Re: BUG #15947: Worse plan is chosen after giving the planner more freedom (partitionwise join)

On 2019-08-11 00:28, PG Bug reporting form wrote:
> The following bug has been logged on the website:
>
> Bug reference: 15947
> I'm trying the following query:
>
> EXPLAIN
> SELECT COUNT(*)
> FROM sc
> WHERE EXISTS (
> SELECT 1
> FROM sg
> WHERE sc.id = sg.sc_fk
> AND sc.a = sg.a
> AND sc.o = sg.o
> AND sc.sl = sg.sl
> );
>
> Which produces the plan with this cost estimation (top node):
> -- Aggregate (cost=147.25..147.26 rows=1 width=8)
>
> But after:
> SET enable_partitionwise_join = true;
>
> The new plan is more expensive:
> -- Aggregate (cost=175.00..175.01 rows=1 width=8)
>
> This shouldn't be happening, right?

Why not?

The 'worse plan' cnsistently executes faster.

I don't think the cost units of different queries can/should be compared
as if they are the same.