From 928df067d1e6ca5b747722d32c79bc3efa891a32 Mon Sep 17 00:00:00 2001 From: David Rowley Date: Tue, 7 Oct 2025 14:15:04 +1300 Subject: [PATCH] Fix incorrect targetlist in dummy UNIONs The prior code, added in 03d40e4b5 attempted to use the targetlist of the first UNION child when all UNION children were proven as dummy rels. That's not going to work when some operation atop of the Result node must find target entries within the Result's targetlist. This could have been something as simple as trying to sort the results of the UNION operation, which would lead to: ERROR: could not find pathkey item to sort Instead, use the top-level UNION's targetlist and fix the varnos in setrefs.c. Because set operation targetlists always use varno==0, we can rewrite those to become varno==1, i.e. use the Vars from the first UNION child. This does result in showing Vars from relations that are not present in the final plan, but that's no different to what we see when normal base relations are proven dummy. Without this fix it would be possible to see the following error in EXPLAIN VERBOSE when all UNION inputs were proven empty. ERROR: bogus varno: 0 Author: David Rowley Discussion: https://postgr.es/m/CAApHDvrUASy9sfULMEsM2udvZJP6AoBRCZvHYXYxZTy2tX9FYw@mail.gmail.com --- src/backend/optimizer/plan/setrefs.c | 27 ++++++++++++++++++++++---- src/backend/optimizer/prep/prepunion.c | 1 - src/test/regress/expected/union.out | 18 ++++++++++------- src/test/regress/sql/union.sql | 3 ++- 4 files changed, 36 insertions(+), 13 deletions(-) diff --git a/src/backend/optimizer/plan/setrefs.c b/src/backend/optimizer/plan/setrefs.c index 6950eff2c5b..ccdc9bc264a 100644 --- a/src/backend/optimizer/plan/setrefs.c +++ b/src/backend/optimizer/plan/setrefs.c @@ -1034,16 +1034,35 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset) * expected to occur here, it seems safer to special-case * it here and keep the assertions that ROWID_VARs * shouldn't be seen by fix_scan_expr. + * + * We also must handle the case where set operations have + * been short-circuited resulting in a dummy Result node. + * prepunion.c uses varno==0 for the set op targetlist. + * See generate_setop_tlist() and generate_setop_tlist(). + * Here we rewrite these to use varno==1, which is the + * varno of the first set-op child. Without this, EXPLAIN + * will have trouble displaying targetlists of dummy set + * operations. */ foreach(l, splan->plan.targetlist) { TargetEntry *tle = (TargetEntry *) lfirst(l); Var *var = (Var *) tle->expr; - if (var && IsA(var, Var) && var->varno == ROWID_VAR) - tle->expr = (Expr *) makeNullConst(var->vartype, - var->vartypmod, - var->varcollid); + if (var && IsA(var, Var)) + { + if (var->varno == ROWID_VAR) + tle->expr = (Expr *) makeNullConst(var->vartype, + var->vartypmod, + var->varcollid); + else if (var->varno == 0) + tle->expr = (Expr *) makeVar(1, + var->varattno, + var->vartype, + var->vartypmod, + var->varcollid, + var->varlevelsup); + } } splan->plan.targetlist = diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c index 547dbd53540..da9431108a2 100644 --- a/src/backend/optimizer/prep/prepunion.c +++ b/src/backend/optimizer/prep/prepunion.c @@ -826,7 +826,6 @@ generate_union_paths(SetOperationStmt *op, PlannerInfo *root, /* If all UNION children were dummy rels, make the resulting rel dummy */ if (cheapest_pathlist == NIL) { - result_rel->reltarget = create_pathtarget(root, list_nth(tlist_list, 0)); mark_dummy_rel(result_rel); return result_rel; diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out index 7c089e0d598..15931beea3a 100644 --- a/src/test/regress/expected/union.out +++ b/src/test/regress/expected/union.out @@ -1258,14 +1258,18 @@ SELECT two FROM tenk1 WHERE 1=2 UNION SELECT four FROM tenk1 WHERE 1=2 UNION -SELECT ten FROM tenk1 WHERE 1=2; - QUERY PLAN --------------------------------- - Result +SELECT ten FROM tenk1 WHERE 1=2 +ORDER BY 1; + QUERY PLAN +-------------------------------------- + Sort Output: unnamed_subquery.two - Replaces: Aggregate - One-Time Filter: false -(4 rows) + Sort Key: unnamed_subquery.two + -> Result + Output: unnamed_subquery.two + Replaces: Aggregate + One-Time Filter: false +(7 rows) -- Test constraint exclusion of UNION ALL subqueries explain (costs off) diff --git a/src/test/regress/sql/union.sql b/src/test/regress/sql/union.sql index 56bd20e741c..e252316f69b 100644 --- a/src/test/regress/sql/union.sql +++ b/src/test/regress/sql/union.sql @@ -484,7 +484,8 @@ SELECT two FROM tenk1 WHERE 1=2 UNION SELECT four FROM tenk1 WHERE 1=2 UNION -SELECT ten FROM tenk1 WHERE 1=2; +SELECT ten FROM tenk1 WHERE 1=2 +ORDER BY 1; -- Test constraint exclusion of UNION ALL subqueries explain (costs off) -- 2.47.3