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Fix parallel query so it doesn't spoil row estimates above Gather.

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Commit 45be99f8cd5d606086e0a458c9c72910ba8a613d removed GatherPath's
num_workers field, but this is entirely bogus.  Normally, a path's
parallel_workers flag is supposed to indicate the number of workers
that it wants, and should be 0 for a non-partial path.  In that
commit, I mistakenly thought that GatherPath could also use that field
to indicate the number of workers that it would try to start, but
that's disastrous, because then it can propagate up to higher nodes in
the plan tree, which will then get incorrect rowcounts because the
parallel_workers flag is involved in computing those values.  Repair
by putting the separate field back.

Report by Tomas Vondra.  Patch by me, reviewed by Amit Kapila.

Discussion: http://postgr.es/m/f91b4a44-f739-04bd-c4b6-f135bd643669@2ndquadrant.com
This commit is contained in:
Robert Haas 2017-03-31 21:01:20 -04:00
parent 2113ac4cbb
commit 7d8f6986b8
4 changed files with 7 additions and 4 deletions
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1
src/backend/nodes/outfuncs.c
View File

@ -1870,6 +1870,7 @@ _outGatherPath(StringInfo str, const GatherPath *node)
WRITE_NODE_FIELD(subpath);
WRITE_BOOL_FIELD(single_copy);
WRITE_INT_FIELD(num_workers);
}
static void

2
src/backend/optimizer/plan/createplan.c
View File

@ -1446,7 +1446,7 @@ create_gather_plan(PlannerInfo *root, GatherPath *best_path)
gather_plan = make_gather(tlist,
NIL,
best_path->path.parallel_workers,
best_path->num_workers,
best_path->single_copy,
subplan);

7
src/backend/optimizer/util/pathnode.c
View File

@ -1742,16 +1742,17 @@ create_gather_path(PlannerInfo *root, RelOptInfo *rel, Path *subpath,
required_outer);
pathnode->path.parallel_aware = false;
pathnode->path.parallel_safe = false;
pathnode->path.parallel_workers = subpath->parallel_workers;
pathnode->path.parallel_workers = 0;
pathnode->path.pathkeys = NIL; /* Gather has unordered result */
pathnode->subpath = subpath;
pathnode->num_workers = subpath->parallel_workers;
pathnode->single_copy = false;
if (pathnode->path.parallel_workers == 0)
if (pathnode->num_workers == 0)
{
pathnode->path.parallel_workers = 1;
pathnode->path.pathkeys = subpath->pathkeys;
pathnode->num_workers = 1;
pathnode->single_copy = true;
}

1
src/include/nodes/relation.h
View File

@ -1228,6 +1228,7 @@ typedef struct GatherPath
Path path;
Path *subpath; /* path for each worker */
bool single_copy; /* don't execute path more than once */
int num_workers; /* number of workers sought to help */
} GatherPath;
/*