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: <490745AF.2090602@enterprisedb.com>
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Date: Tue, 28 Oct 2008 17:30:24 +0000
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On Tue, 2008-10-28 at 19:02 +0200, Heikki Linnakangas wrote:

> Yes, but there's a problem with recently inserted tuples:
> 
> 1. A query begins in the slave, taking a snapshot with xmax = 100. So 
> the effects of anything more recent should not be seen.
> 2. Transaction 100 inserts a tuple in the master, and commits
> 3. A vacuum comes along. There's no other transactions running in the 
> master. Vacuum sees that all tuples on the page, including the one just 
> inserted, are visible to everyone, and sets PD_ALL_VISIBLE flag.
> 4. The change is replicated to the slave.
> 5. The query in the slave that began at step 1 looks at the page, sees 
> that the PD_ALL_VISIBLE flag is set. Therefore it skips the visibility 
> checks, and erroneously returns the inserted tuple.

Yep. I was thinking about FSM and row removal. So PD_ALL_VISIBLE must be
separately settable on the standby. Another reason why it should be able
to be set without a VACUUM - since there will never be one on standby.

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