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From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Andres Freund <andres@2ndquadrant.com>
Cc: Bruce Momjian <bruce@momjian.us>
Cc: Stephen Frost <sfrost@snowman.net>
Cc: pgsql-hackers@postgresql.org
Subject: Re: MultiXactId error after upgrade to 9.3.4
Date: Wed, 23 Apr 2014 16:30:05 -0300
Message-ID: <20140423193005.GS25695@eldon.alvh.no-ip.org> (raw)
In-Reply-To: <60097392-07aa-4310-8632-43b7b09c1822@email.android.com>
References: <20140330141135.GA4582@tamriel.snowman.net>
	<20140330185349.GD4582@tamriel.snowman.net>
	<20140331115452.GT9567@eldon.alvh.no-ip.org>
	<20140331115713.GA18358@alap3.anarazel.de>
	<20140423180102.GP25695@eldon.alvh.no-ip.org>
	<20140423182804.GA21458@momjian.us>
	<20140423184214.GQ25695@eldon.alvh.no-ip.org>
	<20140423184716.GA22722@momjian.us>
	<20140423185120.GR25695@eldon.alvh.no-ip.org>
	<60097392-07aa-4310-8632-43b7b09c1822@email.android.com>
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Andres Freund wrote:

> I think this patch is a seriously bad idea. For one, it's not actually
> doing anything about the problem - the tuple can be accessed without
> freezing getting involved.

Normal access other than freeze is not a problem, because other code
paths do check for HEAP_XMAX_INVALID and avoid access to Xmax if that's
set.

> For another, it will increase wall traffic for freezing of short lived
> tuples considerably, without any benefit.

True.  I didn't actually try to run this; it was just for demonstration
purposes.  It'd need some cooperation from heap_tuple_needs_freeze in
order to work at all, for one thing.

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


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  Subject: Re: MultiXactId error after upgrade to 9.3.4
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