Message-ID: <46321FBA.3070302@archonet.com>
Date: Fri, 27 Apr 2007 17:07:22 +0100
From: Richard Huxton <dev@archonet.com>
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To: sid@etsy.com
Cc: pgsql-performance@postgresql.org
Subject: Re: [Fwd: ] How
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Siddharth Anand wrote:
> Hi!
> I read the link below and am puzzled by or curious about something.
> http://www.postgresql.org/docs/8.1/interactive/datatype-character.html
> 
> The Tip below is intriguing
> 
> "Tip:  There are no performance differences between these three types,
> apart from the increased storage size when using the blank-padded type.
> While character(n) has performance advantages in some other database
> systems, it has no such advantages in PostgreSQL. In most situations text
> or character varying should be used instead."
> 
> How can a field that doesn't have a limit like "text" perform similarly to
> char varying(128), for example? At some point, we need to write data to
> disk. The more data that needs to be written, the longer the disk write
> will take, especially when it requires finding free sectors to write to.

That's no difference *for the same amount of data*. So, char(128), 
varchar(128) with 128 characters and text with 128 characters in it are 
the same. This isn't always the case with other systems.

> Another interesting quote from the same page is the following:
> 
> "Long values are also stored in background tables so they do not interfere
> with rapid access to the shorter column values. "
> 
> If the long values are stored in a separate table, on a different part of
> the disk, doesn't this imply an extra disk seek? Won't it therefore take
> longer?

Yes. But you gain every time you read from the table and aren't 
interested in that column. Typically large text columns contain 
descriptive text and aren't used in joins, so it pays for itself quite 
easily.

-- 
   Richard Huxton
   Archonet Ltd