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Re: Incorrect description of xmax and xip in functions docs

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: pgsql-docs <pgsql-docs(at)postgresql(dot)org>
Subject: Re: Incorrect description of xmax and xip in functions docs
Date: 2008-09-07 01:31:02
Message-ID: 200809070131.m871V2j00862@momjian.us (view raw or flat)
Thread:
Lists: pgsql-docs
Simon Riggs wrote:
> 
> On Fri, 2008-09-05 at 16:14 +0100, Simon Riggs wrote:
> > http://developer.postgresql.org/pgdocs/postgres/functions-info.html
> > 
> > xip_list is described as
> > 
> > "Active txids at the time of the snapshot... "
> > 
> > 
> > This is incorrect. The xip_list is the list of transactions that are in
> > progress *and* less than xmax. There may be transactions in progress
> > with an xid higher than xmax. This will happen frequently in fact. This
> > is because xmax is defined as the highest/latest completed xid, not the
> > highest running xid.
> > 
> > Note that there is no way to discover the list of running xids at the
> > time of the snapshot, from the data we hold about snapshots. Nor can the
> > snapshot data be used to monitor the number of transactions in progress.
> > 
> > Anyone disagree? If not, I'll patch.
> 
> My rewording would be:
> "Active txids at the time of the snapshot. The list includes only those
> active txids between xmin and xmax; there may be active txids higher
> than xmax. A txid that is xmin <= txid < xmax and not in this list was
> already completed at the time of the snapshot, and thus either visible
> or dead according to its commit status. The list does not include txids
> of subtransactions."

Applied, and attached.

> And for txid_visible_in_snapshot() comment added:
> "Function should not be used with subtransaction xids. It is possible
> that this function will return a true result for a subtransaction xid
> that was actually still in progress at the time of the snapshot".

I think the cleaner solution is to throw an appropriate error if a
subtransaction xid is used, rather than adding documentation.

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

  + If your life is a hard drive, Christ can be your backup. +

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