4 files changed, 70 insertions, 13 deletions

diff --git a/src/backend/executor/execMain.c b/src/backend/executor/execMain.c
index 0493b7d5365..e9bd98c7738 100644
--- a/src/backend/executor/execMain.c
+++ b/src/backend/executor/execMain.c
@@ -1006,7 +1006,7 @@ InitPlan(QueryDesc *queryDesc, int eflags)
 				case ROW_MARK_SHARE:
 				case ROW_MARK_KEYSHARE:
 				case ROW_MARK_REFERENCE:
-					relation = ExecGetRangeTableRelation(estate, rc->rti);
+					relation = ExecGetRangeTableRelation(estate, rc->rti, false);
 					break;
 				case ROW_MARK_COPY:
 					/* no physical table access is required */
diff --git a/src/backend/executor/execPartition.c b/src/backend/executor/execPartition.c
index 5cd5e2eeb80..84ccd7d457d 100644
--- a/src/backend/executor/execPartition.c
+++ b/src/backend/executor/execPartition.c
@@ -1819,6 +1819,7 @@ adjust_partition_colnos_using_map(List *colnos, AttrMap *attrMap)
 void
 ExecDoInitialPruning(EState *estate)
 {
+	PlannedStmt *stmt = estate->es_plannedstmt;
 	ListCell   *lc;
 	List	   *locked_relids = NIL;
 
@@ -1868,6 +1869,34 @@ ExecDoInitialPruning(EState *estate)
 	}
 
 	/*
+	 * Lock the first result relation of each ModifyTable node, even if it was
+	 * pruned.  This is required for ExecInitModifyTable(), which keeps its
+	 * first result relation if all other result relations have been pruned,
+	 * because some executor paths (e.g., in nodeModifyTable.c and
+	 * execPartition.c) rely on there being at least one result relation.
+	 *
+	 * There's room for improvement here --- we actually only need to do this
+	 * if all other result relations of the ModifyTable node were pruned, but
+	 * we don't have an easy way to tell that here.
+	 */
+	if (stmt->resultRelations && ExecShouldLockRelations(estate))
+	{
+		foreach(lc, stmt->firstResultRels)
+		{
+			Index		firstResultRel = lfirst_int(lc);
+
+			if (!bms_is_member(firstResultRel, estate->es_unpruned_relids))
+			{
+				RangeTblEntry *rte = exec_rt_fetch(firstResultRel, estate);
+
+				Assert(rte->rtekind == RTE_RELATION && rte->rellockmode != NoLock);
+				LockRelationOid(rte->relid, rte->rellockmode);
+				locked_relids = lappend_int(locked_relids, firstResultRel);
+			}
+		}
+	}
+
+	/*
 	 * Release the useless locks if the plan won't be executed.  This is the
 	 * same as what CheckCachedPlan() in plancache.c does.
 	 */
@@ -2076,7 +2105,7 @@ CreatePartitionPruneState(EState *estate, PartitionPruneInfo *pruneinfo,
 			 * because that entry will be held open and locked for the
 			 * duration of this executor run.
 			 */
-			partrel = ExecGetRangeTableRelation(estate, pinfo->rtindex);
+			partrel = ExecGetRangeTableRelation(estate, pinfo->rtindex, false);
 
 			/* Remember for InitExecPartitionPruneContext(). */
 			pprune->partrel = partrel;
diff --git a/src/backend/executor/execUtils.c b/src/backend/executor/execUtils.c
index 39d6f4d819e..55ab18fb826 100644
--- a/src/backend/executor/execUtils.c
+++ b/src/backend/executor/execUtils.c
@@ -746,7 +746,7 @@ ExecOpenScanRelation(EState *estate, Index scanrelid, int eflags)
 	Relation	rel;
 
 	/* Open the relation. */
-	rel = ExecGetRangeTableRelation(estate, scanrelid);
+	rel = ExecGetRangeTableRelation(estate, scanrelid, false);
 
 	/*
 	 * Complain if we're attempting a scan of an unscannable relation, except
@@ -815,18 +815,22 @@ ExecInitRangeTable(EState *estate, List *rangeTable, List *permInfos,
  *
  * The Relations will be closed in ExecEndPlan().
  *
- * Note: The caller must ensure that 'rti' refers to an unpruned relation
- * (i.e., it is a member of estate->es_unpruned_relids) before calling this
- * function. Attempting to open a pruned relation will result in an error.
+ * If isResultRel is true, the relation is being used as a result relation.
+ * Such a relation might have been pruned, which is OK for result relations,
+ * but not for scan relations; see the details in ExecInitModifyTable(). If
+ * isResultRel is false, the caller must ensure that 'rti' refers to an
+ * unpruned relation (i.e., it is a member of estate->es_unpruned_relids)
+ * before calling this function. Attempting to open a pruned relation for
+ * scanning will result in an error.
  */
 Relation
-ExecGetRangeTableRelation(EState *estate, Index rti)
+ExecGetRangeTableRelation(EState *estate, Index rti, bool isResultRel)
 {
 	Relation	rel;
 
