The location table has an index that is clustered on the
time column to improve the query performance of date range
queries. If the table becomes large and performance degrades, run the
psql cluster command from time-to-time to re-cluster it. Note an
exclusive lock is placed on the table for the duration of the cluster
command execution.
See http://dba.stackexchange.com/questions/39589/optimizing-queries-on-a-range-of-timestamps-two-columns for more information.