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

2 files changed, 67 insertions, 0 deletions

diff --git a/src/test/regress/expected/merge.out b/src/test/regress/expected/merge.out
index e3ebf46e611..1a6f6ad43d1 100644
--- a/src/test/regress/expected/merge.out
+++ b/src/test/regress/expected/merge.out
@@ -1473,6 +1473,56 @@ 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
diff --git a/src/test/regress/sql/merge.sql b/src/test/regress/sql/merge.sql
index 79a18684e48..dbbba2a1cdd 100644
--- a/src/test/regress/sql/merge.sql
+++ b/src/test/regress/sql/merge.sql
@@ -939,6 +939,23 @@ 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