Question

I have a SQL function called get_forecast_history(integer,integer) that takes two arguments, a month and a year. The function returns a CUSTOM TYPE created with:

CREATE TYPE fcholder AS (y integer, m integer, product varchar, actual real);

The first line of the function definition is:

CREATE OR REPLACE FUNCTION get_forecast_history(integer, integer)
  RETURNS SETOF fcholder AS $$

Calling:

SELECT * FROM get_forecast_history(10, 2011);

For example produces the following table (the result type of the function is a table i.e. SETOF):

  y   m product  actual
---- -- -------- ------
2011 10 Product1  29
2011 10 Product2  10
2011 10 Product3  8
2011 10 Product4  0
2011 10 Product5  2

etc. (about 30 products total). This is the history for the given month.

I also have another query that generates a series of months:

SELECT to_char(DATE '2008-01-01'
            + (interval '1 month' * generate_series(0,57)), 'YYYY-MM-DD') AS ym

Which products a list like this:

ym
----------
2008-01-01
2008-02-01
2008-03-01
2008-04-01
...
2011-10-01

I need to somehow LEFT JOIN the results of the generate_series of year/month combinations on the function above by taking the results of the generate_series and passing them as arguments to the function. This way I’ll get the results of the function, but for every year/month combination from the generate_series. At this point I’m stuck.

I’m using PostgreSQL 8.3.14.

Answer 1

What you are trying to to could work like this:

Edit with additional info

CREATE OR REPLACE FUNCTION f_products_per_month()
  RETURNS SETOF fcholder AS
$BODY$
DECLARE
    r fcholder;
BEGIN

FOR r.y, r.m IN
    SELECT to_char(x, 'YYYY')::int4  -- AS y
          ,to_char(x, 'MM')::int4    -- AS m
    FROM  (SELECT '2008-01-01 0:0'::timestamp
        + (interval '1 month' * generate_series(0,57)) AS x) x
LOOP
    RETURN QUERY
    SELECT *    -- use '*' in this case to stay in sync
    FROM   get_forecast_history(r.m, r.y);

    IF NOT FOUND THEN
       RETURN NEXT r;
    END IF;
END LOOP;

END;
$BODY$
  LANGUAGE plpgsql;

Call:

SELECT * FROM f_products_per_month();

Major points:

• Final edit to include an otherwise empty row for months without products.
• You wrote “LEFT JOIN”, but that’s not how it can work.
• There are several ways to do this, but RETURN QUERY is the most elegant.
• Use the same return type as your function get_forecast_history() uses.
• Avoid naming conflicts with the OUT parameters by table-qualifying the column names (not applicable any more in the final version).
• Don’t use DATE '2008-01-01', use a timestamp like I did, it has to be converted for to_char() anyway. Less casting, performs better (not that it matters much in this case).
• '2008-01-01 0:0'::timestamp and timestamp '2008-01-01 0:0' are just two syntax variants doing the same.
• For older versions of PostgreSQL the language plpgsql is not installed by defualt. You may have to issue CREATE LANGUAGE plpgsql; once in your database. See the manual here.

You could probably simplify your two functions into one query or function if you wanted.