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From: Tom Lane <[email protected]>
To: Nathan Bossart <[email protected]>
Cc: [email protected]
Cc: alex work <[email protected]>
Cc: Robert Haas <[email protected]>
Subject: Re: Slow GRANT ROLE on PostgreSQL 16 with thousands of ROLEs
Date: Thu, 21 Mar 2024 22:07:19 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <20240322015251.GA2193900@nathanxps13>
References: <CAGvXd3OSMbJQwOSc-Tq-Ro1CAz=vggErdSG7pv2s6vmmTOLJSg@mail.gmail.com>
	<[email protected]>
	<[email protected]>
	<[email protected]>
	<20240321204012.GA1992418@nathanxps13>
	<20240322005139.GA2060816@nathanxps13>
	<[email protected]>
	<20240322010332.GA2109205@nathanxps13>
	<20240322015251.GA2193900@nathanxps13>

Nathan Bossart <[email protected]> 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

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