Support PostgreSQL native inheritance

[roji]

PostgreSQL natively supports table inheritance in the database (see docs here, and also some current caveats). In theory EF Core could map CLR inheritance relations to this scheme as an additional alternative to TPH/TPC/TPT.

• You use a special DDL to specify that one table (capitals) INHERITS FROM another table (cities).
• The child table (capitals) implicitly contains fields from the parent (cities), so to get capitals you only need to select from the child table. In that sense it's like TPC.
• However, tables need to be created for both child and parent, in that sense it's like TPT.
• Selecting from the parent table will by default return children rows, so selecting from cities will return all cities. A special SELECT ... FROM ONLY CITIES syntax can return only the parents.

Requested in npgsql/efcore.pg#59 and this SO.

Disclaimer: I'm opening this after a conversation with @divega in which he expressed interest, but I imagine the internal impact would be really non-trivial and beyond what you'd want to provide.

[roji]

Note that table inheritance is no longer recommended by PostgreSQL: https://wiki.postgresql.org/wiki/Don%27t_Do_This#Don.27t_use_table_inheritance

[brianjlowry]

I don't understand why this "fad" is a bad idea. How would you map foreign keys/tables to hierarchical domain models?

[ajcvickers]

Make sure to talk to @roji before starting any work on this.

[AndriySvyryd]

To allow this and other similar patterns the creation of RelationalModel needs to be factored out to a service that the providers can override.