Re: [PoC] Reducing planning time when tables have many partitions

On Wed, 27 Jul 2022 at 18:07, Yuya Watari <watari(dot)yuya(at)gmail(dot)com> wrote:
> I agree that introducing a Bitmapset field may solve this problem. I
> will try this approach in addition to previous ones.

I've attached a very half-done patch that might help you get started
on this. There are still 2 failing regression tests which seem to be
due to plan changes. I didn't expend any effort looking into why these
plans changed.

The attached does not contain any actual optimizations to find the
minimal set of EMs to loop through by masking the Bitmapsets that I
mentioned in my post last night. I just quickly put it together to
see if there's some hole in the idea. I don't think there is.

I've not really considered all of the places that we'll want to do the
bit twiddling to get the minimal set of EquivalenceMember. I did see
there's a couple more functions in postgres_fdw.c that could be
optimized.

One thing I've only partially thought about is what if you want to
also find EquivalenceMembers with a constant value. If there's a
Const, then you'll lose the bit for that when you mask the ec's
ec_member_indexes with the RelOptInfos. If there are some places
where we need to keep those then I think we'll need to add another
field to EquivalenceClass to mark the index into PlannerInfo's
eq_members for the EquivalenceMember with the Const. That bit would
have to be bms_add_member()ed back into the Bitmapset of matching
EquivalenceMembers after masking out RelOptInfo's ec_member_indexes.

When adding the optimizations to find the minimal set of EM bits to
search through, you should likely add some functions similar to the
get_eclass_indexes_for_relids() and get_common_eclass_indexes()
functions to help you find the minimal set of bits. You can also
probably get some other inspiration from [1], in general.

David

[1] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=3373c715535