From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-Id: <200210021706.g92H6Re29922@candle.pha.pa.us>
Subject: Re: indexing on char vs varchar
In-Reply-To: <004101c26a32$3bce7390$0c00000a@bethvizx>
To: Beth Gatewood <beth@vizxlabs.com>
Date: Wed, 2 Oct 2002 13:06:27 -0400 (EDT)
Cc: pgsql-sql@postgresql.org
MIME-Version: 1.0
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We store all the text/char/varchar types with the length at the front so
we don't have such optimizations.  We do have "char", in quotes, which
is a single character, but that's about it.

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

Beth Gatewood wrote:
> Hi-
> 
> This is more just trying to understand what is going on under the hood of
> pgsql.  I have read through the archives that there is no difference between
> index on char, varchar or text.  I am wondering why?  I understand all the
> arguments about saving space but I am specifically asking about index
> performance and wondering about the underworkings of indices based on char
> and varchar.
> 
> Othe RDBMS have clear leanings that indexing on chars are a better way to
> go.
> 
> In MySQL this is due to a static table characteristics
> (http://www.mysql.com/doc/en/Static_format.html) and speed for an index
> look-up (row number X row length). and the ease to read a constant number of
> records with each disk.
> 
>  In the case of Oracle, the suggestion for char is based on if using
> varchar2 that takes 5 char and then there is a subsequent update to this
> field to now take 20 char, but now the record can not grow physically...so
> they essentially mark the old one as deleted and create a new record at the
> top (in an entirely new block) but the problem is that the index points to
> the deleted block...so the index has to query the old block and then the
> new....(info from:
> http://groups.google.com/groups?q=oracle+char+vs+varchar+index&hl=en&lr=&ie=
> UTF-8&oe=UTF-8&selm=3a791aa3%40news.iprimus.com.au&rnum=1)
> 
> Thanks for explaining this to me....
> -Beth
> 
> 
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