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 <[email protected]>
Diagnozed-by: Tender Wang <[email protected]>
Diagnozed-by: Dean Rasheed <[email protected]>
Co-authored-by: Dean Rasheed <[email protected]>
Reviewed-by: Tender Wang <[email protected]>
Reviewed-by: Dean Rasheed <[email protected]>
Discussion: https://postgr.es/m/18830-1f31ea1dc930d444%40postgresql.org

1 files changed, 12 insertions, 2 deletions

diff --git a/src/backend/commands/explain.c b/src/backend/commands/explain.c
index 22616cf7add..33a16d2d8e2 100644
--- a/src/backend/commands/explain.c
+++ b/src/backend/commands/explain.c
@@ -4575,10 +4575,20 @@ show_modifytable_info(ModifyTableState *mtstate, List *ancestors,
 			break;
 	}
 
-	/* Should we explicitly label target relations? */
+	/*
+	 * Should we explicitly label target relations?
+	 *
+	 * If there's only one target relation, do not list it if it's the
+	 * relation named in the query, or if it has been pruned.  (Normally
+	 * mtstate->resultRelInfo doesn't include pruned relations, but a single
+	 * pruned target relation may be present, if all other target relations
+	 * have been pruned.  See ExecInitModifyTable().)
+	 */
 	labeltargets = (mtstate->mt_nrels > 1 ||
 					(mtstate->mt_nrels == 1 &&
-					 mtstate->resultRelInfo[0].ri_RangeTableIndex != node->nominalRelation));
+					 mtstate->resultRelInfo[0].ri_RangeTableIndex != node->nominalRelation &&
+					 bms_is_member(mtstate->resultRelInfo[0].ri_RangeTableIndex,
+								   mtstate->ps.state->es_unpruned_relids)));
 
 	if (labeltargets)
 		ExplainOpenGroup("Target Tables", "Target Tables", false, es);