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Re: Bug in autovacuum.c?

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: Alvaro Herrera <alvherre(at)commandprompt(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Bug in autovacuum.c?
Date: 2011-04-01 16:14:18
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Fri, Apr 1, 2011 at 11:18 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Robert Haas wrote:
>> Oh, quite right.  Sorry I missed that.  I suppose if we wanted to fix
>> this for real, we'd want to get:
>> 105->5
>> 104->4
>> 103->3
>> 102->max_xid
>> 101->max_xid-1
>> 100->max_xid-2
>> 99->max_xid-3
>> 98->max_xid-4
>> But it doesn't seem worth getting excited about.
> I think (?) the problem with that is the every time you wrap around you
> get more out of sync.  :-O

It's not clear to me that it matters a bit, though.

> Thinking more, the problem is that when the xid counter wraps around
> from max_xid to 3, we jump the freeze horizon by three, e.g 5000 to
> 5003.  So when, the freeze horizon wraps, we can either have that jump
> by three, e.g set it to FirstNormalTransactionId, or delay by three,
> e.g. set it to MaxTransactionId.

So what?  :-)

Robert Haas
The Enterprise PostgreSQL Company

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