Re: Extremely slow HashAggregate in simple UNION query

> On 21. Aug 2019, at 20:26, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
>
> As noted elsewhere, v12 thwarts your attempts to deliberately design the bad estimates. You can still get them, you just have to work a bit harder at it:
>
> CREATE FUNCTION j (bigint, bigint) returns setof bigint as $$ select generate_series($1,$2) $$ rows 1000 language sql;

Yeah, that's awesome! I didn't know about this until I ran into this issue, I'll definitely be using it for future estimation problems that are difficult to fix otherwise!

> I've made an extension which has a function which always returns true, but lies about how often it is expected to return true. See the attachment. With that, you can fine-tune the planner.
>
> CREATE EXTENSION pg_selectivities ;

Very cool and useful : )!

I think in most cases I'll be okay with declaring a function with a static ROWS estimate, but I'll consider your extension if I need more flexibility in the future!

Thanks
Felix