Re: [PERFORM] unusual performance for vac following 8.2upgrade

On Thu, Jan 11, 2007 at 09:51:39PM -0500, Tom Lane wrote:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> > Can we actually get rid of pg_class entries for temp tables. Maybe
> > creating a "temp pg_class" which would be local to each session? Heck,
> > it doesn't even have to be an actual table -- it just needs to be
> > somewhere from where we can load entries into the relcache.
>
> A few things to think about:
>
> 1. You'll break a whole lotta client-side code if temp tables disappear
> from pg_class. This is probably solvable --- one thought is to give
> pg_class an inheritance child that is a view on a SRF that reads out the
> stored-in-memory rows for temp pg_class entries. Likewise for
> pg_attribute and everything else related to a table definition.
>
> 2. How do you keep the OIDs for temp tables (and their associated
> rowtypes) from conflicting with OIDs for real tables? Given the way
> that OID generation works, there wouldn't be any real problem unless a
> temp table survived for as long as it takes the OID counter to wrap all
> the way around --- but in a database that has WITH OIDS user tables,
> that might not be impossibly long ...
>
> 3. What about dependencies on user-defined types, functions, etc?
> How will you get things to behave sanely if one backend tries to drop a
> type that some other backend is using in a column of a temp table? Even
> if you put entries into pg_depend, which would kind of defeat the point
> of not having on-disk catalog entries for temp tables, I don't see how
> the other backend figures out what the referencing object is.
>
> I don't really see any solution to that last point :-(

Perhaps it would be better to partition pg_class and _attributes based
on whether an object is temporary or not. Granted, that still means
vacuuming is a consideration, but at least it wouldn't be affecting
pg_class itself. Separating temp objects out would also make it more
reasonable to have the system automatically vacuum those tables after
every X number of dropped objects.

Unfortunately, that still wouldn't help with the OID issue. :( Unless
there was a SERIAL column in pg_class_temp and other parts of the system
could differentiate between temp and non-temp objects.
--
Jim Nasby jim(at)nasby(dot)net
EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)