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Re: Bug in autovacuum.c?

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
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 15:18:11
Message-ID: 201104011518.p31FIBO19338@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
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

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.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +

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