How to create a function that passes through another function in PostgreSQL?

Question

I am using PostgreSQL 9.3.9, and I have a procedure called list_all_upsells that takes the beginning of the month and the end of the month. (see sqlfiddle.com/#!15/abd02 for example data). For example, the code below will list the number of deleted accounts in October:

select COUNT(up.*) as "Total Upsell Accounts in October" from 
list_all_upsells('2015-10-01 00:00:00'::timestamp, '2015-10-31 23:59:59'::timestamp) as up
where up.user_id not in
(select distinct user_id from paid_users_no_more 
where concat(extract(month from payment_stop_date),'-',extract(year from payment_stop_date))<>
concat(extract(month from payment_start_date),'-',extract(year from payment_start_date)));

The list_all_upsells procedure is as follows:

DECLARE
payor_email_2 text;
   BEGIN
FOR payor_email_2 in select distinct payor_email from paid_users LOOP
return query
execute
'select paid_users.* from paid_users,
(
select payment_start_date as first_time from paid_users
where payor_email = $3
order by payment_start_date limit 1
) as dummy
where payor_email = $3
and payment_start_date > first_time
and payment_start_date between $1 and $2
and first_time < $1'
using a, b, payor_email_2;
END LOOP;
return;
END

I want to be able to run this for all the months when we have records, and query the data together in one table as follows:

Month   | Total Upselled Accounts
---------------------------------
08/2014 | 23
09/2014 | 35
ETC...
10/2015 | 56

I have a request to capture the first of each month and the last month of each month during which we have been in business:

select distinct date_trunc('month', payment_start_date)::date as startmonth
from paid_users ORDER BY startmonth;

Last month:

SELECT distinct (date_trunc('MONTH', payment_start_date) + 
INTERVAL '1 MONTH - 1 day')::date as endmonth from paid_users 
ORDER BY endmonth;

, list_all_upsells ? startmonth 2014-03-01, 2014-04-01,... 2015-10-01, endmonth 2014-03-31, 2014-04- 30,... 2015-10-31. list_all_sells ,

My paid_users :

CREATE TABLE paid_users
(
  user_id integer,
  user_email character varying(255),
  payor_id integer,
  payor_email character varying(255),
  payment_start_date timestamp without time zone DEFAULT now()
)

paid_users_no_more:

CREATE TABLE paid_users_no_more
(
  user_id integer,
  payment_stop_date timestamp without time zone DEFAULT now()
)

Answer 1

, . , (1) , , , ; (2) , ( ); (3) ; (4) . , PostgreSQL CREATE FUNCTION list_all_upsells(...), .

: , , , payment_start_date payment_stop_date . , , , . payment_start_date > first_time , (first_time < $1) (payment_start_date BETWEEN $1 AND $2).

(1), (2) (3) :

CREATE FUNCTION list_all_upsells(timestamp) RETURNS SETOF paid_users AS $$
  SELECT paid_users.*
  FROM paid_users
  JOIN (  -- This JOIN keeps only those rows where the payor_email has a prior subscription
    SELECT DISTINCT payor_email,
           first_value(payment_start_date) OVER (PARTITION BY payor_email ORDER BY payment_start_date) AS dummy
    FROM paid_users
    WHERE payment_start_date < date_trunc('month', $1)
  ) dummy USING (payor_email)
  -- This filter keeps only those rows with new subscriptions in the month
  WHERE date_trunc('month', payment_start_date) = date_trunc('month', $1)
$$ LANGUAGE sql STRICT;

SQL, sql, , plpgsql. , , , list_all_upsells(LOCALTIMESTAMP) . , , :

SELECT count(up.*) AS "Total Upsell Accounts in October"
FROM list_all_upsells(LOCALTIMESTAMP) up
WHERE up.user_id NOT IN 
  (SELECT DISTINCT user_id FROM paid_users_no_more 
   WHERE date_trunc('month', payment_stop_date) <>
         date_trunc('month', up.payment_start_date)
  );

, , , paid_users_no_more. payment_stop_date paid_users? NULL, - . , list_all_upsells() , - ?

:

SELECT months.m "Month", coalesce(count(up.*), 0) "Total Upselled Accounts"
FROM generate_series('2014-08-01'::timestamp,
                     date_trunc('month', LOCALTIMESTAMP),
                     '1 month') AS months(m)
LEFT JOIN list_all_upsells(months.m) AS up ON date_trunc('month', payment_start_date) = m
GROUP BY 1
ORDER BY 1;

, , , 0.

SQLFiddle