Fix mistaken failure to allow parallelism in corner case.

robertmhaas · robertmhaas · commit 639c1a6bb9ee · 2017-10-27T16:04:01.000+02:00
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
@@ -1702,6 +1702,7 @@ ExecutePlan(EState *estate,
 	if (!execute_once)
 		use_parallel_mode = false;
 
+	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
@@ -156,6 +156,8 @@ CreateExecutorState(void)
 	estate->es_epqScanDone = NULL;
 	estate->es_sourceText = 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
@@ -150,7 +150,7 @@ ExecGather(PlanState *pstate)
 		 * 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/backend/executor/nodeGatherMerge.c b/src/backend/executor/nodeGatherMerge.c
@@ -194,7 +194,7 @@ ExecGatherMerge(PlanState *pstate)
 		 * Sometimes we might have to run without parallelism; but if parallel
 		 * mode is active then we can try to fire up some workers.
 		 */
-		if (gm->num_workers > 0 && IsInParallelMode())
+		if (gm->num_workers > 0 && estate->es_use_parallel_mode)
 		{
 			ParallelContext *pcxt;
 
diff --git a/src/include/nodes/execnodes.h b/src/include/nodes/execnodes.h
@@ -507,6 +507,8 @@ typedef struct EState
 	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? */
+
 	/* The per-query shared memory area to use for parallel execution. */
 	struct dsa_area *es_query_dsa;
 } EState;

Original file line number	Diff line number	Diff line change
`@@ -150,7 +150,7 @@ ExecGather(PlanState *pstate)`
`150`	`150`	`* Sometimes we might have to run without parallelism; but if parallel`
`151`	`151`	`* mode is active then we can try to fire up some workers.`
`152`	`152`	`*/`
`153`		`- if (gather->num_workers > 0 && IsInParallelMode())`
	`153`	`+ if (gather->num_workers > 0 && estate->es_use_parallel_mode)`
`154`	`154`	`{`
`155`	`155`	`ParallelContext *pcxt;`
`156`	`156`
Original file line number	Diff line number	Diff line change
`@@ -194,7 +194,7 @@ ExecGatherMerge(PlanState *pstate)`
`194`	`194`	`* Sometimes we might have to run without parallelism; but if parallel`
`195`	`195`	`* mode is active then we can try to fire up some workers.`
`196`	`196`	`*/`
`197`		`- if (gm->num_workers > 0 && IsInParallelMode())`
	`197`	`+ if (gm->num_workers > 0 && estate->es_use_parallel_mode)`
`198`	`198`	`{`
`199`	`199`	`ParallelContext *pcxt;`
`200`	`200`