To: wweng@kencast.com
cc: pgsql-sql@postgresql.org
Subject: Re: create index on function - why? 
In-reply-to: <3C6D3684.9040101@kencast.com> 
References: <3C6D3684.9040101@kencast.com>
Comments: In-reply-to Wei Weng <wweng@kencast.com>
	message dated "Fri, 15 Feb 2002 11:25:40 -0500"
Date: Fri, 15 Feb 2002 11:46:46 -0500
Message-ID: <14409.1013791606@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>

Wei Weng <wweng@kencast.com> writes:
> But what is the rationale behind creating index on a function?

To speed up searching.

For example, given

	create table foo (f1 text);

	create index fooi on foo (upper(f1));

the index can be used for queries like

	select * from foo where upper(f1) = 'HELLO';

Without the index, there'd be no way to avoid a sequential scan --- not
to mention evaluation of the function at every row.  With the index,
the above query actually performs zero evaluations of upper() --- the
work got pushed into row insertion, instead.

A functional index is sort of like a hidden, precomputed column added to
your table.

			regards, tom lane