Fix mistaken failure to allow parallelism in corner case.

robertmhaas · robertmhaas · commit 036b6bd50365 · 2017-10-27T16:12:16.000+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
@@ -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
@@ -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
@@ -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
@@ -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;
 
 

Original file line number	Diff line number	Diff line change
`@@ -150,7 +150,7 @@ ExecGather(GatherState *node)`
`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`