From bfd28bb07a1fd83f42c6a37ccfc33b1e4959db52 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Thu, 11 Jan 2024 15:28:13 -0500 Subject: [PATCH] Allow subquery pullup to wrap a PlaceHolderVar in another one. The code for wrapping subquery output expressions in PlaceHolderVars believed that if the expression already was a PlaceHolderVar, it was never necessary to wrap that in another one. That's wrong if the expression is underneath an outer join and involves a lateral reference to outside that scope: failing to add an additional PHV risks evaluating the expression at the wrong place and hence not forcing it to null when the outer join should do so. This is an oversight in commit 9e7e29c75, which added logic to forcibly wrap lateral-reference Vars in PlaceHolderVars, but didn't see that the adjacent case for PlaceHolderVars needed the same treatment. The test case we have for this doesn't fail before 4be058fe9, but now that I see the problem I wonder if it is possible to demonstrate related errors before that. That's moot though, since all such branches are out of support. Per bug #18284 from Holger Reise. Back-patch to all supported branches. Discussion: https://postgr.es/m/18284-47505a20c23647f8@postgresql.org --- src/backend/optimizer/prep/prepjointree.c | 9 ++++++-- src/test/regress/expected/join.out | 26 +++++++++++++++++++++++ src/test/regress/sql/join.sql | 12 +++++++++++ 3 files changed, 45 insertions(+), 2 deletions(-) diff --git a/src/backend/optimizer/prep/prepjointree.c b/src/backend/optimizer/prep/prepjointree.c index 999bd3ec56..388694e709 100644 --- a/src/backend/optimizer/prep/prepjointree.c +++ b/src/backend/optimizer/prep/prepjointree.c @@ -2365,8 +2365,13 @@ pullup_replace_vars_callback(Var *var, else if (newnode && IsA(newnode, PlaceHolderVar) && ((PlaceHolderVar *) newnode)->phlevelsup == 0) { - /* No need to wrap a PlaceHolderVar with another one, either */ - wrap = false; + /* The same rules apply for a PlaceHolderVar */ + if (rcon->target_rte->lateral && + !bms_is_subset(((PlaceHolderVar *) newnode)->phrels, + rcon->relids)) + wrap = true; + else + wrap = false; } else if (rcon->wrap_non_vars) { diff --git a/src/test/regress/expected/join.out b/src/test/regress/expected/join.out index d7153a6fd9..0638c2d3ce 100644 --- a/src/test/regress/expected/join.out +++ b/src/test/regress/expected/join.out @@ -5696,6 +5696,32 @@ select * from Output: (COALESCE((COALESCE(b.q2, '42'::bigint)), d.q2)) (24 rows) +-- another case requiring nested PlaceHolderVars +explain (verbose, costs off) +select * from + (select 0 as val0) as ss0 + left join (select 1 as val) as ss1 on true + left join lateral (select ss1.val as val_filtered where false) as ss2 on true; + QUERY PLAN +-------------------------------- + Nested Loop Left Join + Output: 0, (1), ((1)) + -> Result + Output: 1 + -> Result + Output: (1) + One-Time Filter: false +(7 rows) + +select * from + (select 0 as val0) as ss0 + left join (select 1 as val) as ss1 on true + left join lateral (select ss1.val as val_filtered where false) as ss2 on true; + val0 | val | val_filtered +------+-----+-------------- + 0 | 1 | +(1 row) + -- case that breaks the old ph_may_need optimization explain (verbose, costs off) select c.*,a.*,ss1.q1,ss2.q1,ss3.* from diff --git a/src/test/regress/sql/join.sql b/src/test/regress/sql/join.sql index 92ae4ca79c..1795ed8235 100644 --- a/src/test/regress/sql/join.sql +++ b/src/test/regress/sql/join.sql @@ -1948,6 +1948,18 @@ select * from ) on c.q2 = ss2.q1, lateral (select ss2.y offset 0) ss3; +-- another case requiring nested PlaceHolderVars +explain (verbose, costs off) +select * from + (select 0 as val0) as ss0 + left join (select 1 as val) as ss1 on true + left join lateral (select ss1.val as val_filtered where false) as ss2 on true; + +select * from + (select 0 as val0) as ss0 + left join (select 1 as val) as ss1 on true + left join lateral (select ss1.val as val_filtered where false) as ss2 on true; + -- case that breaks the old ph_may_need optimization explain (verbose, costs off) select c.*,a.*,ss1.q1,ss2.q1,ss3.* from