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This will replace the manual calculation in Admin [^1] for SMS and
also in API [^2] for letters.
Doing the calculation here also means we correctly attribute free
allowance to the earliest rows in the billing table - Admin doesn't
know when a given rate was applied so can't do this with the data
currently returned from the API.
Since the calculation now depends on annual billing, we need to
change all the tests to make sure a suitable row exists.
Note about "OVER" clause
========================
Using "rows=" ("ROWS BETWEEN") makes more sense than "range=" as
we want the remainder to be incremental within each group in a
"GROUP BY" clause, as well as between groups i.e
# ROWS BETWEEN (arbitrary numbers to illustrate)
date=2021-04-03, units=3, cost=3.29
date=2021-04-03, units=2, cost=4.17
date=2021-04-04, units=2, cost=5.10
vs.
# RANGE BETWEEN
date=2021-04-03, units=3, cost=4.17
date=2021-04-03, units=2, cost=4.17
date=2021-04-04, units=2, cost=5.10
See [^3] for more details and examples.
[^1]: https://github.com/alphagov/notifications-admin/blob/master/app/templates/views/usage.html#L60
[^2]: 072c3b2079/app/billing/billing_schemas.py (L37)
[^3]: https://learnsql.com/blog/difference-between-rows-range-window-functions/