Re: Shouldn't cost_append() also scale the partial path's cost?

At Wed, 14 Jun 2023 14:36:54 +0200, Antonin Houska <ah@cybertec.at> wrote in 
> Like in cost_seqscan(), I'd expect the subpath cost to be divided among
> parallel workers. The patch below shows what I mean. Am I right?

If I've got it right, the total cost of a partial seqscan path
comprises a distributed CPU run cost and an undistributed disk run
cost. If we want to adjust for a different worker number, we should
only tweak the CPU component of the total cost. By default, if one
page contains 100 rows (I guess a moderate ratio), these two costs are
balanced at a 1:1 ratio and the CPU run cost and disk run cost in a
partial seqscan path is 1:n (n = #workers).  If we adjust the run cost
in the porposed manner, it adjusts the CPU run cost correctly but in
turn the disk run cost gets wrong (by a larger error in this premise).

In short, it will get wrong in a different way.

Actually it looks strange that rows are adjusted but cost is not, so
we might want to add an explanation in this aspect.

regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center