Using a variable period in an interval in Postgres

I have a relation that maintains monthly historical data. This data is added to the table on the last day of each month. A service I am writing can then be called specifying a month and a number of months prior for which to retrieve the historical data. I am doing this by creating startDate and endDate variables, and then returning data between the two. The problem I am having is that startDate is a variable number of months before endDate, and I cannot figure out how to use a variable period in an interval.

Here is what I have:

    DECLARE
      endDate   TIMESTAMP := (DATE_TRUNC('MONTH',$2) + INTERVAL '1 MONTH') - INTERVAL '1 DAY';
      startDate TIMESTAMP := endDate - INTERVAL $3 'MONTH';

I know that the line for startDate is not correct. How is this properly done?

Use this line:

startDate TIMESTAMP := endDate - ($3 || ' MONTH')::INTERVAL;

and note the space before MONTH. Basically: You construct a string with like 4 MONTH and cast it with ::type into a proper interval.

Edit: I' have found another solution: You can calculate with interval like this:

startDate TIMESTAMP := endDate - $3 * INTERVAL '1 MONTH';

This looks a little bit nicer to me.

This code has nothing directly to do with your situation, but it does illustrate how to use variables in INTERVAL arithmetic. My table's name is "calendar".

CREATE OR REPLACE FUNCTION test_param(num_months integer)
  RETURNS SETOF calendar AS
$BODY$

    select * from calendar
    where cal_date <= '2008-12-31 00:00:00'
    and cal_date > date '2008-12-31' - ($1 || ' month')::interval;

$BODY$
  LANGUAGE sql VOLATILE
  COST 100
  ROWS 1000;