Subject: Re: Visibility map, partial vacuums
From: Simon Riggs <simon@2ndQuadrant.com>
To: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>
Cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
In-Reply-To: <49070C29.9090508@enterprisedb.com>
References: <4905AE17.7090305@enterprisedb.com>
	<1225193108.3971.154.camel@ebony.2ndQuadrant>
	<49070C29.9090508@enterprisedb.com>
Content-Type: text/plain
Date: Tue, 28 Oct 2008 14:22:15 +0000
Message-Id: <1225203735.3971.167.camel@ebony.2ndQuadrant>
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit


On Tue, 2008-10-28 at 14:57 +0200, Heikki Linnakangas wrote:
> Simon Riggs wrote:
> > On Mon, 2008-10-27 at 14:03 +0200, Heikki Linnakangas wrote:
> >> One option would be to just ignore that problem for now, and not 
> >> WAL-log.
> > 
> > Probably worth skipping for now, since it will cause patch conflicts if
> > you do. Are there any other interactions with Hot Standby? 
> > 
> > But it seems like we can sneak in an extra flag on a HEAP2_CLEAN record
> > to say "page is now all visible", without too much work.
> 
> Hmm. Even if a tuple is visible to everyone on the master, it's not 
> necessarily yet visible to all the read-only transactions in the slave.

Never a problem. No query can ever see the rows removed by a cleanup
record, enforced by the recovery system.

> > Does the PD_ALL_VISIBLE flag need to be set at the same time as updating
> > the VM? Surely heapgetpage() could do a ConditionalLockBuffer exclusive
> > to set the block flag (unlogged), but just not update VM. Separating the
> > two concepts should allow the visibility check speed gain to more
> > generally available. 
> 
> Yes, that should be possible in theory. There's no version of 
> ConditionalLockBuffer() for conditionally upgrading a shared lock to 
> exclusive, but it should be possible in theory. I'm not sure if it would 
> be safe to set the PD_ALL_VISIBLE_FLAG while holding just a shared lock, 
> though. If it is, then we could do just that.

To be honest, I'm more excited about your perf results for that than I
am about speeding up some VACUUMs.

-- 
 Simon Riggs           www.2ndQuadrant.com
 PostgreSQL Training, Services and Support