From: Tomas Vondra <tv@fuzzy.cz>
To: Andrus <kobruleht2@hot.ee>
Cc: pgsql-performance@postgresql.org
Subject: Re: Using index for IS NULL query
Date: Tue, 11 Nov 2008 22:01:58 +0100
Message-ID: <4919F2C6.4000303@fuzzy.cz> (raw)
In-Reply-To: <gfcprb$m40$1@news.hub.org>
References: <gfcprb$m40$1@news.hub.org>
> Index is not used for
>
> is null
>
> condition:
>
> create index makse_dokumnr_idx on makse(dokumnr);
> explain select
> sum( summa)
> from MAKSE
> where dokumnr is null
>
> "Aggregate (cost=131927.95..131927.96 rows=1 width=10)"
> " -> Seq Scan on makse (cost=0.00..131927.94 rows=1 width=10)"
> " Filter: (dokumnr IS NULL)"
>
>
> Table makse contains 1200000 rows and about 800 rows with dokumnr is
> null so using index is much faster that seq scan.
> How to fix ?
Yes, NULL values are not stored in the index, but you may create
functional index on
(CASE WHEN dokumnr IS NULL THEN -1 ELSE dokumnr END)
and then use the same expression in the WHERE clause. You may replace
the '-1' value by something that's not used in the dokumnr column.
regards
Tomas
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To: pgsql-performance@postgresql.org
Cc: tv@fuzzy.cz, kobruleht2@hot.ee
Subject: Re: Using index for IS NULL query
In-Reply-To: <4919F2C6.4000303@fuzzy.cz>
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