Don't delete system catalog tables in Postgres

[viggyfresh]

• See diff
• We've been bumping into this recently - I'm sure there is a cleaner way to check for system catalog tables, but I couldn't figure it out.

[Jaymon]

I'm apprehensive about having this in the core for a few reasons:

• most people don't create tables with the same table owner as the table owner of the pg_* tables.
• Just filtering pg_* isn't sufficient, there are also sql_* tables and maybe others, that ambiguity scares me.

• After a bit of research, I can't figure out a way to definitively tell the difference between the system tables and the prom tables, I tried looking at the output of these queries but nothing jumped out at me:

  * `SELECT tablename FROM pg_tables`
  * `SELECT * FROM information_schema.tables`

Because of the above reasons, my inclination is to not merge this unless we can figure out a bulletproof way to tell the system tables apart from the prom tables.

But since you definitely need this, my proposal would be to override prom's Interface with a custom version, along the lines of:

# custom.py
from prom.interface.postgres import PostgreSQL

class CustomPostgres(PostgresSQL):
    def _delete_table(self, schema, **kwargs):
       table_name = self._normalize_table_name(schema)
       if table_name.startswith("pg_"): return
       super(CustomPostgres, self)._delete_table(schema, **kwargs)

Then you could just modify your PROM_DSN to use your custom interface instead:

custom.CustomPostgres://user:pw@localhost/db"

Hopefully that can get you pointed in the right direction.

[viggyfresh]

Thanks Jay - will look into it.