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Message-ID: <464C7A2B.5030105@enterprisedb.com>
Date: Thu, 17 May 2007 16:52:11 +0100
From: Heikki Linnakangas <heikki@enterprisedb.com>
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To: kadams@mxlogic.com
CC: pgsql-performance@postgresql.org
Subject: Re: WAL log performance/efficiency question
References: <1179410671.26604.3.camel@MXLRMT-208.corp.mxlogic.com>
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	<1179416083.26604.15.camel@MXLRMT-208.corp.mxlogic.com>
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Keaton Adams wrote:
> So for every data page there is a 20 byte header, for every row there is
> a 4 byte identifier (offset into the page), AND there is also a 28 byte
> fixed-size header (27 + optional null bitmap)?? (I did find the section
> in the 8.1 manual that give the physical page layout.)  The other RDBMS
> platforms I have worked with have a header in the 28 byte range and a
> row pointer of 4 bytes, and that's it.  I find it a bit surprising that
> PostgreSQL would need another 28 bytes per row to track its contents.

Yes, it is more than many other DBMSs. It contains mostly MVCC-related 
visibility information that other DBMSs store at page level etc, or 
don't have MVCC at all.

As Tom mentioned, that's going to be a bit better in 8.3. We reduced the 
header size from 27 + null bitmap to 23 + null bitmap, which makes a big 
difference especially on 64-bit architectures, where the header used to 
be padded up to 32 bytes, and now it's only 24 bytes.

For character fields, including CHAR(100) like you have, we also store a 
4 bytes length header per field. That's been reduced to 1 byte for 
string shorter than 127 bytes in 8.3.

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