Re: Extremely slow HashAggregate in simple UNION query

Hi,

On 2019-08-20 19:55:56 +0200, Felix Geisendörfer wrote:
> > On 20. Aug 2019, at 19:32, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > FWIW, that's not a mis-estimate I'm getting on master ;). Obviously
> > that doesn't actually address your concern...
>
> I suppose this is thanks to the new optimizer support functions
> mentioned by Michael and Pavel?

Right.

> > Under-sizing the hashtable just out of caution will have add overhead to
> > a lot more common cases. That requires copying data around during
> > growth, which is far far from free. Or you can use hashtables that don't
> > need to copy, but they're also considerably slower in the more common
> > cases.
>
> How does PostgreSQL currently handle the case where the initial hash
> table is under-sized due to the planner having underestimated things?
> Are the growth costs getting amortized by using an exponential growth
> function?

Yes. But that's still far from free.

Greetings,

Andres Freund