`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
f22cb07c76