Arnau,
> As the number of rows grows the time needed to execute this query takes
> longer. What'd I should do improve the performance of this query?
Tip #1) add an index to the timestamp column
Tip #2) make sure that you VACUUM and ANALYZE regularly
Tip #3) You will get better performance if you pass the "current_date - 1
month" as a constant from the client instead of in the query. This is a
known issue, expected to be fixed in 7.5.
--
-Josh Berkus
Aglio Database Solutions
San Francisco