You can truncate a date in Postgres with the function date_trunc. In a simple way it allows you to ignore a part of the date/time and focus only on the part you want.
An example of use:
SELECT date_trunc('month', current_date) as este_mes;
este_mes
------------------------
2014-04-01 00:00:00-03
Note that he truncated today’s date and the result was the first day of this month because the keyword was used month, but you can truncate on time, minute or even year. It’s a good alternative so you don’t need to use the fução extract more than once.
So you can use this feature for your query more or less like this:
SELECT sum(...) as soma
FROM ...
WHERE date_trunc('month', data) = date_trunc('month', current_date);
For more date and time manipulation functions see http://www.postgresql.org/docs/current/static/functions-datetime.html.