Simplify executor's determination of whether to use parallelism.

Our parallel-mode code only works when we are executing a query
in full, so ExecutePlan must disable parallel mode when it is
asked to do partial execution.  The previous logic for this
involved passing down a flag (variously named execute_once or
run_once) from callers of ExecutorRun or PortalRun.  This is
overcomplicated, and unsurprisingly some of the callers didn't
get it right, since it requires keeping state that not all of
them have handy; not to mention that the requirements for it were
undocumented.  That led to assertion failures in some corner
cases.  The only state we really need for this is the existing
QueryDesc.already_executed flag, so let's just put all the
responsibility in ExecutePlan.  (It could have been done in
ExecutorRun too, leading to a slightly shorter patch -- but if
there's ever more than one caller of ExecutePlan, it seems better
to have this logic in the subroutine than the callers.)

This makes those ExecutorRun/PortalRun parameters unnecessary.
In master it seems okay to just remove them, returning the
API for those functions to what it was before parallelism.
Such an API break is clearly not okay in stable branches,
but for them we can just leave the parameters in place after
documenting that they do nothing.

Per report from Yugo Nagata, who also reviewed and tested
this patch.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/20241206062549.710dc01cf91224809dd6c0e1@sraoss.co.jp

1 files changed, 22 insertions, 31 deletions

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 5ca856fd279..13f3fcdaee9 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -77,14 +77,12 @@ static void InitPlan(QueryDesc *queryDesc, int eflags);
 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,
+static void ExecutePlan(QueryDesc *queryDesc,
 						CmdType operation,
 						bool sendTuples,
 						uint64 numberTuples,
 						ScanDirection direction,
-						DestReceiver *dest,
-						bool execute_once);
+						DestReceiver *dest);
 static bool ExecCheckOneRelPerms(RTEPermissionInfo *perminfo);
 static bool ExecCheckPermissionsModified(Oid relOid, Oid userid,
 										 Bitmapset *modifiedCols,
@@ -294,18 +292,17 @@ standard_ExecutorStart(QueryDesc *queryDesc, int eflags)
  */
 void
 ExecutorRun(QueryDesc *queryDesc,
-			ScanDirection direction, uint64 count,
-			bool execute_once)
+			ScanDirection direction, uint64 count)
 {
 	if (ExecutorRun_hook)
-		(*ExecutorRun_hook) (queryDesc, direction, count, execute_once);
+		(*ExecutorRun_hook) (queryDesc, direction, count);
 	else
-		standard_ExecutorRun(queryDesc, direction, count, execute_once);
+		standard_ExecutorRun(queryDesc, direction, count);
 }
 
 void
 standard_ExecutorRun(QueryDesc *queryDesc,
-					 ScanDirection direction, uint64 count, bool execute_once)
+					 ScanDirection direction, uint64 count)
 {
 	EState	   *estate;
 	CmdType		operation;
@@ -354,21 +351,12 @@ standard_ExecutorRun(QueryDesc *queryDesc,
 	 * run plan
 	 */
 	if (!ScanDirectionIsNoMovement(direction))
-	{
-		if (execute_once && queryDesc->already_executed)
-			elog(ERROR, "can't re-execute query flagged for single execution");
-		queryDesc->already_executed = true;
-
-		ExecutePlan(estate,
-					queryDesc->planstate,
-					queryDesc->plannedstmt->parallelModeNeeded,
+		ExecutePlan(queryDesc,
 					operation,
 					sendTuples,
 					count,
 					direction,
-					dest,
-					execute_once);
-	}
+					dest);
 
 	/*
 	 * Update es_total_processed to keep track of the number of tuples
@@ -1601,22 +1589,19 @@ ExecCloseRangeTableRelations(EState *estate)
  *		moving in the specified direction.
  *
  *		Runs to completion if numberTuples is 0
- *
- * Note: the ctid attribute is a 'junk' attribute that is removed before the
- * user can see it
  * ----------------------------------------------------------------
  */
 static void
-ExecutePlan(EState *estate,
-			PlanState *planstate,
-			bool use_parallel_mode,
+ExecutePlan(QueryDesc *queryDesc,
 			CmdType operation,
 			bool sendTuples,
 			uint64 numberTuples,
 			ScanDirection direction,
-			DestReceiver *dest,
-			bool execute_once)
+			DestReceiver *dest)
 {
+	EState	   *estate = queryDesc->estate;
+	PlanState  *planstate = queryDesc->planstate;
+	bool		use_parallel_mode;
 	TupleTableSlot *slot;
 	uint64		current_tuple_count;
 
@@ -1631,11 +1616,17 @@ ExecutePlan(EState *estate,
 	estate->es_direction = direction;
 
 	/*
-	 * If the plan might potentially be executed multiple times, we must force
-	 * it to run without parallelism, because we might exit early.
+	 * Set up parallel mode if appropriate.
+	 *
+	 * Parallel mode only supports complete execution of a plan.  If we've
+	 * already partially executed it, or if the caller asks us to exit early,
+	 * we must force the plan to run without parallelism.
 	 */
-	if (!execute_once)
+	if (queryDesc->already_executed || numberTuples != 0)
 		use_parallel_mode = false;
+	else
+		use_parallel_mode = queryDesc->plannedstmt->parallelModeNeeded;
+	queryDesc->already_executed = true;
 
 	estate->es_use_parallel_mode = use_parallel_mode;
 	if (use_parallel_mode)