Why scan all columns when we select distinct c1?

[Yongtao Huang]

PostgreSQL version: 16.1
Operating system:   centos7
Description:

Let me show these explain results first, in PG9.4 and PG16.1.

### Behavior in PG9.4
``` SQL
gpadmin=# create table t1 (c1 int, c2 text);
CREATE TABLE
gpadmin=# explain (costs off, verbose) select distinct c1 from t1;
         QUERY PLAN
-----------------------------
 HashAggregate
   Output: c1
   Group Key: t1.c1
   ->  Seq Scan on public.t1
         Output: c1 <---- pay attention <----  !!!
(5 rows)
```

### Behavior in PG 16.1
``` SQL
gpadmin=# create table t1 (c1 int, c2 text);
CREATE TABLE
gpadmin=# explain (costs off, verbose) select distinct c1 from t1;
         QUERY PLAN
-----------------------------
 HashAggregate
   Output: c1
   Group Key: t1.c1
   ->  Seq Scan on public.t1
         Output: c1, c2  <---- pay attention <---- !!!
(5 rows)
```

My question is why scan all columns in PG 16.01?
If `select distinct c1`, scan the column `c1` is enough, like PG 9.4.

Related GPDB issue link: https://github.com/greenplum-db/gpdb/issues/15266

Reporter: David Kimura and Yongtao Huang