Re: Partial aggregates pushdown - Mailing list pgsql-hackers

On Wed, Nov 22, 2023 at 1:32 AM Alexander Pyhalov
<a.pyhalov@postgrespro.ru> wrote:
> Hi. HAVING is also a problem. Consider the following query
>
> SELECT count(a) FROM t HAVING count(a) > 10 - we can't push it down to
> foreign server as HAVING needs full aggregate result, but foreign server
> don't know it.

I don't see it that way. What we would push to the foreign server
would be something like SELECT count(a) FROM t. Then, after we get the
results back and combine the various partial counts locally, we would
locally evaluate the HAVING clause afterward. That is, partial
aggregation is a barrier to pushing down HAVING clause itself, but it
doesn't preclude pushing down the aggregation.

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