Date: Sat, 05 Jul 2008 11:04:53 -0400
From: Lew <lew@lwsc.ehost-services.com>
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Subject: Re: performance cost for varchar(20), varchar(255), and text
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To: pgsql-admin@postgresql.org

Jessica Richard wrote:
> I am tuning a database created by someone else.
> 
> I noticed that some column lengths were defined longer than needed.
> 
> For example, an Id column is holding a stand length of 20 characters but 
> was defined as varchar(255).
> 
> On some other columns, for example, a Description column is supposed to 
> hold less than 100 characters but defined as text.
> 
> I am trying to understand the performance impact if a column is over 
> defined in the following cases:
> 
> 1. char(20) vs varchar(20)
> 
> 2. varchar(20) vs varchar(255)
> 
> 3. varchar(255) vs text

Interestingly, the Postgres documentation has the answer:
<http://www.postgresql.org/docs/8.3/interactive/datatype-character.html>
> Tip:  There are no performance differences between these three types, apart 
> from increased storage size when using the blank-padded type, and a few extra 
> cycles to check the length when storing into a length-constrained column. 
> 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. 

Performance is not the only issue.  The semantics of CHAR and VARCHAR differ. 
  You cannot use them equivalently.

I recommend searching the docs as a first approach to finding such answers.

-- 
Lew