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Date: Mon, 15 Oct 2012 19:44:46 +0200
From: =?ISO-8859-1?Q?Pedro_Jim=E9nez?= <p.jimenez@ismsolar.com>
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To: pgsql-performance@postgresql.org
Subject: limit order by performance issue
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Hello,
   I'm trying to do a simple SQL query over Postgresl 9.0 running on Ubuntu.

I have a large table (over 100 million records) with three fields, 
id_signal (bigint), time_stamp (timestamp) and var_value (float).

My query looks like this:

select var_value from ism_floatvalues where id_signal = 29660 order by 
time_stamp desc limit 1;

So I want to select the last value from a determinated ID (is_signal).

This query runs FOREVER, while if I delete "limit 1" it runs instantly....

Any help?

Regards.