`report_journal_ledger` is auto-vacuumed as any transient model, but has some
ondelete="cascade" constraints that auto-remove subtables when a record is removed,
doing this operation very slow when selecting these sub-records.
Letting default ondelete="set null" would result in same performance bottleneck,
as the select on sub-table is performed the same for setting "null" value on them.
As a solution, and for avoiding a costly index operation, we delete by SQL sub-tables
rows in advance.
A bit of extra logic has been added for avoiding to remove that records if it's not
the turn of vacuum the parent table.
Pedro M. Baeza
Isaac Gallart Bochons
mreficent
Jordi Ballester Alomar