Message-ID: <44A53532.50402@logix-tt.com>
Date: Fri, 30 Jun 2006 16:29:06 +0200
From: Markus Schaber <schabi@logix-tt.com>
Organization: Logical Tracking and Tracing International AG, Switzerland
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To: pgsql-performance@postgresql.org
Subject: Re: Index Being Ignored?
References: <D2575270-BD66-4654-A9E7-11A4207A00D2@sweetwater.com>
In-Reply-To: <D2575270-BD66-4654-A9E7-11A4207A00D2@sweetwater.com>
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Hi, Joe,

Joe Lester wrote:
> Aggregate  (cost=22695.28..22695.28 rows=1 width=0) (actual
> time=2205.688..2205.724 rows=1 loops=1)
>   ->  Seq Scan on purchase_order_items  (cost=0.00..21978.08 rows=286882
> width=0) (actual time=0.535..2184.405 rows=7458 loops=1)
>         Filter: (expected_quantity > 0)

The query planner estimates that your filter will hit 286882 rows, while
in reality it hits only 7458 rows. That's why the query planer chooses a
sequential scan.

It seems that the statistics for the column expected_quantity are off.

My suggestions:

- make shure that the statistics are current by analyzing the table
appropriately (e. G. by using the autovacuum daemon from contrib).

- increase the statistics target for this column.

- if you run this query very often, an conditional index might make sense:

CREATE INDEX purchase_order_having_quantity_idx ON purchase_order_items
(expected_quantity) WHERE expected_quantity > 0;


HTH,
Markus

-- 
Markus Schaber | Logical Tracking&Tracing International AG
Dipl. Inf.     | Software Development GIS

Fight against software patents in EU! www.ffii.org www.nosoftwarepatents.org