Re: anti-join chosen even when slower than old plan

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> Let's back up a moment and talk about what the overall goal is, here.
> Ideally, we would like PostgreSQL to have excellent performance at all
> times, under all circumstances, with minimal tuning. Therefore, we do
> NOT want to add variables that will, by design, need constant manual
> adjustment. That is why I suggested that Tom's idea of an
> assume_cached GUC is probably not what we really want to do. On the
> other hand, what I understand Mladen to be suggesting is something
> completely different. He's basically saying that, of course, he wants
> it to work out of the box most of the time, but since there are
> guaranteed to be cases where it doesn't, how about providing some
> knobs that aren't intended to be routinely twaddled but which are
> available in case of emergency? Bravo, I say!

Um ... those are exactly the same thing. You're just making different
assumptions about how often you will need to twiddle the setting.
Neither assumption is based on any visible evidence, unfortunately.
I was thinking of assume_cached as something that could be
set-and-forget most of the time, and you're entirely right to criticize
it on the grounds that maybe it wouldn't. But to support a proposal
that doesn't even exist yet on the grounds that it *would* be
set-and-forget seems a tad inconsistent. We can't make that judgment
without a whole lot more details than have been provided yet for any
idea in this thread.

I do think that something based around a settable-per-table caching
percentage might be a reasonable way to proceed. But the devil is in
the details, and we don't have those yet.

regards, tom lane