Add introspection of special table and view types.

[Nick Pope]

Originally this ticket was aimed at PostgreSQL:

Looking at the documentation for ​pg_class there are the following types that we might be interested in introspecting for generation of models:

• r — ordinary table — obviously this is already supported.
• v — views — support was implemented in #29004.
• f — foreign table — being addressed by #29719.
• m — materialized views — could be handled much like #29004, mapping m to v.
• p — partitioned table — could be handled mapping p to t, but gets more complicated, read on below...

For partitioned tables, we are unlikely to want to generate models for all of the individual partitions, although this could be supported with an --include-partitions flag.

From ​this link we can see an example of output for partitioned tables:

 relid |      relname       | relsize | relispartition | relkind
-------+--------------------+---------+----------------+---------
 16410 | population         |       0 | f              | p
 16417 | population_s       |    8192 | t              | r
 16424 | population_t       |       0 | t              | p
 16431 | population_t_10_20 |    8192 | t              | r
 16445 | population_t_20_30 |    8192 | t              | r

On PostgreSQL 10+ we need to filter on NOT relispartition to ignore all of the partitions and only include the parent (which is the table that is usually interacted with).

Regarding support for all backends:

Feature support by backend:

	PostgreSQL	MySQL	Oracle	SQLite
Partitions	✔	✔	✔	✘
Foreign Tables	✔	✘	✘	✘
Materialized Views	✔	✘	✔	✘

It looks partitions can be introspected using INFORMATION_SCHEMA.PARTITIONS for ​MySQL and there are other options for ​Oracle.

For introspection of materialized views, I think we could use USER_MVIEWS for ​Oracle.

[Nick Pope]

​PR