From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200210200259.g9K2xr423634@candle.pha.pa.us>
Subject: Re: indexing on char vs varchar
In-Reply-To: <200210021055.22641.josh@agliodbs.com>
To: josh@agliodbs.com
Date: Sat, 19 Oct 2002 22:59:53 -0400 (EDT)
Cc: Beth Gatewood <beth@vizxlabs.com>, pgsql-sql@postgresql.org
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I have updated the FAQ to mention that char()/varchar()/text/bytea have
similar performance characteristics.

---------------------------------------------------------------------------

Josh Berkus wrote:
> Beth,
> 
> > Sorry....I don't understand.  The length is at the front of what?
> 
> In some RDBMSs, the VARCHAR data type has a 2 or 4-byte indicator of the 
> length of the stored string before the data itself, while CHAR does not 
> require this information because it is fixed-length.  This makes the CHAR 
> datatype marginally smaller, and thus faster, than the VARCHAR data type on 
> those databases.   This difference goes back to much older databases and 
> computers, where every byte of a row counted in terms of performance.
> 
> In my experience, even though MS SQL Server still functions this way, the 
> performance difference between CHAR and VARCHAR is not measurable unless you 
> are getting close to the 8K data page limit that MSSQL imposes.  YMMV.
> 
> Postgres does not materially differentiate between CHAR, VARCHAR, and TEXT, 
> except that CHAR is padded by spaces and VARCHAR often has a length limit.   
> However, in terms of storage efficiency (and indexing efficiency), they are 
> identical.  In Postgres, the character count is included in all string data 
> types.
> 
> Thus, you should use the data type most appropriate to the data you are 
> storing, ignoring performance issues.  If the data is a fixed-length string 
> (such as a required zip code) use CHAR; if it's variable but limited, use 
> varchar; if it's a long description, use TEXT.
> 
> -- 
> -Josh Berkus
>  Aglio Database Solutions
>  San Francisco
> 
> 

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