Rewrite interaction of parallel mode with parallel executor support.

In the previous coding, before returning from ExecutorRun, we'd shut
down all parallel workers.  This was dead wrong if ExecutorRun was
called with a non-zero tuple count; it had the effect of truncating
the query output.  To fix, give ExecutePlan control over whether to
enter parallel mode, and have it refuse to do so if the tuple count
is non-zero.  Rewrite the Gather logic so that it can cope with being
called outside parallel mode.

Commit 7aea8e4f2daa4b39ca9d1309a0c4aadb0f7ed81b is largely to blame
for this problem, though this patch modifies some subsequently-committed
code which relied on the guarantees it purported to make.

1 files changed, 24 insertions, 13 deletions

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 37b7bbd413b..a55022e0a80 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -76,6 +76,7 @@ static void CheckValidRowMarkRel(Relation rel, RowMarkType markType);
 static void ExecPostprocessPlan(EState *estate);
 static void ExecEndPlan(PlanState *planstate, EState *estate);
 static void ExecutePlan(EState *estate, PlanState *planstate,
+			bool use_parallel_mode,
 			CmdType operation,
 			bool sendTuples,
 			long numberTuples,
@@ -243,11 +244,6 @@ standard_ExecutorStart(QueryDesc *queryDesc, int eflags)
 	if (!(eflags & (EXEC_FLAG_SKIP_TRIGGERS | EXEC_FLAG_EXPLAIN_ONLY)))
 		AfterTriggerBeginQuery();
 
-	/* Enter parallel mode, if required by the query. */
-	if (queryDesc->plannedstmt->parallelModeNeeded &&
-		!(eflags & EXEC_FLAG_EXPLAIN_ONLY))
-		EnterParallelMode();
-
 	MemoryContextSwitchTo(oldcontext);
 }
 
@@ -341,15 +337,13 @@ standard_ExecutorRun(QueryDesc *queryDesc,
 	if (!ScanDirectionIsNoMovement(direction))
 		ExecutePlan(estate,
 					queryDesc->planstate,
+					queryDesc->plannedstmt->parallelModeNeeded,
 					operation,
 					sendTuples,
 					count,
 					direction,
 					dest);
 
-	/* Allow nodes to release or shut down resources. */
-	(void) ExecShutdownNode(queryDesc->planstate);
-
 	/*
 	 * shutdown tuple receiver, if we started it
 	 */
@@ -482,11 +476,6 @@ standard_ExecutorEnd(QueryDesc *queryDesc)
 	 */
 	MemoryContextSwitchTo(oldcontext);
 
-	/* Exit parallel mode, if it was required by the query. */
-	if (queryDesc->plannedstmt->parallelModeNeeded &&
-		!(estate->es_top_eflags & EXEC_FLAG_EXPLAIN_ONLY))
-		ExitParallelMode();
-
 	/*
 	 * Release EState and per-query memory context.  This should release
 	 * everything the executor has allocated.
@@ -1529,6 +1518,7 @@ ExecEndPlan(PlanState *planstate, EState *estate)
 static void
 ExecutePlan(EState *estate,
 			PlanState *planstate,
+			bool use_parallel_mode,
 			CmdType operation,
 			bool sendTuples,
 			long numberTuples,
@@ -1549,6 +1539,20 @@ ExecutePlan(EState *estate,
 	estate->es_direction = direction;
 
 	/*
+	 * If a tuple count was supplied, we must force the plan to run without
+	 * parallelism, because we might exit early.
+	 */
+	if (numberTuples != 0)
+		use_parallel_mode = false;
+
+	/*
+	 * If a tuple count was supplied, we must force the plan to run without
+	 * parallelism, because we might exit early.
+	 */
+	if (use_parallel_mode)
+		EnterParallelMode();
+
+	/*
 	 * Loop until we've processed the proper number of tuples from the plan.
 	 */
 	for (;;)
@@ -1566,7 +1570,11 @@ ExecutePlan(EState *estate,
 		 * process so we just end the loop...
 		 */
 		if (TupIsNull(slot))
+		{
+			/* Allow nodes to release or shut down resources. */
+			(void) ExecShutdownNode(planstate);
 			break;
+		}
 
 		/*
 		 * If we have a junk filter, then project a new tuple with the junk
@@ -1603,6 +1611,9 @@ ExecutePlan(EState *estate,
 		if (numberTuples && numberTuples == current_tuple_count)
 			break;
 	}
+
+	if (use_parallel_mode)
+		ExitParallelMode();
 }