Re: Confusing comment in pg_upgrade in regards to VACUUM FREEZE

From: Craig Ringer <craig(at)2ndquadrant(dot)com>
To: David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Confusing comment in pg_upgrade in regards to VACUUM FREEZE
Date: 2016-04-18 04:52:17
Message-ID: CAMsr+YHLaWRc6R5j5Kb2+bXJp+Ftje3QGNs7nKxzUYAvfE61hA@mail.gmail.com
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On 18 April 2016 at 12:11, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
wrote:

> this is not helping me much as I don't really understand why
> pg_statistic need to be frozen?
>

Yeah, in particular it's not clear to me why pg_upgrade goes to such
efforts to freeze everything when it copies pg_clog over in
copy_clog_xlog_xid() .

Is it mainly a defense against the multixact format change?

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

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