Re: Why scan all columns when we select distinct c1?

On Sun, Jan 14, 2024 at 6:18 AM Yongtao Huang <yongtaoh2022(at)gmail(dot)com>
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.