From: | Lew <lew(at)lwsc(dot)ehost-services(dot)com> |
---|---|
To: | pgsql-admin(at)postgresql(dot)org |
Subject: | Re: performance cost for varchar(20), varchar(255), and text |
Date: | 2008-07-05 15:04:53 |
Message-ID: | p9WdnYHPPPgIEPLVnZ2dnUVZ_tydnZ2d@comcast.com |
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Lists: | pgsql-admin |
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
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