Re: Why hash OIDs?

On Wed, Aug 29, 2018 at 2:09 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Mon, Aug 27, 2018 at 10:12 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
> > Huh? Oids between, say, 1 and FirstNormalObjectId, are vastly more
> > common than the rest. And even after that, individual tables get large
> > clusters of sequential values to the global oid counter.
>
> Sure, but if you get a large cluster of sequential values, a straight
> mod-N bucket mapping works just fine. I think the bigger problem is
> that you might get a large cluster of values separated by exactly a
> power of 2. For instance, say you have one serial column and one
> index:
>
> rhaas=# create table a (x serial primary key);
> CREATE TABLE
> rhaas=# create table b (x serial primary key);
> CREATE TABLE
> rhaas=# select 'a'::regclass::oid, 'b'::regclass::oid;
> oid | oid
> -------+-------
> 16422 | 16430
> (1 row)
>
> If you have a lot of tables like that, bad things are going to happen
> to your hash table.

Right. I suppose that might happen accidentally when creating a lot
of partitions.

Advance the OID generator by some prime number after every CREATE TABLE?

/me ducks

--
Thomas Munro
http://www.enterprisedb.com