From: Robert Haas <rhaas@postgresql.org>
Date: Sat, 1 Apr 2017 01:01:20 +0000 (-0400)
Subject: Fix parallel query so it doesn't spoil row estimates above Gather.
X-Git-Tag: REL9_6_3~58
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=fb1879c374581c0f0e251e94c0087c8385b91f5b;p=postgresql

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

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
---

diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 0ccd75b74a..f8d43dbf99 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -1795,6 +1795,7 @@ _outGatherPath(StringInfo str, const GatherPath *node)
 
 	WRITE_NODE_FIELD(subpath);
 	WRITE_BOOL_FIELD(single_copy);
+	WRITE_INT_FIELD(num_workers);
 }
 
 static void
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index 427eedea91..71678d08dc 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -1395,7 +1395,7 @@ 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);
 
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index ce7ad545a9..fc22951e68 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -1681,16 +1681,17 @@ 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;
 	}
 
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index b97b700ee4..48862d93ae 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -1189,6 +1189,7 @@ typedef struct GatherPath
 	Path		path;
 	Path	   *subpath;		/* path for each worker */
 	bool		single_copy;	/* path must not be executed >1x */
+	int			num_workers;	/* number of workers sought to help */
 } GatherPath;
 
 /*