Re: Eager aggregation, take 3

On Tue, Oct 29, 2024 at 3:51 AM Richard Guo <guofenglinux(at)gmail(dot)com> wrote:
> > 2. join type is FULL JOIN, (i am not sure about other Semijoins and
> > anti-semijoins types).
>
> The presence of a FULL JOIN does not preclude the use of eager
> aggregation. We still can push a partial aggregation down to a level
> that is above the FULL JOIN.

I think you can also push a partial aggregation step through a FULL
JOIN. Consider this:

SELECT p.name, string_agg(c.name, ', ') FROM parents p FULL JOIN
children c ON p.id = c.parent_id GROUP BY 1;

I don't see why it matters here that this is a FULL JOIN. If it were
an inner join, we'd have one group for every parent that has at least
one child. Since it's a full join, we'll also get one group for every
parent with no children, and also one group for the null parent. But
that should work fine with a partially aggregated c.

--
Robert Haas
EDB: http://www.enterprisedb.com