ne 23. 8. 2020 v 23:08 odesílatel Tom Lane <tgl@sss.pgh.pa.us> napsal:
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.
This patch is just a workaround that works well 10 years (but for one special use case) - nothing more. Without this patch that application cannot work ever.
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.
I'll check it
I think it can work. It is a good enough solution for people who need a different behaviour with minimal impact on people who don't need a change.