Message-ID: <49070C29.9090508@enterprisedb.com>
Date: Tue, 28 Oct 2008 14:57:13 +0200
Organization: EnterpriseDB
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To: Simon Riggs <simon@2ndQuadrant.com>
CC: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Visibility map, partial vacuums
References: <4905AE17.7090305@enterprisedb.com>
	<1225193108.3971.154.camel@ebony.2ndQuadrant>
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From: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>

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.

> 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.

-- 
   Heikki Linnakangas
   EnterpriseDB   http://www.enterprisedb.com