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From: Mark Mielke <mark@mark.mielke.cc>
To: Robins Tharakan <tharakan@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: count * performance issue
Date: Mon, 10 Mar 2008 23:01:51 -0400
Message-ID: <47D5F61F.4080305@mark.mielke.cc> (raw)
In-Reply-To: <36af4bed0803101957u6078127fq78dc3384fe78de5d@mail.gmail.com>
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	<200803071727.36146.josh@agliodbs.com>
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	<36af4bed0803101957u6078127fq78dc3384fe78de5d@mail.gmail.com>

Robins Tharakan wrote:
> Hi,
>
> I have been reading this conversation for a few days now and I just 
> wanted to ask this. From the release notes, one of the new additions 
> in 8.3 is (Allow col IS NULL to use an index (Teodor)).
>
> Sorry, if I am missing something here, but shouldn't something like 
> this allow us to get a (fast) accurate count ?
>
> SELECT COUNT(*) from table WHERE indexed_field IS NULL
> +
> SELECT COUNT(*) from table WHERE indexed_field IS NOT NULL

For PostgreSQL: You still don't know whether the row is visible until 
you check the row. That it's NULL or NOT NULL does not influence this truth.

Cheers,
mark

-- 
Mark Mielke <mark@mielke.cc>

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