Re: Remove useless GROUP BY columns considering unique index

On Fri, Nov 22, 2024 at 9:24 PM jian he <jian(dot)universality(at)gmail(dot)com> wrote:
>
> overall i come up with the attached patch.
in v2:

create table t1 (a int, b int not null, c int, unique(b, c));
explain(costs off) select count(*) from t1 group by b,c,a;
QUERY PLAN
----------------------
HashAggregate
Group Key: b
-> Seq Scan on t1

so the v2 implementation is wrong.
I overlooked cases like: only part of the unique-key is not-null.
In that situation, we cannot remove useless groupby columns.

v3 attached. in v3:
QUERY PLAN
----------------------
HashAggregate
Group Key: b, c, a
-> Seq Scan on t1