Re: Slow GRANT ROLE on PostgreSQL 16 with thousands of ROLEs

On Thu, Mar 21, 2024 at 04:31:45PM -0400, Tom Lane wrote:
> I wrote:
>> ... I still see the problematic GRANT taking ~250ms, compared
>> to 5ms in v15. roles_is_member_of is clearly on the hook for that.
>
> Ah: looks like that is mainly the fault of the list_append_unique_oid
> calls in roles_is_member_of. That's also an O(N^2) cost of course,
> though with a much smaller constant factor.
>
> I don't think we have any really cheap way to de-duplicate the role
> OIDs, especially seeing that it has to be done on-the-fly within the
> collection loop, and the order of roles_list is at least potentially
> interesting. Not sure how to make further progress without a lot of
> work.

Assuming these are larger lists, this might benefit from optimizations
involving SIMD intrinsics. I looked into that a while ago [0], but the
effort was abandoned because we didn't have any concrete use-cases at the
time. (I'm looking into some additional optimizations in a separate thread
[1] that would likely apply here, too.)

[0] https://postgr.es/m/20230308002502.GA3378449%40nathanxps13
[1] https://postgr.es/m/20240321183823.GA1800896%40nathanxps13

--
Nathan Bossart
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