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Commit3915178

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Fix testing of parallel-safety of SubPlans.
is_parallel_safe() supposed that the only relevant property of a SubPlanwas the parallel safety of the referenced subplan tree. This is wrong:the testexpr or args subtrees might contain parallel-unsafe stuff, asdemonstrated by the test case added here. However, just recursing into thesubtrees fails in a different way: we'll typically find PARAM_EXEC Paramsrepresenting the subplan's output columns in the testexpr. The previouscoding supposed that any Param must be treated as parallel-restricted, sothat a naive attempt at fixing this disabled parallel pushdown of SubPlansaltogether. We must instead determine, for any visited Param, whether itis one that would be computed by a surrounding SubPlan node; if so, it'ssafe to push down along with the SubPlan node.We might later be able to extend this logic to cope with Params used forcorrelated subplans and other cases; but that's a task for v11 or beyond.Tom Lane and Amit KapilaDiscussion:https://postgr.es/m/7064.1492022469@sss.pgh.pa.us
1 parent539f670 commit3915178

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4 files changed

+57
-6
lines changed

4 files changed

+57
-6
lines changed

‎src/backend/optimizer/util/clauses.c

Lines changed: 39 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -93,6 +93,7 @@ typedef struct
9393
{
9494
charmax_hazard;/* worst proparallel hazard found so far */
9595
charmax_interesting;/* worst proparallel hazard of interest */
96+
List*safe_param_ids;/* PARAM_EXEC Param IDs to treat as safe */
9697
}max_parallel_hazard_context;
9798

9899
staticboolcontain_agg_clause_walker(Node*node,void*context);
@@ -1056,6 +1057,7 @@ max_parallel_hazard(Query *parse)
10561057

10571058
context.max_hazard=PROPARALLEL_SAFE;
10581059
context.max_interesting=PROPARALLEL_UNSAFE;
1060+
context.safe_param_ids=NIL;
10591061
(void)max_parallel_hazard_walker((Node*)parse,&context);
10601062
returncontext.max_hazard;
10611063
}
@@ -1084,6 +1086,7 @@ is_parallel_safe(PlannerInfo *root, Node *node)
10841086
/* Else use max_parallel_hazard's search logic, but stop on RESTRICTED */
10851087
context.max_hazard=PROPARALLEL_SAFE;
10861088
context.max_interesting=PROPARALLEL_RESTRICTED;
1089+
context.safe_param_ids=NIL;
10871090
return !max_parallel_hazard_walker(node,&context);
10881091
}
10891092

@@ -1171,18 +1174,49 @@ max_parallel_hazard_walker(Node *node, max_parallel_hazard_context *context)
11711174
return true;
11721175
}
11731176

1174-
/* We can push the subplans only if they are parallel-safe. */
1177+
/*
1178+
* Only parallel-safe SubPlans can be sent to workers. Within the
1179+
* testexpr of the SubPlan, Params representing the output columns of the
1180+
* subplan can be treated as parallel-safe, so temporarily add their IDs
1181+
* to the safe_param_ids list while examining the testexpr.
1182+
*/
11751183
elseif (IsA(node,SubPlan))
1176-
return !((SubPlan*)node)->parallel_safe;
1184+
{
1185+
SubPlan*subplan= (SubPlan*)node;
1186+
List*save_safe_param_ids;
1187+
1188+
if (!subplan->parallel_safe&&
1189+
max_parallel_hazard_test(PROPARALLEL_RESTRICTED,context))
1190+
return true;
1191+
save_safe_param_ids=context->safe_param_ids;
1192+
context->safe_param_ids=list_concat(list_copy(subplan->paramIds),
1193+
context->safe_param_ids);
1194+
if (max_parallel_hazard_walker(subplan->testexpr,context))
1195+
return true;/* no need to restore safe_param_ids */
1196+
context->safe_param_ids=save_safe_param_ids;
1197+
/* we must also check args, but no special Param treatment there */
1198+
if (max_parallel_hazard_walker((Node*)subplan->args,context))
1199+
return true;
1200+
/* don't want to recurse normally, so we're done */
1201+
return false;
1202+
}
11771203

11781204
/*
11791205
* We can't pass Params to workers at the moment either, so they are also
1180-
* parallel-restricted.
1206+
* parallel-restricted, unless they are PARAM_EXEC Params listed in
1207+
* safe_param_ids, meaning they could be generated within the worker.
11811208
*/
11821209
elseif (IsA(node,Param))
11831210
{
1184-
if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED,context))
1185-
return true;
1211+
Param*param= (Param*)node;
1212+
1213+
if (param->paramkind!=PARAM_EXEC||
1214+
!list_member_int(context->safe_param_ids,param->paramid))
1215+
{
1216+
if (max_parallel_hazard_test(PROPARALLEL_RESTRICTED,context))
1217+
return true;
1218+
}
1219+
return false;/* nothing to recurse to */
11861220
}
11871221

11881222
/*

‎src/include/nodes/primnodes.h

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -699,7 +699,8 @@ typedef struct SubPlan
699699
boolunknownEqFalse;/* TRUE if it's okay to return FALSE when the
700700
* spec result is UNKNOWN; this allows much
701701
* simpler handling of null values */
702-
boolparallel_safe;/* OK to use as part of parallel plan? */
702+
boolparallel_safe;/* is the subplan parallel-safe? */
703+
/* Note: parallel_safe does not consider contents of testexpr or args */
703704
/* Information for passing params into and out of the subselect: */
704705
/* setParam and parParam are lists of integers (param IDs) */
705706
List*setParam;/* initplan subqueries have to set these

‎src/test/regress/expected/select_parallel.out

Lines changed: 12 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -126,6 +126,18 @@ select count(*) from tenk1 where (two, four) not in
126126
10000
127127
(1 row)
128128

129+
-- this is not parallel-safe due to use of random() within SubLink's testexpr:
130+
explain (costs off)
131+
select * from tenk1 where (unique1 + random())::integer not in
132+
(select ten from tenk2);
133+
QUERY PLAN
134+
------------------------------------
135+
Seq Scan on tenk1
136+
Filter: (NOT (hashed SubPlan 1))
137+
SubPlan 1
138+
-> Seq Scan on tenk2
139+
(4 rows)
140+
129141
alter table tenk2 reset (parallel_workers);
130142
-- test parallel index scans.
131143
set enable_seqscan to off;

‎src/test/regress/sql/select_parallel.sql

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -46,6 +46,10 @@ explain (costs off)
4646
(select hundred, thousandfrom tenk2where thousand>100);
4747
selectcount(*)from tenk1where (two, four) notin
4848
(select hundred, thousandfrom tenk2where thousand>100);
49+
-- this is not parallel-safe due to use of random() within SubLink's testexpr:
50+
explain (costs off)
51+
select*from tenk1where (unique1+ random())::integer notin
52+
(select tenfrom tenk2);
4953
altertable tenk2 reset (parallel_workers);
5054

5155
-- test parallel index scans.

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