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

Ron Johnson <ronljohnsonjr@gmail.com> writes:
> On Sun, Jan 14, 2024 at 6:18 AM Yongtao Huang <yongtaoh2022@gmail.com>
> wrote:
>> 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?

> You can't scan just one column of a row-oriented table.
> The real question is why it mentions c2.

The planner did that so that the SeqScan step doesn't have to
perform a projection: it can just return (a pointer to)
the physical tuple it found in the table, without doing extra
work to form a tuple containing only c1.  The upper HashAgg
step won't really care.  See use_physical_tlist() in createplan.c.

What I'm confused about is why 9.4 didn't do the same.
That optimization heuristic is very old, and certainly
would be applied by 9.4 in some circumstances.  Testing
says the behavior in this specific case changed at 9.6.
I'm not quite interested enough to drill down further...

            regards, tom lane