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Check parallel safety in generate_useful_gather_paths

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Commit ebb7ae839d ensured we ignore pathkeys with volatile expressions
when considering adding a sort below a Gather Merge. Turns out we need
to care about parallel safety of the pathkeys too, otherwise we might
try sorting e.g. on results of a correlated subquery (as demonstrated
by a report from Luis Roberto).

Initial investigation by Tom Lane, patch by James Coleman. Backpatch
to 13, where the code was instroduced (as part of Incremental Sort).

Reported-by: Luis Roberto
Author: James Coleman
Reviewed-by: Tomas Vondra
Backpatch-through: 13
Discussion: https://postgr.es/m/622580997.37108180.1604080457319.JavaMail.zimbra%40siscobra.com.br
Discussion: https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com
This commit is contained in:
Tomas Vondra
2020-12-21 18:29:46 +01:00
parent f4a3c0b062
commit 86b7cca72d
5 changed files with 73 additions and 5 deletions
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9
src/backend/optimizer/path/equivclass.c
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@@ -803,7 +803,8 @@ find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel)
* applied in prepare_sort_from_pathkeys.
*/
Expr *
find_em_expr_usable_for_sorting_rel(EquivalenceClass *ec, RelOptInfo *rel)
find_em_expr_usable_for_sorting_rel(PlannerInfo *root, EquivalenceClass *ec,
RelOptInfo *rel, bool require_parallel_safe)
{
ListCell *lc_em;
@@ -833,6 +834,12 @@ find_em_expr_usable_for_sorting_rel(EquivalenceClass *ec, RelOptInfo *rel)
if (!bms_is_subset(em->em_relids, rel->relids))
continue;
/*
* If requested, reject expressions that are not parallel-safe.
*/
if (require_parallel_safe && !is_parallel_safe(root, (Node *) em_expr))
continue;
/*
* As long as the expression isn't volatile then
* prepare_sort_from_pathkeys is able to generate a new target entry,