 	Assert(rti > 0 && rti <= estate->es_range_table_size);
 
-	if (!bms_is_member(rti, estate->es_unpruned_relids))
+	if (!isResultRel && !bms_is_member(rti, estate->es_unpruned_relids))
 		elog(ERROR, "trying to open a pruned relation");
 
 	rel = estate->es_relations[rti - 1];
@@ -880,7 +884,7 @@ ExecInitResultRelation(EState *estate, ResultRelInfo *resultRelInfo,
 {
 	Relation	resultRelationDesc;
 
-	resultRelationDesc = ExecGetRangeTableRelation(estate, rti);
+	resultRelationDesc = ExecGetRangeTableRelation(estate, rti, true);
 	InitResultRelInfo(resultRelInfo,
 					  resultRelationDesc,
 					  rti,
diff --git a/src/backend/executor/nodeModifyTable.c b/src/backend/executor/nodeModifyTable.c
index b0fe50075ad..87c820276a8 100644
--- a/src/backend/executor/nodeModifyTable.c
+++ b/src/backend/executor/nodeModifyTable.c
@@ -4471,6 +4471,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	ModifyTableState *mtstate;
 	Plan	   *subplan = outerPlan(node);
 	CmdType		operation = node->operation;
+	int			total_nrels = list_length(node->resultRelations);
 	int			nrels;
 	List	   *resultRelations = NIL;
 	List	   *withCheckOptionLists = NIL;
@@ -4490,13 +4491,35 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	/*
 	 * Only consider unpruned relations for initializing their ResultRelInfo
 	 * struct and other fields such as withCheckOptions, etc.
+	 *
+	 * Note: We must avoid pruning every result relation.  This is important
+	 * for MERGE, since even if every result relation is pruned from the
+	 * subplan, there might still be NOT MATCHED rows, for which there may be
+	 * INSERT actions to perform.  To allow these actions to be found, at
+	 * least one result relation must be kept.  Also, when inserting into a
+	 * partitioned table, ExecInitPartitionInfo() needs a ResultRelInfo struct
+	 * as a reference for building the ResultRelInfo of the target partition.
+	 * In either case, it doesn't matter which result relation is kept, so we
+	 * just keep the first one, if all others have been pruned.  See also,
+	 * ExecDoInitialPruning(), which ensures that this first result relation
+	 * has been locked.
 	 */
 	i = 0;
 	foreach(l, node->resultRelations)
 	{
 		Index		rti = lfirst_int(l);
+		bool		keep_rel;
+
+		keep_rel = bms_is_member(rti, estate->es_unpruned_relids);
+		if (!keep_rel && i == total_nrels - 1 && resultRelations == NIL)
+		{
+			/* all result relations pruned; keep the first one */
+			keep_rel = true;
+			rti = linitial_int(node->resultRelations);
+			i = 0;
+		}
 
-		if (bms_is_member(rti, estate->es_unpruned_relids))
+		if (keep_rel)
 		{
 			resultRelations = lappend_int(resultRelations, rti);
 			if (node->withCheckOptionLists)
@@ -4537,6 +4560,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 		i++;
 	}
 	nrels = list_length(resultRelations);
+	Assert(nrels > 0);
 
 	/*
 	 * create state structure
@@ -4735,7 +4759,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	 */
 	mtstate->mt_resultOidAttno =
 		ExecFindJunkAttributeInTlist(subplan->targetlist, "tableoid");
-	Assert(AttributeNumberIsValid(mtstate->mt_resultOidAttno) || nrels == 1);
+	Assert(AttributeNumberIsValid(mtstate->mt_resultOidAttno) || total_nrels == 1);
 	mtstate->mt_lastResultOid = InvalidOid; /* force lookup at first tuple */
 	mtstate->mt_lastResultIndex = 0;	/* must be zero if no such attr */
 
@@ -4832,7 +4856,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	if (node->onConflictAction != ONCONFLICT_NONE)
 	{
 		/* insert may only have one relation, inheritance is not expanded */
-		Assert(nrels == 1);
+		Assert(total_nrels == 1);
 		resultRelInfo->ri_onConflictArbiterIndexes = node->arbiterIndexes;
 	}
 
@@ -4979,7 +5003,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 	if (operation == CMD_INSERT)
 	{
 		/* insert may only have one relation, inheritance is not expanded */
-		Assert(nrels == 1);
+		Assert(total_nrels == 1);
 		resultRelInfo = mtstate->resultRelInfo;
 		if (!resultRelInfo->ri_usesFdwDirectModify &&
 			resultRelInfo->ri_FdwRoutine != NULL &&