From: Tom Lane <tgl@sss.pgh.pa.us>
To: Pavel Stehule <pavel.stehule@gmail.com>
cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>,
        PostgreSQL Hackers <pgsql-hackers@postgresql.org>,
        Aleksander Alekseev <a.alekseev@postgrespro.ru>
Subject: Re: [Patch] Temporary tables that do not bloat pg_catalog (a.k.a fast
 temp tables)
In-reply-to: 
 <CAFj8pRAv+vPxzTQMviCsYqQrfpYei5bTvX80zRUp1Q3xAbTWyg@mail.gmail.com>
References: <20160729141552.4062e19b@fujitsu>
 <39b0f423-a4df-3564-3b4a-b1de6f9eea80@2ndquadrant.com>
 <CAFj8pRAv+vPxzTQMviCsYqQrfpYei5bTvX80zRUp1Q3xAbTWyg@mail.gmail.com>
Comments: In-reply-to Pavel Stehule <pavel.stehule@gmail.com>
	message dated "Sat, 30 Jul 2016 06:49:05 +0200"
Date: Sat, 30 Jul 2016 10:47:10 -0400
Message-ID: <1184.1469890030@sss.pgh.pa.us>
Precedence: bulk
Sender: pgsql-hackers-owner@postgresql.org

Pavel Stehule <pavel.stehule@gmail.com> writes:
> But there are some patterns used with work with temp tables,that should not
> working, and we would to decide if we prepare workaround or not.

> -- problematic pattern (old code)
> IF NOT EXISTS(SELECT * FROM pg_class WHERE ....) THEN
>   CREATE TEMP TABLE xxx()
> ELSE
>   TRUNCATE TABLE xxx;
> END IF;

> -- modern patter (new code)
> BEGIN
>   TRUNCATE TABLE xxx;
> EXCEPTION WHEN ..... THEN
>   CREATE TEMP TABLE(...)
> END;

If the former stops working, that's a sufficient reason to reject the
patch: it hasn't been thought through carefully enough.  The key reason
why I don't think that's negotiable is that if there aren't (apparently)
catalog entries corresponding to the temp tables, that will almost
certainly break many things in the backend and third-party extensions,
not only user code patterns like this one.  We'd constantly be fielding
bug reports that "feature X doesn't work with temp tables anymore".

In short, I think that the way to make something like this work is to
figure out how to have "virtual" catalog rows describing a temp table.
Or maybe to partition the catalogs so that vacuuming away temp-table
rows is easier/cheaper than today.

			regards, tom lane


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