we had an issue where the notification postage constraint command ran
into a deadlock, after trying to acquire two exclusive access locks on
large frequently modified/read tables.
To avoid this happening, we've had to split the upgrade script into
three - one script to apply the not-valid constraint to notifications
table, one for notification_history, and a third to validate the two
constraints.
Note: The first two scripts acquire exclusive access locks, but the
third only needs a row by row lock.
since this involves changing the exsiting alembic upgrades, if you've
upgraded your db you'll need to run the following three commands to
revert your database to a previous good state.
```
alter table notifications drop constraint chk_notifications_postage_null;
alter table notification_history drop constraint chk_notification_history_postage_null;
update alembic_version set version_num = '0229_new_letter_rates';
```
2b354eb5df