Fix EXPLAIN output for subplans in MERGE.

Given a subplan in a MERGE query, EXPLAIN would sometimes fail to
properly display expressions involving Params referencing variables in
other parts of the plan tree.

This would affect subplans outside the topmost join plan node, for
which expansion of Params would go via the top-level ModifyTable plan
node.  The problem was that "inner_tlist" for the ModifyTable node's
deparse_namespace was set to the join node's targetlist, but
"inner_plan" was set to the ModifyTable node itself, rather than the
join node, leading to incorrect results when descending to the
referenced variable.

Fix and backpatch to v15, where MERGE was introduced.

Discussion: https://postgr.es/m/CAEZATCWAv-sZuH%2BwG5xJ-%2BGt7qGNGX8wUQd3XYydMFDKgRB9nw%40mail.gmail.com

src/backend/utils/adt/ruleutils.c

@@ -4973,8 +4973,11 @@ set_deparse_plan(deparse_namespace *dpns, Plan *plan)
 	 * For a WorkTableScan, locate the parent RecursiveUnion plan node and use
 	 * that as INNER referent.
 	 *
-	 * For MERGE, make the inner tlist point to the merge source tlist, which
-	 * is same as the targetlist that the ModifyTable's source plan provides.
+	 * For MERGE, pretend the ModifyTable's source plan (its outer plan) is
+	 * INNER referent.  This is the join from the target relation to the data
+	 * source, and all INNER_VAR Vars in other parts of the query refer to its
+	 * targetlist.
+	 *
 	 * For ON CONFLICT .. UPDATE we just need the inner tlist to point to the
 	 * excluded expression's tlist. (Similar to the SubqueryScan we don't want
 	 * to reuse OUTER, it's used for RETURNING in some modify table cases,
@@ -4989,17 +4992,17 @@ set_deparse_plan(deparse_namespace *dpns, Plan *plan)
 		dpns->inner_plan = find_recursive_union(dpns,
 												(WorkTableScan *) plan);
 	else if (IsA(plan, ModifyTable))
-		dpns->inner_plan = plan;
+	{
+		if (((ModifyTable *) plan)->operation == CMD_MERGE)
+			dpns->inner_plan = outerPlan(plan);
+		else
+			dpns->inner_plan = plan;
+	}
 	else
 		dpns->inner_plan = innerPlan(plan);
 
-	if (IsA(plan, ModifyTable))
-	{
-		if (((ModifyTable *) plan)->operation == CMD_MERGE)
-			dpns->inner_tlist = dpns->outer_tlist;
-		else
-			dpns->inner_tlist = ((ModifyTable *) plan)->exclRelTlist;
-	}
+	if (IsA(plan, ModifyTable) && ((ModifyTable *) plan)->operation == CMD_INSERT)
+		dpns->inner_tlist = ((ModifyTable *) plan)->exclRelTlist;
 	else if (dpns->inner_plan)
 		dpns->inner_tlist = dpns->inner_plan->targetlist;
 	else

src/test/regress/expected/merge.out

@@ -1483,6 +1483,56 @@ WHEN MATCHED AND t.a < 10 THEN
 
 DROP TABLE ex_msource, ex_mtarget;
 DROP FUNCTION explain_merge(text);
+-- EXPLAIN SubPlans and InitPlans
+CREATE TABLE src (a int, b int, c int, d int);
+CREATE TABLE tgt (a int, b int, c int, d int);
+CREATE TABLE ref (ab int, cd int);
+EXPLAIN (verbose, costs off)
+MERGE INTO tgt t
+USING (SELECT *, (SELECT count(*) FROM ref r
+                   WHERE r.ab = s.a + s.b
+                     AND r.cd = s.c - s.d) cnt
+         FROM src s) s
+ON t.a = s.a AND t.b < s.cnt
+WHEN MATCHED AND t.c > s.cnt THEN
+  UPDATE SET (b, c) = (SELECT s.b, s.cnt);
+                                     QUERY PLAN                                      
+-------------------------------------------------------------------------------------
+ Merge on public.tgt t
+   ->  Hash Join
+         Output: t.ctid, s.a, s.b, s.c, s.d, s.ctid
+         Hash Cond: (t.a = s.a)
+         Join Filter: (t.b < (SubPlan 1))
+         ->  Seq Scan on public.tgt t
+               Output: t.ctid, t.a, t.b
+         ->  Hash
+               Output: s.a, s.b, s.c, s.d, s.ctid
+               ->  Seq Scan on public.src s
+                     Output: s.a, s.b, s.c, s.d, s.ctid
+         SubPlan 1
+           ->  Aggregate
+                 Output: count(*)
+                 ->  Seq Scan on public.ref r
+                       Output: r.ab, r.cd
+                       Filter: ((r.ab = (s.a + s.b)) AND (r.cd = (s.c - s.d)))
+   SubPlan 4
+     ->  Aggregate
+           Output: count(*)
+           ->  Seq Scan on public.ref r_2
+                 Output: r_2.ab, r_2.cd
+                 Filter: ((r_2.ab = (s.a + s.b)) AND (r_2.cd = (s.c - s.d)))
+   SubPlan 3 (returns $9,$10)
+     ->  Result
+           Output: s.b, $8
+           InitPlan 2 (returns $8)
+             ->  Aggregate
+                   Output: count(*)
+                   ->  Seq Scan on public.ref r_1
+                         Output: r_1.ab, r_1.cd
+                         Filter: ((r_1.ab = (s.a + s.b)) AND (r_1.cd = (s.c - s.d)))
+(32 rows)
+
+DROP TABLE src, tgt, ref;
 -- Subqueries
 BEGIN;
 MERGE INTO sq_target t

src/test/regress/sql/merge.sql

@@ -948,6 +948,23 @@ WHEN MATCHED AND t.a < 10 THEN
 DROP TABLE ex_msource, ex_mtarget;
 DROP FUNCTION explain_merge(text);
 
+-- EXPLAIN SubPlans and InitPlans
+CREATE TABLE src (a int, b int, c int, d int);
+CREATE TABLE tgt (a int, b int, c int, d int);
+CREATE TABLE ref (ab int, cd int);
+
+EXPLAIN (verbose, costs off)
+MERGE INTO tgt t
+USING (SELECT *, (SELECT count(*) FROM ref r
+                   WHERE r.ab = s.a + s.b
+                     AND r.cd = s.c - s.d) cnt
+         FROM src s) s
+ON t.a = s.a AND t.b < s.cnt
+WHEN MATCHED AND t.c > s.cnt THEN
+  UPDATE SET (b, c) = (SELECT s.b, s.cnt);
+
+DROP TABLE src, tgt, ref;
+
 -- Subqueries
 BEGIN;
 MERGE INTO sq_target t