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Commit2ccd8fb

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Consider parallel awareness when removing single-child Appends
8edd0e7 added some code to remove Append and MergeAppend nodes when theycontained a single child node. As it turned out, this was unsafe to dowhen the Append/MergeAppend was parallel_aware and the child node was not.Removing the Append/MergeAppend, in this case, could lead to the child planbeing called multiple times by parallel workers when it was unsafe to doso.Here we fix this by just not removing the Append/MergeAppend when theparallel_aware flag of the parent and child node don't match.Reported-by: Yura SokolovBug: #17335Discussion:https://postgr.es/m/b59605fecb20ba9ea94e70ab60098c237c870628.camel%40postgrespro.ruBackpatch-through: 12, where8edd0e7 was first introduced
1 parentfcc4340 commit2ccd8fb

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‎src/backend/optimizer/plan/setrefs.c

Lines changed: 20 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1350,8 +1350,16 @@ set_append_references(PlannerInfo *root,
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lfirst(l)=set_plan_refs(root, (Plan*)lfirst(l),rtoffset);
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}
13521352

1353-
/* Now, if there's just one, forget the Append and return that child */
1354-
if (list_length(aplan->appendplans)==1)
1353+
/*
1354+
* See if it's safe to get rid of the Append entirely. For this to be
1355+
* safe, there must be only one child plan and that child plan's parallel
1356+
* awareness must match that of the Append's. The reason for the latter
1357+
* is that the if the Append is parallel aware and the child is not then
1358+
* the calling plan may execute the non-parallel aware child multiple
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* times.
1360+
*/
1361+
if (list_length(aplan->appendplans)==1&&
1362+
((Plan*)linitial(aplan->appendplans))->parallel_aware==aplan->plan.parallel_aware)
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returnclean_up_removed_plan_level((Plan*)aplan,
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(Plan*)linitial(aplan->appendplans));
13571365

@@ -1412,8 +1420,16 @@ set_mergeappend_references(PlannerInfo *root,
14121420
lfirst(l)=set_plan_refs(root, (Plan*)lfirst(l),rtoffset);
14131421
}
14141422

1415-
/* Now, if there's just one, forget the MergeAppend and return that child */
1416-
if (list_length(mplan->mergeplans)==1)
1423+
/*
1424+
* See if it's safe to get rid of the MergeAppend entirely. For this to
1425+
* be safe, there must be only one child plan and that child plan's
1426+
* parallel awareness must match that of the MergeAppend's. The reason
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* for the latter is that the if the MergeAppend is parallel aware and the
1428+
* child is not then the calling plan may execute the non-parallel aware
1429+
* child multiple times.
1430+
*/
1431+
if (list_length(mplan->mergeplans)==1&&
1432+
((Plan*)linitial(mplan->mergeplans))->parallel_aware==mplan->plan.parallel_aware)
14171433
returnclean_up_removed_plan_level((Plan*)mplan,
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(Plan*)linitial(mplan->mergeplans));
14191435

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