Re: Making empty Bitmapsets always be NULL

Thank you for running the profiles.

On Tue, 27 Jun 2023 at 21:11, Yuya Watari <watari(dot)yuya(at)gmail(dot)com> wrote:
> On Sat, Jun 24, 2023 at 1:15 PM David Rowley <dgrowleyml(at)gmail(dot)com> wrote:
> > I think it's also important to check we don't slow anything down for
> > more normal-sized sets. The vast majority of sets will contain just a
> > single word, so we should probably focus on making sure we're not
> > slowing anything down for those.
>
> I agree with you and thank you for sharing the results. I ran
> installcheck with your patch. The result is as follows. The speedup
> was 0.33%. At least in my environment, I did not observe any
> regression with this test. So, the patch looks very good.
>
> Master: 2.559648 seconds
> Patched: 2.551116 seconds (0.33% faster)

I wondered if the common case could be made slightly faster by
checking the 0th word before checking the word count before going onto
check the remaining words. For bms_equal(), that's something like:

if (a->words[0] != b->words[0] || a->nwords != b->nwords)
return false;

/* check all the remaining words match */
for (int i = 1; i < a->nwords; i++) ...

I wrote the patch and tried it out, but it seems slightly slower than
the v4 patch.

Linux with AMD 3990x, again using the patch from [1] with make installcheck

master: 1.41720145 seconds
v4: 1.392969606 seconds (1.74% faster than master)
v4 with 0th word check: 1.404199748 seconds (0.93% faster than master)

I've attached a delta patch of what I used to test. Since it's not
any faster, I don't think it's worth doing. It'll also produce
slightly more compiled code.

David

[1] https://postgr.es/m/CAApHDvo68m_0JuTHnEHFNsdSJEb2uPphK6BWXStj93u_QEi2rg@mail.gmail.com