How to find if a SELECT is reading from buffer or disk ? - Mailing list pgsql-admin

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 ?

Thanks

Bala