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Commit86b7cca

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Check parallel safety in generate_useful_gather_paths
Commitebb7ae8 ensured we ignore pathkeys with volatile expressionswhen considering adding a sort below a Gather Merge. Turns out we needto care about parallel safety of the pathkeys too, otherwise we mighttry sorting e.g. on results of a correlated subquery (as demonstratedby a report from Luis Roberto).Initial investigation by Tom Lane, patch by James Coleman. Backpatchto 13, where the code was instroduced (as part of Incremental Sort).Reported-by: Luis RobertoAuthor: James ColemanReviewed-by: Tomas VondraBackpatch-through: 13Discussion:https://postgr.es/m/622580997.37108180.1604080457319.JavaMail.zimbra%40siscobra.com.brDiscussion:https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com
1 parentf4a3c0b commit86b7cca

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5 files changed

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5 files changed

+73
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‎src/backend/optimizer/path/allpaths.c

Lines changed: 10 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -2802,14 +2802,18 @@ generate_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_rows)
28022802
* This allows us to do incremental sort on top of an index scan under a gather
28032803
* merge node, i.e. parallelized.
28042804
*
2805+
* If the require_parallel_safe is true, we also require the expressions to
2806+
* be parallel safe (which allows pushing the sort below Gather Merge).
2807+
*
28052808
* XXX At the moment this can only ever return a list with a single element,
28062809
* because it looks at query_pathkeys only. So we might return the pathkeys
28072810
* directly, but it seems plausible we'll want to consider other orderings
28082811
* in the future. For example, we might want to consider pathkeys useful for
28092812
* merge joins.
28102813
*/
28112814
staticList*
2812-
get_useful_pathkeys_for_relation(PlannerInfo*root,RelOptInfo*rel)
2815+
get_useful_pathkeys_for_relation(PlannerInfo*root,RelOptInfo*rel,
2816+
boolrequire_parallel_safe)
28132817
{
28142818
List*useful_pathkeys_list=NIL;
28152819

@@ -2839,8 +2843,11 @@ get_useful_pathkeys_for_relation(PlannerInfo *root, RelOptInfo *rel)
28392843
* meet criteria of EC membership in the current relation, we
28402844
* enable not just an incremental sort on the entirety of
28412845
* query_pathkeys but also incremental sort below a JOIN.
2846+
*
2847+
* If requested, ensure the expression is parallel safe too.
28422848
*/
2843-
if (!find_em_expr_usable_for_sorting_rel(pathkey_ec,rel))
2849+
if (!find_em_expr_usable_for_sorting_rel(root,pathkey_ec,rel,
2850+
require_parallel_safe))
28442851
break;
28452852

28462853
npathkeys++;
@@ -2894,7 +2901,7 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
28942901
generate_gather_paths(root,rel,override_rows);
28952902

28962903
/* consider incremental sort for interesting orderings */
2897-
useful_pathkeys_list=get_useful_pathkeys_for_relation(root,rel);
2904+
useful_pathkeys_list=get_useful_pathkeys_for_relation(root,rel, true);
28982905

28992906
/* used for explicit (full) sort paths */
29002907
cheapest_partial_path=linitial(rel->partial_pathlist);

‎src/backend/optimizer/path/equivclass.c

Lines changed: 8 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -803,7 +803,8 @@ find_em_expr_for_rel(EquivalenceClass *ec, RelOptInfo *rel)
803803
* applied in prepare_sort_from_pathkeys.
804804
*/
805805
Expr*
806-
find_em_expr_usable_for_sorting_rel(EquivalenceClass*ec,RelOptInfo*rel)
806+
find_em_expr_usable_for_sorting_rel(PlannerInfo*root,EquivalenceClass*ec,
807+
RelOptInfo*rel,boolrequire_parallel_safe)
807808
{
808809
ListCell*lc_em;
809810

@@ -833,6 +834,12 @@ find_em_expr_usable_for_sorting_rel(EquivalenceClass *ec, RelOptInfo *rel)
833834
if (!bms_is_subset(em->em_relids,rel->relids))
834835
continue;
835836

837+
/*
838+
* If requested, reject expressions that are not parallel-safe.
839+
*/
840+
if (require_parallel_safe&& !is_parallel_safe(root, (Node*)em_expr))
841+
continue;
842+
836843
/*
837844
* As long as the expression isn't volatile then
838845
* prepare_sort_from_pathkeys is able to generate a new target entry,

