I have a database on PostgreSQL 9.2 that has a main schema with around 70 tables and a variable number of identically structured per-client schemas of 30 tables each. The client schemas have foreign keys referencing the main schema and not the other way around.
I just started filling the database with some real data taken from the previous version. The DB had reached about 1.5 GB (it's expected to grow to several 10s GB within weeks) when I had to do a bulk delete in a very central table in the main schema. All concerned foreign keys are marked ON DELETE CASCADE.
It was no surprise that this would take a long time but after 12 hours it became clear that I was better off starting over, dropping the DB and launching the migration again. But what if I need to repeat this operation later when the DB is live and much larger? Are there alternative, faster methods?
Would it be much faster if I wrote a script that will browse the dependent tables, starting at the table furthest from the central table, deleting the dependent rows table by table?