How to create a function that loops through another function in PostgreSQL? - postgresql

Question

I'm using PostgreSQL 9.3.9 and I have a procedure called list_all_upsells that takes in the beginning of a month and the end of a month. (see sqlfiddle.com/#!15/abd02 for sample data) For example, the below code would list the count of upselled accounts for the month of 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 looks like this:
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 months that we have records and query the data together in one table like this:
Month | Total Upselled Accounts
---------------------------------
08/2014 | 23
09/2014 | 35
ETC...
10/2015 | 56
I have a query to grab the first of each month and last of each month for the months we have been in business:
select distinct date_trunc('month', payment_start_date)::date as startmonth
from paid_users ORDER BY startmonth;
Last of month:
SELECT distinct (date_trunc('MONTH', payment_start_date) +
INTERVAL '1 MONTH - 1 day')::date as endmonth from paid_users
ORDER BY endmonth;
Now how would I create a function to loop through the list_all_upsells and grab the count for each of these months? I.e. the first query for startmonth gives me 2014-03-01, 2014-04-01, ...to 2015-10-01 whereas the second query for endmonth gives me 2014-03-31, 2014-04-30, ...to 2015-10-31. I want to run the list_all_sells on each of these months so that I can get an aggregate count each month of how many upselled accounts we have
My paid_users table looks like this:
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

You have a couple of issues with your function, so let's start there. The short of it is that (1) you need only a single parameter to indicate the month, using beginning and ending of the month is setting yourself up for problems; (2) you do not need a dynamic query because you are not changing identifiers (table or column names); (3) you do not need a loop; and (4) your logic is wrong. I could also mention that PostgreSQL uses functions and that they all start with a line like CREATE FUNCTION list_all_upsells(...) but that would be just too picky.
To start with the logic: Apparently a user identified by his email address takes out a subscription from a certain payment_start_date until a certain payment_stop_date and can do this multiple times. You are looking for those users who took out their first subscription before the month in question, and who started a new subscription in the month in question but not a first subscription. In that case the filter payment_start_date > first_time is useless because you already filter for a first subscription being prior to the month in question (first_time < $1) and a new subscription (payment_start_date BETWEEN $1 AND $2).
Points (1), (2) and (3) really only become obvious when rewriting the query inside the function:
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;
Since the body of the function has reduced to a single SQL statement, the function is now a sql language function, which is more efficient than plpgsql. You now supply only a single parameter, which can be any moment in the month you want the data for, so list_all_upsells(LOCALTIMESTAMP) will give you the results for the current month. In terms of the query you posted it would be:
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)
);
This, incidentally, really begs the question why you have the table paid_users_no_more. Why not simply add a column payment_stop_date to table paid_users? Where that column is NULL the user is still subscribed. But the whole query is rather odd, because list_all_upsells() returns new subscriptions during the month, so why bother with cancelled subscriptions at some other time?
Now on to your real question:
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;
Generate a series of months from some starting month until the current month, then count the new subscriptions for each month, possibly 0.
SQLFiddle