Understanding Hash Join performance

From: Robert James <srobertjames(at)gmail(dot)com>
To: pgsql-performance <pgsql-performance(at)postgresql(dot)org>
Cc: srobertjames <srobertjames(at)gmail(dot)com>
Subject: Understanding Hash Join performance
Date: 2011-06-02 00:10:32
Message-ID: BANLkTikh3k=TcezOUk86k6CFw39oF6=uGA@mail.gmail.com
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A query I has spends a long time on Hash Joins (and Hash Left Joins).
I have a few questions:

1. When does Postgres decide to do a Hash Join, over another type of Join?
2. Do Hash Joins normally perform poorly? What can I do to speed them up?
3. What can I do to enable Postgres to use a faster type of join?

If there's a good resource for me to read on this, please let me know.

Thanks!

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