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

Nathan Bossart <nathandbossart@gmail.com> writes:
> The Bloom filter appears to help a bit, although it regresses the
> create-roles.sql portion of the test.  I'm assuming that's thanks to all
> the extra pallocs and pfrees, which are probably avoidable if we store the
> filter in a long-lived context and just clear it at the beginning of each
> call to roles_is_member_of().

The zero-fill to reinitialize the filter probably costs a good deal
all by itself, considering we're talking about at least a megabyte.
Maybe a better idea is to not enable the filter till we're dealing
with at least 1000 or so entries in roles_list, though obviously that
will complicate the logic.

+            if (bloom_lacks_element(bf, (unsigned char *) &otherid, sizeof(Oid)))
+                roles_list = lappend_oid(roles_list, otherid);
+            else
+                roles_list = list_append_unique_oid(roles_list, otherid);
+            bloom_add_element(bf, (unsigned char *) &otherid, sizeof(Oid));

Hmm, I was imagining something more like

        if (bloom_lacks_element(bf, (unsigned char *) &otherid, sizeof(Oid)) ||
            !list_member_oid(roles_list, otherid))
        {
            roles_list = lappend_oid(roles_list, otherid);
            bloom_add_element(bf, (unsigned char *) &otherid, sizeof(Oid));
        }

to avoid duplicate bloom_add_element calls.  Probably wouldn't move
the needle much in this specific test case, but this formulation
would simplify letting the filter kick in later.  Very roughly,

        if ((bf && bloom_lacks_element(bf, (unsigned char *) &otherid, sizeof(Oid))) ||
            !list_member_oid(roles_list, otherid))
        {
            if (bf == NULL && list_length(roles_list) > 1000)
            {
                ... create bf and populate with existing list entries
            }
            roles_list = lappend_oid(roles_list, otherid);
            if (bf)
                bloom_add_element(bf, (unsigned char *) &otherid, sizeof(Oid));
        }


            regards, tom lane