Allow fields to be part of the RETURNING clause during INSERT

[Johannes Maron]

Dependency for #27452

The SQL RETURNING statement is currently used for inserts of new objects. Mainly it is responsible to return the primary key in a single query.

That's great but currently in the postgres database backend this value is hard coded to only the primary key. I would like to propose to make this configurable.

There are plenty of uses cases for such a feature ultimately even enabling database defaults, but I would suggest to keep it very simple for now.

I would suggest to simply allow model fields to specify if they should be included in the returning statement or not. The default being not (except PK) to maintain the current behavior.

The implementation as well as the related code lines have been already discussed here:
​https://github.com/django/django/pull/7525#issuecomment-354269077

I highly suggest to keep this a Postgres ONLY feature for now. That way we can see what sticks and consider extending this functionality into django.db in the future after we have seen how it's being used and gathered feedback.

[Tim Graham]

A number of Oracle test failures remain.

[Johannes Maron]

I fixed a couple of things. Nothing fails for me now locally that doesn't fail on master. Can you trigger it again?

[Tim Graham <timograham@…>]

In b131f9c7:

Refs #29444 -- Renamed DatabaseFeatures.can_return_id* to be generic for other columns.

[Tim Graham]

You should be able to trigger the Oracle build yourself as described on wiki:Jenkins. I ran the tests locally and put the failures in a PR comment.

[Mariusz Felisiak <felisiak.mariusz@…>]

In bc91f27a:

Refs #29444 -- Added support for fetching a returned non-integer insert values on Oracle.

This is currently not actively used, since the ORM will ask the
SQL compiler to only return auto fields.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 003bb34b:

Refs #29444 -- Made db.backends.oracle.utils.InsertVar use str as default.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 736e7d44:

Refs #29444 -- Fixed DateField constructor in db.backends.oracle.utils.InsertVar.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 7254f113:

Refs #29444 -- Allowed returning multiple fields from INSERT statements on PostgreSQL.

Thanks Florian Apolloner, Tim Graham, Simon Charette, Nick Pope, and
Mariusz Felisiak for reviews.

[Mariusz Felisiak]

This ticket is doable on Oracle with cx_Oracle 6.0+ that's why I'm leaving it open. First attempt of fixing this on Oracle was a part of the original PR (see a diff).

[Mariusz Felisiak]

​PR with adding support on Oracle

[Mariusz Felisiak <felisiak.mariusz@…>]

In b31e6387:

Fixed #29444 -- Allowed returning multiple fields from INSERT statements on Oracle.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 0110436:

Refs #29444 -- Removed redundant DatabaseFeatures.can_return_multiple_columns_from_insert.

Unnecessary since b31e63879eb5d9717e9f890401f7222e4f00c910.