Peter Geoghegan <pg(at)heroku(dot)com> writes:
> I think that we *can* refine this guess, and should, because random
> I/O is really quite unlikely to be a large cost these days (I/O in
> general often isn't a large cost, actually). More fundamentally, I
> think it's a problem that cost_sort() thinks that external sorts are
> far more expensive than internal sorts in general. There is good
> reason to think that that does not reflect the reality. I think we can
> expect external sorts to be *faster* than internal sorts with
> increasing regularity in Postgres 10.
TBH, if that's true, haven't you broken something?
regards, tom lane