x, b.x; + QUERY PLAN +--------------------------------------------------------------------------------------------------------------------------------- + Sort + Sort Key: "*VALUES*".column1, "*VALUES*_1".column1 + -> Hash Full Join + Hash Cond: (CASE WHEN ("*VALUES*".column1 IS NULL) THEN 0 ELSE "*VALUES*".column1 END = "*VALUES*_1".column1) + Filter: (COALESCE(CASE WHEN ("*VALUES*".column1 IS NULL) THEN 0 ELSE "*VALUES*".column1 END, "*VALUES*_1".column1) = 0) + -> Values Scan on "*VALUES*" + -> Hash + -> Values Scan on "*VALUES*_1" +(8 rows) + +select * from (values (0), (1)) a(x) + full join (values (null::int), (1)) b(x) + on (case when a.x is null then 0 else a.x end) = b.x + where coalesce(case when a.x is null then 0 else a.x end, b.x) = 0 + order by a.x, b.x; + x | x +---+--- + 0 | + | +(2 rows) + +-- nested merged columns and precomputed expressions remain null-preserving +explain (costs off) +select * from (select coalesce(f1, 0) as f1 from int4_tbl) a + full join int4_tbl b using (f1) + full join int4_tbl c using (f1) + where f1 = 0; + QUERY PLAN +--------------------------------------------- + Merge Full Join + -> Merge Full Join + -> Seq Scan on int4_tbl + Filter: (COALESCE(f1, 0) = 0) + -> Materialize + -> Seq Scan on int4_tbl b + Filter: (f1 = 0) + -> Materialize + -> Seq Scan on int4_tbl c + Filter: (f1 = 0) +(10 rows) + +select * from (select coalesce(f1, 0) as f1 from int4_tbl) a + full join int4_tbl b using (f1) + full join int4_tbl c using (f1) + where f1 = 0; + f1 +---- + 0 +(1 row) + +-- strict expressions of precomputed values remain null-preserving +explain (costs off) +select * from (select coalesce(x, 0) as x from (values (null::int), (1)) v(x)) a + full join (values (1), (2)) b(x) on a.x + 0 = b.x + where coalesce(a.x + 0, b.x) = 0 + order by a.x, b.x; + QUERY PLAN +--------------------------------------------------------------------- + Sort + Sort Key: (COALESCE("*VALUES*".column1, 0)), "*VALUES*_1".column1 + -> Merge Full Join + -> Values Scan on "*VALUES*" + Filter: ((COALESCE(column1, 0) + 0) = 0) + -> Materialize + -> Values Scan on "*VALUES*_1" + Filter: (column1 = 0) +(8 rows) + +select * from (select coalesce(x, 0) as x from (values (null::int), (1)) v(x)) a + full join (values (1), (2)) b(x) on a.x + 0 = b.x + where coalesce(a.x + 0, b.x) = 0 + order by a.x, b.x; + x | x +---+--- + 0 | +(1 row) + -- -- check deductions from FULL JOIN clauses and COALESCE() = constant: the -- join clause's operands may appear in either order, but must come from diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql index 27b44016717..de8690e2426 100644 --- a/src/test/regress/sql/join.sql +++ b/src/test/regress/sql/join.sql @@ -2278,6 +2278,55 @@ select * from int8_tbl t1 left join int8_tbl t2 on t1.q2 = t2.q1, select * from int8_tbl t1 left join int8_tbl t2 on t1.q2 = t2.q1, lateral (select * from int8_tbl t3 where t2.q1 = t2.q2) ss; +-- non-strict operands must not cause unmatched rows to be filtered out +explain (costs off) +select * from (values (null::int), (1)) a(x) + full join (values (0), (1)) b(x) on a.x = coalesce(b.x, 0) + where coalesce(a.x, coalesce(b.x, 0)) = 0 + order by a.x, b.x; + +select * from (values (null::int), (1)) a(x) + full join (values (0), (1)) b(x) on a.x = coalesce(b.x, 0) + where coalesce(a.x, coalesce(b.x, 0)) = 0 + order by a.x, b.x; + +explain (costs off) +select * from (values (0), (1)) a(x) + full join (values (null::int), (1)) b(x) + on (case when a.x is null then 0 else a.x end) = b.x + where coalesce(case when a.x is null then 0 else a.x end, b.x) = 0 + order by a.x, b.x; + +select * from (values (0), (1)) a(x) + full join (values (null::int), (1)) b(x) + on (case when a.x is null then 0 else a.x end) = b.x + where coalesce(case when a.x is null then 0 else a.x end, b.x) = 0 + order by a.x, b.x; + +-- nested merged columns and precomputed expressions remain null-preserving +explain (costs off) +select * from (select coalesce(f1, 0) as f1 from int4_tbl) a + full join int4_tbl b using (f1) + full join int4_tbl c using (f1) + where f1 = 0; + +select * from (select coalesce(f1, 0) as f1 from int4_tbl) a + full join int4_tbl b using (f1) + full join int4_tbl c using (f1) + where f1 = 0; + +-- strict expressions of precomputed values remain null-preserving +explain (costs off) +select * from (select coalesce(x, 0) as x from (values (null::int), (1)) v(x)) a + full join (values (1), (2)) b(x) on a.x + 0 = b.x + where coalesce(a.x + 0, b.x) = 0 + order by a.x, b.x; + +select * from (select coalesce(x, 0) as x from (values (null::int), (1)) v(x)) a + full join (values (1), (2)) b(x) on a.x + 0 = b.x + where coalesce(a.x + 0, b.x) = 0 + order by a.x, b.x; + -- -- check deductions from FULL JOIN clauses and COALESCE() = constant: the -- join clause's operands may appear in either order, but must come from -- 2.56.0