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Re: [COMMITTERS] pgsql: Augment WAL records for btree delete with GetOldestXmin() to

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Stefan Kaltenbrunner <stefan(at)kaltenbrunner(dot)cc>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: [COMMITTERS] pgsql: Augment WAL records for btree delete with GetOldestXmin() to
Date: 2010-03-27 19:36:47
Message-ID: 1269718607.3684.1967.camel@ebony (view raw or flat)
Thread:
Lists: pgsql-committerspgsql-hackers
On Sat, 2010-03-27 at 19:15 +0000, Greg Stark wrote:
> On Sat, Mar 27, 2010 at 10:10 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
> > It appears that in practice many of the index items point to heap items
> > that are LP_DEAD. So for the purposes of accessing a heap tuple's xmin,
> > then we're both right. To the current purpose the tuple has been
> > removed, though you are also right: its stub remains.
> 
> If we're pruning an index entry to a heap tuple that has been HOT
> pruned wouldn't the HOT pruning record have already conflicted with
> any queries that could see  it?

Quite probably, but a query that started after that record arrived might
slip through. We have to treat each WAL record separately.

Do you agree with the conjecture? That LP_DEAD items can be ignored
because their xid would have been earlier than the latest LP_NORMAL
tuple we find? (on any page).

Or is a slightly less strong condition true: we can ignore LP_DEAD items
on a page that is also referenced by an LP_NORMAL item.

-- 
 Simon Riggs           www.2ndQuadrant.com


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