Ensure first ModifyTable rel initialized if all are pruned

Commit cbc127917e introduced tracking of unpruned relids to avoid
processing pruned relations, and changed ExecInitModifyTable() to
initialize only unpruned result relations. As a result, MERGE
statements that prune all target partitions can now lead to crashes
or incorrect behavior during execution.

The crash occurs because some executor code paths rely on
ModifyTableState.resultRelInfo[0] being present and initialized,
even when no result relations remain after pruning. For example,
ExecMerge() and ExecMergeNotMatched() use the first resultRelInfo
to determine the appropriate action. Similarly,
ExecInitPartitionInfo() assumes that at least one result relation
exists.

To preserve these assumptions, ExecInitModifyTable() now includes the
first result relation in the initialized result relation list if all
result relations for that ModifyTable were pruned. To enable that,
ExecDoInitialPruning() ensures the first relation is locked if it was
pruned and locking is necessary.

To support this exception to the pruning logic, PlannedStmt now
includes a list of RT indexes identifying the first result relation
of each ModifyTable node in the plan. This allows
ExecDoInitialPruning() to check whether each such relation was
pruned and, if so, lock it if necessary.

Bug: #18830
Reported-by: Robins Tharakan <tharakan@gmail.com>
Diagnozed-by: Tender Wang <tndrwang@gmail.com>
Diagnozed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Co-authored-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Reviewed-by: Tender Wang <tndrwang@gmail.com>
Reviewed-by: Dean Rasheed <dean.a.rasheed@gmail.com>
Discussion: https://postgr.es/m/18830-1f31ea1dc930d444%40postgresql.org

1 files changed, 28 insertions, 4 deletions

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 &&