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: <4905AE17.7090305@enterprisedb.com>
References: <4905AE17.7090305@enterprisedb.com>
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Date: Tue, 28 Oct 2008 17:03:15 +0000
Message-Id: <1225213395.3971.217.camel@ebony.2ndQuadrant>
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On Mon, 2008-10-27 at 14:03 +0200, Heikki Linnakangas wrote:

> Lazy VACUUM only needs to visit pages that are '0' in the visibility 
> map. This allows partial vacuums, where we only need to scan those parts 
> of the table that need vacuuming, plus all indexes.

Just realised that this means we still have to visit each block of a
btree index with a cleanup lock.

That means the earlier idea of saying I don't need a cleanup lock if the
page is not in memory makes a lot more sense with a partial vacuum.

1. Scan all blocks in memory for the index (and so, don't do this unless
the index is larger than a certain % of shared buffers), 
2. Start reading in new blocks until you've removed the correct number
of tuples
3. Work through the rest of the blocks checking that they are either in
shared buffers and we can get a cleanup lock, or they aren't in shared
buffers and so nobody has them pinned.

If you step (2) intelligently with regard to index correlation you might
not need to do much I/O at all, if any.

(1) has a good hit ratio because mostly only active tables will be
vacuumed so are fairly likely to be in memory.

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