Re: Why hash OIDs?

On Mon, Aug 27, 2018 at 10:12 PM, Andres Freund <andres@anarazel.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.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company