‎src/include/optimizer/paths.h

Lines changed: 4 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -135,7 +135,10 @@ extern EquivalenceClass *get_eclass_for_sort_expr(PlannerInfo *root,
135135
Relidsrel,
136136
boolcreate_it);
137137
externExpr*find_em_expr_for_rel(EquivalenceClass*ec,RelOptInfo*rel);
138-
externExpr*find_em_expr_usable_for_sorting_rel(EquivalenceClass*ec,RelOptInfo*rel);
138+
externExpr*find_em_expr_usable_for_sorting_rel(PlannerInfo*root,
139+
EquivalenceClass*ec,
140+
RelOptInfo*rel,
141+
boolrequire_parallel_safe);
139142
externvoidgenerate_base_implied_equalities(PlannerInfo*root);
140143
externList*generate_join_implied_equalities(PlannerInfo*root,
141144
Relidsjoin_relids,

‎src/test/regress/expected/incremental_sort.out

Lines changed: 40 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1551,6 +1551,46 @@ order by 1, 2;
15511551
-> Function Scan on generate_series
15521552
(7 rows)
15531553

1554+
-- Parallel sort but with expression (correlated subquery) that
1555+
-- is prohibited in parallel plans.
1556+
explain (costs off) select distinct
1557+
unique1,
1558+
(select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
1559+
from tenk1 t, generate_series(1, 1000);
1560+
QUERY PLAN
1561+
---------------------------------------------------------------------------------
1562+
Unique
1563+
-> Sort
1564+
Sort Key: t.unique1, ((SubPlan 1))
1565+
-> Gather
1566+
Workers Planned: 2
1567+
-> Nested Loop
1568+
-> Parallel Index Only Scan using tenk1_unique1 on tenk1 t
1569+
-> Function Scan on generate_series
1570+
SubPlan 1
1571+
-> Index Only Scan using tenk1_unique1 on tenk1
1572+
Index Cond: (unique1 = t.unique1)
1573+
(11 rows)
1574+
1575+
explain (costs off) select
1576+
unique1,
1577+
(select t.unique1 from tenk1 where tenk1.unique1 = t.unique1)
1578+
from tenk1 t, generate_series(1, 1000)
1579+
order by 1, 2;
1580+
QUERY PLAN
1581+
---------------------------------------------------------------------------
1582+
Sort
1583+
Sort Key: t.unique1, ((SubPlan 1))
1584+
-> Gather
1585+
Workers Planned: 2
1586+
-> Nested Loop
1587+
-> Parallel Index Only Scan using tenk1_unique1 on tenk1 t
1588+
-> Function Scan on generate_series
1589+
SubPlan 1
1590+
-> Index Only Scan using tenk1_unique1 on tenk1
1591+
Index Cond: (unique1 = t.unique1)
1592+
(10 rows)
1593+
15541594
-- Parallel sort but with expression not available until the upper rel.
15551595
explain (costs off) select distinct sub.unique1, stringu1 || random()::text
15561596
from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub;

‎src/test/regress/sql/incremental_sort.sql

Lines changed: 11 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -250,6 +250,17 @@ from tenk1, lateral (select tenk1.unique1 from generate_series(1, 1000)) as sub;
250250
explain (costs off)selectsub.unique1, md5(stringu1)
251251
from tenk1, lateral (selecttenk1.unique1from generate_series(1,1000))as sub
252252
order by1,2;
253+
-- Parallel sort but with expression (correlated subquery) that
254+
-- is prohibited in parallel plans.
255+
explain (costs off)select distinct
256+
unique1,
257+
(selectt.unique1from tenk1wheretenk1.unique1=t.unique1)
258+
from tenk1 t, generate_series(1,1000);
259+
explain (costs off)select
260+
unique1,
261+
(selectt.unique1from tenk1wheretenk1.unique1=t.unique1)
262+
from tenk1 t, generate_series(1,1000)
263+
order by1,2;
253264
-- Parallel sort but with expression not available until the upper rel.
254265
explain (costs off)select distinctsub.unique1, stringu1|| random()::text
255266
from tenk1, lateral (selecttenk1.unique1from generate_series(1,1000))as sub;

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