From: | Daniel CAUNE <d(dot)caune(at)free(dot)fr> |
---|---|
To: | 'Tom Lane' <tgl(at)sss(dot)pgh(dot)pa(dot)us>, 'Daniel Caune' <daniel(dot)caune(at)ubisoft(dot)com> |
Cc: | pgsql-sql(at)postgresql(dot)org |
Subject: | Re: Index on nullable column |
Date: | 2006-03-25 12:38:29 |
Message-ID: | 0IWO00E7BOG2MP90@VL-MH-MR001.ip.videotron.ca |
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Thread: | |
Lists: | pgsql-sql |
> > Is an index on a nullable column useful for retrieving rows having that
> > column null?
>
> Nope, because IS NULL isn't an indexable operator.
>
> You can make an end-run around that with a partial index, eg
>
> create index fooi on foo(f1) where f1 is null
>
> This can be used to satisfy queries using "where f1 is null", but it's
> not any good for any other purpose.
>
> If you often do "where f1 is null and something-about-f2",
> you might get better mileage with
>
> create index fooi on foo(f2) where f1 is null
>
> but it's still a very specialized index.
>
Thanks Tom. I will use such an index even if it's very specialized; OLAP world is somewhat full of specialized index anyway... :-)
--
Daniel
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