Postgres is reporting that it is out of disk space while performing a rather large aggregation query:

Caused by: org.postgresql.util.PSQLException: ERROR: could not write block 31840050 of temporary file: No space left on device
    at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:1592)
    at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1327)
    at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:192)
    at org.postgresql.jdbc2.AbstractJdbc2Statement.execute(AbstractJdbc2Statement.java:451)
    at org.postgresql.jdbc2.AbstractJdbc2Statement.executeWithFlags(AbstractJdbc2Statement.java:350)
    at org.postgresql.jdbc2.AbstractJdbc2Statement.executeUpdate(AbstractJdbc2Statement.java:304)
    at org.hibernate.engine.query.NativeSQLQueryPlan.performExecuteUpdate(NativeSQLQueryPlan.java:189)
    ... 8 more

However the disk has quite a lot of space:

Filesystem            Size Used Avail Use% Mounted on
/dev/sda1             386G  123G  243G  34% /
udev                  5.9G  172K  5.9G   1% /dev
none                  5.9G     0  5.9G   0% /dev/shm
none                  5.9G  628K  5.9G   1% /var/run
none                  5.9G     0  5.9G   0% /var/lock
none                  5.9G     0  5.9G   0% /lib/init/rw

The query is doing the following:

INSERT INTO summary_table SELECT t.a, t.b, SUM(t.c) AS c, COUNT(t.*) AS count,
    t.d, t.e, DATE_TRUNC('month', t.start) AS month, tt.type AS type, FALSE, tt.duration
FROM detail_table_1 t, detail_table_2 tt
WHERE t.trid=tt.id  AND tt.type='a'
    AND DATE_PART('hour', t.start AT TIME ZONE 'Australia/Sydney' AT TIME ZONE 'America/New_York')>=23
        OR DATE_PART('hour', t.start AT TIME ZONE 'Australia/Sydney' AT TIME ZONE 'America/New_York')<13
GROUP BY month, type, t.a, t.b, t.d, t.e, FALSE, tt.duration

any tips?