prohibit_shutdown_backward_scans_v2.patch

From cd4a6348dc2b925a6da1c4464417a94074687e5a Mon Sep 17 00:00:00 2001
From: Amit Kapila <akapila@postgresql.org>
Date: Fri, 3 Aug 2018 13:34:41 +0530
Subject: [PATCH] Prohibit shutting down resources if there is a possibility of
 back up.

Reported-by: Robert Haas
Analysed-by: Robert Haas and Amit Kapila
Author: Amit Kapila
Reviewed-by: Robert Haas
Backpatch-through: 9.6 where this code was introduced
Discussion: https://postgr.es/m/86137f17-1dfb-42f9-7421-82fd786b04a1@anayrat.info
---
 src/backend/executor/execMain.c  | 16 ++++++++++++----
 src/backend/executor/nodeLimit.c | 10 ++++++++--
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 01e1a46..0a58688 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -1726,8 +1726,12 @@ ExecutePlan(EState *estate,
 		 */
 		if (TupIsNull(slot))
 		{
-			/* Allow nodes to release or shut down resources. */
-			(void) ExecShutdownNode(planstate);
+			/*
+			 * If we know we won't need to back up, we can release
+			 * resources at this point.
+			 */
+			if (!(estate->es_top_eflags & EXEC_FLAG_BACKWARD))
+				(void) ExecShutdownNode(planstate);
 			break;
 		}
 
@@ -1773,8 +1777,12 @@ ExecutePlan(EState *estate,
 		current_tuple_count++;
 		if (numberTuples && numberTuples == current_tuple_count)
 		{
-			/* Allow nodes to release or shut down resources. */
-			(void) ExecShutdownNode(planstate);
+			/*
+			 * If we know we won't need to back up, we can release
+			 * resources at this point.
+			 */
+			if (!(estate->es_top_eflags & EXEC_FLAG_BACKWARD))
+				(void) ExecShutdownNode(planstate);
 			break;
 		}
 	}
diff --git a/src/backend/executor/nodeLimit.c b/src/backend/executor/nodeLimit.c
index 66ea6aa..bb28cf7 100644
--- a/src/backend/executor/nodeLimit.c
+++ b/src/backend/executor/nodeLimit.c
@@ -134,8 +134,14 @@ ExecLimit(PlanState *pstate)
 					node->position - node->offset >= node->count)
 				{
 					node->lstate = LIMIT_WINDOWEND;
-					/* Allow nodes to release or shut down resources. */
-					(void) ExecShutdownNode(outerPlan);
+
+					/*
+					 * If we know we won't need to back up, we can release
+					 * resources at this point.
+					 */
+					if (!(node->ps.state->es_top_eflags & EXEC_FLAG_BACKWARD))
+						(void) ExecShutdownNode(outerPlan);
+
 					return NULL;
 				}
 
-- 
1.8.3.1