Re: PostgreSQL Disk Usage and Page Size

From: Stephen Robert Norris <srn(at)commsecure(dot)com(dot)au>
To: Seum-Lim Gan <slgan(at)lucent(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org, ahsaleh(at)lucent(dot)com
Subject: Re: PostgreSQL Disk Usage and Page Size
Date: 2004-03-18 00:07:35
Message-ID: 1079568455.12785.3.camel@ws12.commsecure.com.au
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On Thu, 2004-03-18 at 10:52, Seum-Lim Gan wrote:
> Hi all,
>
> we have a question about the pagesize in PostgreSQL:
>
> Using different pagesizes: 4K, 8K, 16K, 32K, when we store different
> record sizes
> such as in the following example:
>
> CREATE TABLE TEST_1 (
> F1 VARCHAR(10),
> F2 VARCHAR(5) );
>
> CREATE TABLE TEST_2 (
> F1 VARCHAR(10),
> F2 VARCHAR(10) );
>
> we're consistently having the following storage behavior:
>
> 60 records / 4k_page
> 120 records / 8k_page
> 240 records / 16k_page
> 480 records / 32k_page.
>
> So it seems that it doesn't matter whether the record size is
> 15 bytes or 20 bytes, there's maximum number of records per page
> as shown above.
>
> Any clues if there's any parameter or bug causing that?
>
> Gan (for Amgad)

Well, you're size counts are completely wrong, for starters.

Each varchar uses 4 bytes + length of the string, so that's 8 more bytes
per row. Then you may have an OID as well for another 4 bytes. I'd also
not be surprised if the length of the string is rounded up to the
nearest word (although I don't know if it is), and I'd be amazed if the
length of the record isn't rounded to some boundary too.

There's a handy page in the documentation that talks about how to know
how big rows are, I suggest you start there...

Stephen

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