You might try the geqo_threshhold setting increasing it to 24 or so to see if it speeds up this particular query.
geqo_threshold (integer)
Use genetic query optimization to plan queries with at least this many FROM items involved. (Note that a FULL OUTER JOIN construct counts as only one FROM item.) The default is 12. For simpler queries it is usually best to use the deterministic, exhaustive planner, but for queries with many tables the deterministic planner takes too long.
See: http://www.postgresql.org/docs/8.2/static/runtime-config-query.html#RUNTIME-CONFIG-QUERY-GEQO
