Re: Why hash OIDs? - Mailing list pgsql-hackers

From Thomas Munro
Subject Re: Why hash OIDs?
Date
Msg-id CAEepm=3X3EFQXKfbbVTZPRmPhrixd7TPK89R6sQKztegM=z6ow@mail.gmail.com
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In response to Re: Why hash OIDs?  (Robert Haas <robertmhaas@gmail.com>)
Responses Re: Why hash OIDs?
List pgsql-hackers
On Wed, Aug 29, 2018 at 2:09 AM Robert Haas <robertmhaas@gmail.com> wrote:
> 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.

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


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