Postgresql 9.1 query to find queries that are blocked by locks

blocked_queries.sql

SELECT
    l1.relation::regclass       AS blocked_relation,
    l1.pid                      AS blocked_pid,
    a1.usename                  AS blocked_user,
    a1.current_query            AS blocked_query,
    now() - a1.query_start      AS blocked_time,
    l2.pid                      AS blocking_pid,
    a2.usename                  AS blocking_user,
    a2.current_query            AS blocking_query,
    l2.virtualtransaction,
    l2.mode,
    l2.granted
FROM
    pg_locks l1
JOIN
    pg_locks l2 ON (
        (
            l1.locktype,
            l1.DATABASE,
            l1.relation,
            l1.page,
            l1.tuple,
            l1.virtualxid,
            l1.transactionid,
            l1.classid,
            l1.objid,
            l1.objsubid
        )
    IS NOT DISTINCT FROM
        (
            l2.locktype,
            l2.DATABASE,
            l2.relation,
            l2.page,
            l2.tuple,
            l2.virtualxid,
            l2.transactionid,
            l2.classid,
            l2.objid,
            l2.objsubid
        )
    )
JOIN pg_stat_activity a1 ON a1.procpid = l1.pid
JOIN pg_stat_activity a2 ON a2.procpid = l2.pid
WHERE
    NOT l1.granted
AND
    l2.granted
ORDER BY blocked_time DESC
;