Date: Thu, 16 Mar 2006 22:25:19 -0800
From: andrew@pillette.com
Message-Id: <200603170625.k2H6PJ926955@pillette.com>
Subject: Re: Indexes with descending date columns
To: theo@flame.co.za
Cc: pgsql-performance@postgresql.org
MIME-Version: 1.0
Content-Type: text/plain
Content-Transfer-Encoding: 7bit

> I have a performance problem when traversing a table in index order with
> multiple columns including a date column in date reverse order. Below
> follows a simplified description of the table, the index and the
> associated query
> 
> \d prcdedit
>  prcdedit_prcd       | character(20)               |
>  prcdedit_date       | timestamp without time zone |
> 
> Indexes:
>     "prcdedit_idx" btree (prcdedit_prcd, prcdedit_date)

Depending on how you use the table, there are three possible solutions.

First, if it makes sense in the domain, using an ORDER BY where _both_ columns are used descending will make PG search the index in reverse and will be just as fast as when both as searched by the default ascending.

Second possibility: Create a dummy column whose value depends on the negative of prcdedit_date, e.g., -extract(epoch from prcdedit_date), keep the dummy column in sync with the original column using triggers, and rewrite your queries to use ORDER BY prcdedit_prod, dummy_column.

Third: Create an index on a function which sorts in the order you want, and then always sort using the function index (you could use the -extract(epoch...) gimmick for that, among other possibilities.)

HTH.