How to find if a SELECT is reading from buffer or disk ?

From: Balkrishna Sharma <b_ki(at)hotmail(dot)com>
To: <pgsql-admin(at)postgresql(dot)org>
Subject: How to find if a SELECT is reading from buffer or disk ?
Date: 2010-05-25 17:48:34
Message-ID: BAY149-w19CB46743B9D1EC9D68760F0E80@phx.gbl
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Hi,
I am increasing the shared_buffer size in postgresql.conf and want to measure its effect on READ. In essence I want to know if the SELECT queries I am firing repeatedly is reading from the buffer or going directly to the disk.
I am expecting the first SELECT to go to disk and the subsequent call of the same SELECT to read from buffer .
Right now I am just looking at execution time of the SELECTs and trying to conclude. But there should be a direct way to see where the SELECT reads from.
How can I accomplish this ?
ThanksBala
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