On Sun, Jan 14, 2024 at 6:18 AM Yongtao Huang <yongtaoh2022@xxxxxxxxx> wrote:
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.
You can't scan just one column of a row-oriented table.
The real question is why it mentions c2.
