Subject: Re: [Patch] Temporary tables that do not bloat pg_catalog
 (a.k.a fast temp tables)
To: Robert Haas <robertmhaas@gmail.com>, Jim Nasby <Jim.Nasby@bluetreble.com>
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Cc: Aleksander Alekseev <a.alekseev@postgrespro.ru>,
 PostgreSQL Hackers <pgsql-hackers@postgresql.org>
From: Tomas Vondra <tomas.vondra@2ndquadrant.com>
Message-ID: <d6adce51-96f4-c508-d0dc-5311cfb223b6@2ndquadrant.com>
Date: Wed, 24 Aug 2016 00:11:49 +0200
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On 08/22/2016 10:32 PM, Robert Haas wrote:
>
> ...
>
> 1. The number of tables for which we would need to add a duplicate,
> unlogged table is formidable.  You need pg_attribute, pg_attrdef,
> pg_constraint, pg_description, pg_type, pg_trigger, pg_rewrite, etc.
> And the backend changes needed so that we used the unlogged copy for
> temp tables and the permanent copy for regular tables is probably
> really large.
>
> 2. You can't write to unlogged tables on standby servers, so this
> doesn't help solve the problem of wanting to use temporary tables on
> standbys.
>
> 3. While it makes creating temporary tables a lighter-weight
> operation, because you no longer need to write WAL for the catalog
> entries, there's probably still substantially more overhead than just
> stuffing them in backend-local RAM.  So the performance benefits are
> probably fairly modest.
>
> Overall I feel like the development effort that it would take to make
> this work would almost certainly be better-expended elsewhere.  But of
> course I'm not in charge of how people who work for other companies
> spend their time...
>

Could someone please explain how the unlogged tables are supposed to fix 
the catalog bloat problem, as stated in the initial patch submission? 
We'd still need to insert/delete the catalog rows when creating/dropping 
the temporary tables, causing the bloat. Or is there something I'm missing?


-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services


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