Computing the balance or statistics is slow on big projects

[zorun]

Somebody reported that ihatemoney is now much slower than the previous version (4.1.5), and I can confirm using a big project.

I identified two slow actions:

• computing the balance (as shown in the list of bills or in the "settle bills" page)
• computing statistics

For a project with 1000 bills, both actions create thousands of SQL queries like this:

SELECT sum(person.weight) AS sum_1 
FROM person JOIN billowers ON person.id = billowers.person_id JOIN bill ON bill.id = billowers.bill_id 
WHERE bill.id = ?

I traced this query back to the Bill.pay_each_default() function, which is used to basically compute bill_amount / total_owers_weight (what is shown as "42 € each" in the list of bills). Each call to this function triggers the above SQL query.

Some ideas to improve things:

• add a new field Bill.total_owers_weight so that we can cache the result of this expensive computation. We would need to make sure this field is recomputed when necessary.
• rethink the data model, so that simple things don't need complicated JOIN operations.

[zorun]

Ok, as a simpler solution, I managed to implement balance and statistics more efficiently in #890 : with 1000 bills, computing statistics goes from 7 seconds to 500 ms.

pay_each is still used in one place: the list of bills. But thanks to pagination, it's not that costly. It would still be nice to improve it, but that's no longer blocking, so I'm closing this issue.