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Re: idea: global temp tables

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Greg Stark <stark(at)enterprisedb(dot)com>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>, Alvaro Herrera <alvherre(at)commandprompt(dot)com>, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: idea: global temp tables
Date: 2009-04-29 19:59:38
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
Greg Stark <stark(at)enterprisedb(dot)com> writes:
> I don't understand what you mean by a cost once per request. You only
> have to create the temporary table on the first request. If you can't
> tell which is the first request you only have to test whether it
> exists which doesn't incur the consequences that ddl incurs.

This is all based on utterly-unproven assumptions about relative costs.
In particular, ISTM an additional network round trip or two associated
with testing for/creating a temp table could easily swamp any costs
associated with catalog entry creation.  Even if it doesn't,
creating/deleting a few dozen rows in the system catalogs shouldn't
really be something that autovacuum can't deal with.  If it were,
we'd be hearing a lot more complaints about the *existing* temp table
feature being unusable.  (And yes, I know it's come up once or twice,
but not all that often.)

I'm all for eliminating catalog overheads, if we can find a way to do
that.  I don't think that you get to veto implementation of the feature
until we can find a way to optimize it better.  The question is not
about whether having the optimization would be better than not having it
--- it's about whether having the unoptimized feature is better than
having no feature at all (which means people have to implement the same
behavior by hand, and they'll *still* not get the optimization).

			regards, tom lane

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