Incorrect explain output for updates/delete operations with returning-list on partitioned tables

Hi PostgreSQL Community,

I have been working on partitioned tables recently, and I have noticed
something that doesn't seem correct with the EXPLAIN output of an
update/delete query with a returning list.

For example, consider two partitioned tables, "t1" and "t2," with
partitions "t11," "t12," and "t21," "t22," respectively. The table
definitions are as follows:

```sql
postgres=# \d+ t1
Partitioned table "public.t1"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
a | integer | | | | plain |
| |
b | integer | | | | plain |
| |
c | integer | | | | plain |
| |
Partition key: RANGE (a)
Partitions: t11 FOR VALUES FROM (0) TO (1000),
t12 FOR VALUES FROM (1000) TO (10000)

postgres=# \d+ t2
Partitioned table "public.t2"
Column | Type | Collation | Nullable | Default | Storage | Compression
| Stats target | Description
--------+---------+-----------+----------+---------+---------+-------------+--------------+-------------
a | integer | | | | plain |
| |
b | integer | | | | plain |
| |
c | integer | | | | plain |
| |
Partition key: RANGE (a)
Partitions: t21 FOR VALUES FROM (0) TO (1000),
t22 FOR VALUES FROM (1000) TO (10000)
```

The EXPLAIN output for an update query with a returning list doesn't seem
correct to me. Here are the examples (the part that doesn't seem right is
highlighted in bold):

*Query1:*
```
postgres=# explain verbose update t1 set b = 10 from t2 where t1.a = t2.a
returning t1.c;
QUERY PLAN

-------------------------------------------------------------------------------------------
Update on public.t1 (cost=0.00..125187.88 rows=41616 width=14)
*Output: t1_1.c -----> something not right??*
Update on public.t11 t1_1
Update on public.t12 t1_2
-> Append (cost=0.00..125187.88 rows=41616 width=14)
-> Nested Loop (cost=0.00..62489.90 rows=20808 width=14)
Output: 10, t1_1.tableoid, t1_1.ctid
Join Filter: (t1_1.a = t2_1.a)
-> Seq Scan on public.t11 t1_1 (cost=0.00..30.40 rows=2040
width=14)
Output: t1_1.a, t1_1.tableoid, t1_1.ctid
-> Materialize (cost=0.00..40.60 rows=2040 width=4)
Output: t2_1.a
-> Seq Scan on public.t21 t2_1 (cost=0.00..30.40
rows=2040 width=4)
Output: t2_1.a
-> Nested Loop (cost=0.00..62489.90 rows=20808 width=14)
Output: 10, t1_2.tableoid, t1_2.ctid
Join Filter: (t1_2.a = t2_2.a)
-> Seq Scan on public.t12 t1_2 (cost=0.00..30.40 rows=2040
width=14)
Output: t1_2.a, t1_2.tableoid, t1_2.ctid
-> Materialize (cost=0.00..40.60 rows=2040 width=4)
Output: t2_2.a
-> Seq Scan on public.t22 t2_2 (cost=0.00..30.40
rows=2040 width=4)
Output: t2_2.a
(23 rows)
```

*Query2:*

*```*postgres=# explain verbose update t1 set b = 10 from t2 where t1.a =
t2.a returning t2.c;
QUERY PLAN

-------------------------------------------------------------------------------------------
Update on public.t1 (cost=0.00..125187.88 rows=41616 width=18)
*Output: t2.c*
Update on public.t11 t1_1
Update on public.t12 t1_2
-> Append (cost=0.00..125187.88 rows=41616 width=18)
-> Nested Loop (cost=0.00..62489.90 rows=20808 width=18)
Output: 10, t2_1.c, t1_1.tableoid, t1_1.ctid
Join Filter: (t1_1.a = t2_1.a)
-> Seq Scan on public.t11 t1_1 (cost=0.00..30.40 rows=2040
width=14)
Output: t1_1.a, t1_1.tableoid, t1_1.ctid
-> Materialize (cost=0.00..40.60 rows=2040 width=8)
Output: t2_1.c, t2_1.a
-> Seq Scan on public.t21 t2_1 (cost=0.00..30.40
rows=2040 width=8)
Output: t2_1.c, t2_1.a
-> Nested Loop (cost=0.00..62489.90 rows=20808 width=18)
Output: 10, t2_2.c, t1_2.tableoid, t1_2.ctid
Join Filter: (t1_2.a = t2_2.a)
-> Seq Scan on public.t12 t1_2 (cost=0.00..30.40 rows=2040
width=14)
Output: t1_2.a, t1_2.tableoid, t1_2.ctid
-> Materialize (cost=0.00..40.60 rows=2040 width=8)
Output: t2_2.c, t2_2.a
-> Seq Scan on public.t22 t2_2 (cost=0.00..30.40
rows=2040 width=8)
Output: t2_2.c, t2_2.a
(23 rows)
```

After further investigation into the code, I noticed following:

1. In the 'grouping_planner()' function, while generating paths for the
final relation (
https://github.com/postgres/postgres/blob/master/src/backend/optimizer/plan/planner.c#L1857),
we only take care of adjusting the append_rel_attributes in returningList
for resultRelation. Shouldn't we do that for other relations as well in
query? Example for *Query2* above, *adjust_appendrel_attrs_multilevel* is a
no-op.
2. After plan creation (
https://github.com/postgres/postgres/blob/master/src/backend/optimizer/plan/createplan.c#L351),
shouldn't we perform tlist labeling for the `returningList` as well? I
suspect this is resulting in incorrect output in *Query1*.

I suspect that similar issues might also be present for `withCheckOptions`,
`mergeActionList`, and `mergeJoinCondition`.

I would appreciate it if the community could provide insights or
clarifications regarding this observation.

Thank you for your time and consideration.

Regards
Saikiran Avula,
SDE, Amazon Web Services.