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

src/backend/executor/execMain.c

@@ -75,14 +75,12 @@ 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,
@@ -283,6 +281,9 @@ standard_ExecutorStart(QueryDesc *queryDesc, int eflags)
  *		retrieved tuples, not for instance to those inserted/updated/deleted
  *		by a ModifyTable plan node.
  *
+ *		execute_once is ignored, and is present only to avoid an API break
+ *		in stable branches.
+ *
  *		There is no return value, but output tuples (if any) are sent to
  *		the destination receiver specified in the QueryDesc; and the number
  *		of tuples processed at the top level can be found in
@@ -357,21 +358,12 @@ 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
@@ -1600,22 +1592,19 @@ 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;
 
@@ -1630,11 +1619,17 @@ 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)

src/backend/tcop/postgres.c

@@ -1278,7 +1278,7 @@ exec_simple_query(const char *query_string)
 		(void) PortalRun(portal,
 						 FETCH_ALL,
 						 true,	/* always top level */
-						 true,
+						 true,	/* ignored */
 						 receiver,
 						 receiver,
 						 &qc);
@@ -2255,7 +2255,7 @@ exec_execute_message(const char *portal_name, long max_rows)
 	completed = PortalRun(portal,
 						  max_rows,
 						  true, /* always top level */
-						  !execute_is_fetch && max_rows == FETCH_ALL,
+						  true, /* ignored */
 						  receiver,
 						  receiver,
 						  &qc);

src/backend/tcop/pquery.c

@@ -670,6 +670,8 @@ PortalSetResultFormat(Portal portal, int nFormats, int16 *formats)
  * isTopLevel: true if query is being executed at backend "top level"
  * (that is, directly from a client command message)
  *
+ * run_once: ignored, present only to avoid an API break in stable branches.
+ *
  * dest: where to send output of primary (canSetTag) query
  *
  * altdest: where to send output of non-primary queries
@@ -714,10 +716,6 @@ PortalRun(Portal portal, long count, bool isTopLevel, bool run_once,
 	 */
 	MarkPortalActive(portal);
 
-	/* Set run_once flag.  Shouldn't be clear if previously set. */
-	Assert(!portal->run_once || run_once);
-	portal->run_once = run_once;
-
 	/*
 	 * Set up global portal context pointers.
 	 *
@@ -922,7 +920,7 @@ PortalRunSelect(Portal portal,
 		{
 			PushActiveSnapshot(queryDesc->snapshot);
 			ExecutorRun(queryDesc, direction, (uint64) count,
-						portal->run_once);
+						false);
 			nprocessed = queryDesc->estate->es_processed;
 			PopActiveSnapshot();
 		}
@@ -962,7 +960,7 @@ PortalRunSelect(Portal portal,
 		{
 			PushActiveSnapshot(queryDesc->snapshot);
 			ExecutorRun(queryDesc, direction, (uint64) count,
-						portal->run_once);
+						false);
 			nprocessed = queryDesc->estate->es_processed;
 			PopActiveSnapshot();
 		}
@@ -1406,9 +1404,6 @@ PortalRunFetch(Portal portal,
 	 */
 	MarkPortalActive(portal);
 
-	/* If supporting FETCH, portal can't be run-once. */
-	Assert(!portal->run_once);
-
 	/*
 	 * Set up global portal context pointers.
 	 */

src/include/executor/execdesc.h

@@ -48,7 +48,7 @@ typedef struct QueryDesc
 	EState	   *estate;			/* executor's query-wide state */
 	PlanState  *planstate;		/* tree of per-plan-node state */
 
-	/* This field is set by ExecutorRun */
+	/* This field is set by ExecutePlan */
 	bool		already_executed;	/* true if previously executed */
 
 	/* This is always set NULL by the core system, but plugins can change it */

src/include/utils/portal.h

@@ -145,7 +145,7 @@ typedef struct PortalData
 	/* Features/options */
 	PortalStrategy strategy;	/* see above */
 	int			cursorOptions;	/* DECLARE CURSOR option bits */
-	bool		run_once;		/* portal will only be run once */
+	bool		run_once;		/* unused */
 
 	/* Status data */
 	PortalStatus status;		/* see above */