From 7d8f6986b83c9a56f6ea11c959cdd8f52e1b543d Mon Sep 17 00:00:00 2001
From: Robert Haas <rhaas@postgresql.org>
Date: Fri, 31 Mar 2017 21:01:20 -0400
Subject: [PATCH] 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
---
src/backend/nodes/outfuncs.c | 1 +
src/backend/optimizer/plan/createplan.c | 2 +-
src/backend/optimizer/util/pathnode.c | 7 ++++---
src/include/nodes/relation.h | 1 +
4 files changed, 7 insertions(+), 4 deletions(-)
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index bbb63a4bfae..0b45c25a499 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -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
diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index d357479829f..ed06a8de787 100644
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -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);
diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index 999ebcee704..c6298072c9b 100644
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -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;
}
diff --git a/src/include/nodes/relation.h b/src/include/nodes/relation.h
index 8930edf8264..ebf9480f377 100644
--- a/src/include/nodes/relation.h
+++ b/src/include/nodes/relation.h
@@ -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;
/*
--
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