Subject: Re: VACUUMs and WAL
From: Hannu Krosing <hannu@2ndQuadrant.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
In-Reply-To: <1225188645.3971.118.camel@ebony.2ndQuadrant>
References: <1225183785.3971.114.camel@ebony.2ndQuadrant>
	<1225187130.7721.5.camel@huvostro>
	<1225188645.3971.118.camel@ebony.2ndQuadrant>
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Date: Tue, 28 Oct 2008 14:19:04 +0200
Message-Id: <1225196344.7694.2.camel@huvostro>
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On Tue, 2008-10-28 at 10:10 +0000, Simon Riggs wrote:
> On Tue, 2008-10-28 at 11:45 +0200, Hannu Krosing wrote:
> > On Tue, 2008-10-28 at 08:49 +0000, Simon Riggs wrote:
> > > Looking at a VACUUM's WAL records makes me think twice about the way we
> > > issue a VACUUM.
> > > 
> > > 1. First we scan the heap, issuing a HEAP2 clean record for every block
> > > that needs cleaning.
> > 
> > IIRC the first heap pass just collects info and does nothing else. 
> > Is this just an empty/do-nothing WAL record ?
> 
> 8.3 changed that; it used to work that way. I guess I never looked at
> the amount of WAL being generated.


I can't see how it is safe to do anything more than just lookups on
first pass. 

There will be dangling index pointers if the system crashes/is rebooted
or the vacuum is just interrupted after cleaning some heap pages but
before cleaning corresponding index pages.

---------------
Hannu