To: "Steven Flatt" <steven.flatt@gmail.com>
cc: pgsql-performance@postgresql.org
Subject: Re: Insertion to temp table deteriorating over time 
In-reply-to: <357fa7590612131517q375fc0by50cb4257bcce7a94@mail.gmail.com> 
References: <357fa7590612130844q58ef3ca5q31e164f9b006e0fc@mail.gmail.com>
	<9315.1166030958@sss.pgh.pa.us>
	<357fa7590612131517q375fc0by50cb4257bcce7a94@mail.gmail.com>
Comments: In-reply-to "Steven Flatt" <steven.flatt@gmail.com>
	message dated "Wed, 13 Dec 2006 18:17:41 -0500"
Date: Wed, 13 Dec 2006 18:44:47 -0500
Message-ID: <13154.1166053487@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

"Steven Flatt" <steven.flatt@gmail.com> writes:
> Having said that, what kinds of things should I be looking for that could
> deteriorate/bloat over time?  Ordinarily the culprit might be infrequent
> vacuuming or analyzing, but that wouldn't be corrected by a restart of
> Postgres.  In our case, restarting Postgres gives us a huge performance
> improvement (for a short while, anyways).

> By the way, we are using PG 7.4.6 on FreeBSD 5.30.0170.  The temp table has
> 15 columns: a timestamp, a double, and the rest integers.  It has no
> indexes.

Hm, *are* you vacuuming only infrequently?  In particular, what is your
maintenance policy for pg_class?

Some experimentation with TRUNCATE and VACUUM VERBOSE shows that in 7.4,
a TRUNCATE of a temp table with no indexes and no toast table generates
three dead row versions in pg_class.  (I'm surprised that it's as many
as three, but in any case the TRUNCATE would certainly have to do one
update of the table's pg_class entry and thereby generate one dead row
version.)

If you're being sloppy about vacuuming pg_class, then over time the
repeated-truncate pattern would build up a huge number of dead rows
in pg_class, *all with the same OID*.  It's unsurprising that this
would create some slowness in looking up the temp table's pg_class
entry.

If this theory is correct, the reason that starting a fresh backend
makes it fast again is that the new backend creates a whole new temp
table with a new OID assigned, and so the adjacent litter in pg_class
doesn't matter anymore (or not so much anyway).

Solution would be to institute regular vacuuming of the system
catalogs...

			regards, tom lane