On Thu, Jul 18, 2013 at 9:14 AM, Pavel Stehule <pavel.stehule@gmail.com> wrote:
> Hello
>
> I found a slow query with large external sort. I expect, so external
> sort should be penalized. Is it?
It tries to, but it doesn't seem to be much good at it. In
particular, I think it does a poor job of estimating the CPU cost of
an external sort relative to an in-memory sort of a slightly smaller
data set. In my experience adding the single tuple that actually
pushes you over the work_mem limit costs about 3x in CPU.
It is one of those things I started looking into a few times, but
never got anywhere before getting distracted.
Cheers,
Jeff