From: Robert Haas <rhaas@postgresql.org>
Date: Fri, 27 Oct 2017 14:04:01 +0000 (+0200)
Subject: Fix mistaken failure to allow parallelism in corner case.
X-Git-Tag: REL9_6_6~22
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=036b6bd50365b9ae6ec0b4cb21d172510ef440a9;p=postgresql

Fix mistaken failure to allow parallelism in corner case.

If we try to run a parallel plan in serial mode because, for example,
it's going to be scanned via a cursor, but for some reason we're
already in parallel mode (for example because an outer query is
running in parallel), we'd incorrectly try to launch workers.
Fix by adding a flag to the EState, so that we can be certain that
ExecutePlan() and ExecGather()/ExecGatherMerge() will have the same
idea about whether we are executing serially or in parallel.

Report and fix by Amit Kapila with help from Kuntal Ghosh.  A few
tweaks by me.

Discussion: http://postgr.es/m/CAA4eK1+_BuZrmVCeua5Eqnm4Co9DAXdM5HPAOE2J19ePbR912Q@mail.gmail.com
---

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 09b8169b6c..d8b46a11f4 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -1548,10 +1548,7 @@ ExecutePlan(EState *estate,
 	if (numberTuples || dest->mydest == DestIntoRel)
 		use_parallel_mode = false;
 
-	/*
-	 * If a tuple count was supplied, we must force the plan to run without
-	 * parallelism, because we might exit early.
-	 */
+	estate->es_use_parallel_mode = use_parallel_mode;
 	if (use_parallel_mode)
 		EnterParallelMode();
 
diff --git a/src/backend/executor/execUtils.c b/src/backend/executor/execUtils.c
index a3bcb100da..54c60a8d44 100644
--- a/src/backend/executor/execUtils.c
+++ b/src/backend/executor/execUtils.c
@@ -140,6 +140,8 @@ CreateExecutorState(void)
 	estate->es_epqTupleSet = NULL;
 	estate->es_epqScanDone = NULL;
 
+	estate->es_use_parallel_mode = false;
+
 	/*
 	 * Return the executor state structure
 	 */
diff --git a/src/backend/executor/nodeGather.c b/src/backend/executor/nodeGather.c
index 7342aadfbb..216759aeab 100644
--- a/src/backend/executor/nodeGather.c
+++ b/src/backend/executor/nodeGather.c
@@ -150,7 +150,7 @@ ExecGather(GatherState *node)
 		 * Sometimes we might have to run without parallelism; but if parallel
 		 * mode is active then we can try to fire up some workers.
 		 */
-		if (gather->num_workers > 0 && IsInParallelMode())
+		if (gather->num_workers > 0 && estate->es_use_parallel_mode)
 		{
 			ParallelContext *pcxt;
 
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
index 53a247ca5d..7621a17167 100644
--- a/src/include/nodes/execnodes.h
+++ b/src/include/nodes/execnodes.h
@@ -421,6 +421,8 @@ typedef struct EState
 	HeapTuple  *es_epqTuple;	/* array of EPQ substitute tuples */
 	bool	   *es_epqTupleSet; /* true if EPQ tuple is provided */
 	bool	   *es_epqScanDone; /* true if EPQ tuple has been fetched */
+
+	bool		es_use_parallel_mode; /* can we use parallel workers? */
 } EState;