Re: Row estimates for empty tables

Pavel Stehule <pavel.stehule@gmail.com> writes:
> I am sending a patch that is years used in GoodData.

I'm quite unexcited about that.  I'd be the first to agree that the
ten-pages estimate is a hack, but it's not an improvement to ask users
to think of a better value ... especially not as a one-size-fits-
all-relations GUC setting.

I did have an idea that I think is better than my previous one:
rather than lying about the value of relpages, let's represent the
case where we don't know the tuple density by setting reltuples = -1
initially.  This leads to a patch that's a good bit more invasive than
the quick-hack solution, but I think it's a lot cleaner on the whole.

A possible objection is that this changes the FDW API slightly, as
GetForeignRelSize callbacks now need to deal with rel->tuples possibly
being -1.  We could avoid an API break if we made plancat.c clamp
that value to zero; but then FDWs still couldn't tell the difference
between the "empty" and "never analyzed" cases, and I think this is
just as much of an issue for them as for the core code.

I'll add this to the upcoming CF.

			regards, tom lane