Date: Mon, 22 Aug 2016 17:50:11 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Jim Nasby <Jim.Nasby@bluetreble.com>,
	Aleksander Alekseev <a.alekseev@postgrespro.ru>,
	PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: [Patch] Temporary tables that do not bloat pg_catalog
 (a.k.a fast temp tables)
Message-ID: <20160822205011.GA149299@alvherre.pgsql>
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Robert Haas wrote:

> However:
> 
> 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.

Check.  This is the most serious issue, IMV.

> 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.

Check.  We could think about relaxing this restriction, which would
enable the feature to satisfy that use case.  (I think the main
complication there is the init fork of btrees on those catalogs; other
relations could just be truncated to empty on restart.)

> 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.

You also save catalog bloat ... These benefits may not be tremendous,
but I think they may be good enough for many users.

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


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