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

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.

            regards, tom lane