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count(*) slow on large tables

From: Dror Matalon <dror(at)zapatec(dot)com>
To: "pgsql-performance(at)postgresql(dot)org" <pgsql-performance(at)postgresql(dot)org>
Subject: count(*) slow on large tables
Date: 2003-10-02 19:15:47
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Lists: pgsql-hackerspgsql-performance

I have a somewhat large table, 3 million rows, 1 Gig on disk,  and growing. Doing a
count(*) takes around 40 seconds.

Looks like the count(*) fetches the table from disk and goes through it.
Made me wonder, why the optimizer doesn't just choose the smallest index
which in my case is around 60 Megs and goes through it, which it could
do in a fraction of the time.


Dror Matalon
Zapatec Inc 
1700 MLK Way
Berkeley, CA 94709


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