Optimizing multiple joins

Question

I\'m trying to figure out a way to speed up a particularly cumbersome query which aggregates some data by date across a couple of tables. The full (ugly) query is below along w

Answer 1

Building on Craig Young's suggestions, here is the amended query which runs in ~1.8 seconds for the data set I'm working on. That is a slight improvement on the original ~2.0s and a huge improvement on Craig's which took ~22s.

SELECT
    p.period,
    /* The pivot technique... */
    SUM(CASE envelope_command WHEN 1 THEN body_size ELSE 0 END) AS Outbound,
    SUM(CASE envelope_command WHEN 2 THEN body_size ELSE 0 END) AS Inbound
FROM
(
    /* Get days range */
    SELECT date '2009-10-01' + day AS period
    FROM generate_series(0, date '2009-10-31' - date '2009-10-01') AS day
) p
    /* Join message information */
    LEFT OUTER JOIN
    (
        SELECT b.body_size, b.body_time::date, e.envelope_command
        FROM body AS b 
            INNER JOIN envelope e ON e.message_id = b.message_id 
        WHERE
            e.envelope_command IN (2, 1)
            AND b.body_time::date BETWEEN (date '2009-10-01') AND (date '2009-10-31')
    ) d ON d.body_time = p.period
GROUP BY p.period
ORDER BY p.period