Fix corner-case planner failure for MERGE.

MERGE planning could fail with "variable not found in subplan target
list" if the target table is partitioned and all its partitions are
excluded at plan time, or in the case where it has no partitions but
used to have some.  This happened because distribute_row_identity_vars
thought it didn't need to make the target table's reltarget list
fully valid; but if we generate a join plan then that is required
because the dummy Result node's tlist will be made from the reltarget.

The same logic appears in distribute_row_identity_vars in v14,
but AFAICS the problem is unreachable in that branch for lack of
MERGE.  In other updating statements, the target table is always
inner-joined to any other tables, so if the target is known dummy
then the whole plan reduces to dummy, so no join nodes are created.
So I'll refrain from back-patching this code change to v14 for now.

Per report from Alvaro Herrera.

Discussion: https://postgr.es/m/20230328112248.6as34mlx5sr4kltg@alvherre.pgsql

src/backend/optimizer/util/appendinfo.c

@@ -21,6 +21,7 @@
 #include "nodes/nodeFuncs.h"
 #include "optimizer/appendinfo.h"
 #include "optimizer/pathnode.h"
+#include "optimizer/planmain.h"
 #include "parser/parsetree.h"
 #include "utils/lsyscache.h"
 #include "utils/rel.h"
@@ -994,9 +995,10 @@ distribute_row_identity_vars(PlannerInfo *root)
 	 * certainly process no rows.  Handle this edge case by re-opening the top
 	 * result relation and adding the row identity columns it would have used,
 	 * as preprocess_targetlist() would have done if it weren't marked "inh".
-	 * (This is a bit ugly, but it seems better to confine the ugliness and
-	 * extra cycles to this unusual corner case.)  We needn't worry about
-	 * fixing the rel's reltarget, as that won't affect the finished plan.
+	 * Then re-run build_base_rel_tlists() to ensure that the added columns
+	 * get propagated to the relation's reltarget.  (This is a bit ugly, but
+	 * it seems better to confine the ugliness and extra cycles to this
+	 * unusual corner case.)
 	 */
 	if (root->row_identity_vars == NIL)
 	{
@@ -1006,6 +1008,8 @@ distribute_row_identity_vars(PlannerInfo *root)
 		add_row_identity_columns(root, result_relation,
 								 target_rte, target_relation);
 		table_close(target_relation, NoLock);
+		build_base_rel_tlists(root, root->processed_tlist);
+		/* There are no ROWID_VAR Vars in this case, so we're done. */
 		return;
 	}
 

src/test/regress/expected/merge.out

@@ -1849,6 +1849,32 @@ TABLE pa_target;
    2
 (2 rows)
 
+-- Partition-less partitioned table
+-- (the bug we are checking for appeared only if table had partitions before)
+DROP TABLE pa_targetp;
+EXPLAIN (VERBOSE, COSTS OFF)
+MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
+  WHEN NOT MATCHED THEN INSERT VALUES (s.sid);
+                 QUERY PLAN                 
+--------------------------------------------
+ Merge on public.pa_target t
+   ->  Hash Left Join
+         Output: s.sid, t.ctid
+         Inner Unique: true
+         Hash Cond: (s.sid = t.tid)
+         ->  Seq Scan on public.pa_source s
+               Output: s.sid
+         ->  Hash
+               Output: t.tid, t.ctid
+               ->  Result
+                     Output: t.tid, t.ctid
+                     One-Time Filter: false
+(12 rows)
+
+MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
+  WHEN NOT MATCHED THEN INSERT VALUES (s.sid);
+ERROR:  no partition of relation "pa_target" found for row
+DETAIL:  Partition key of the failing row contains (tid) = (1).
 DROP TABLE pa_source;
 DROP TABLE pa_target CASCADE;
 -- some complex joins on the source side

src/test/regress/sql/merge.sql

@@ -1169,6 +1169,18 @@ MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
 
 TABLE pa_target;
 
+-- Partition-less partitioned table
+-- (the bug we are checking for appeared only if table had partitions before)
+
+DROP TABLE pa_targetp;
+
+EXPLAIN (VERBOSE, COSTS OFF)
+MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
+  WHEN NOT MATCHED THEN INSERT VALUES (s.sid);
+
+MERGE INTO pa_target t USING pa_source s ON t.tid = s.sid
+  WHEN NOT MATCHED THEN INSERT VALUES (s.sid);
+
 DROP TABLE pa_source;
 DROP TABLE pa_target CASCADE;