Optimise cross-table update for one-to-one fields

[Malcolm Tredinnick]

When we do an update involving a OneToOneField (model inheritance cases being an obvious example), the update queryset does something akin to filter(pk__in=inner_query) where inner_query describes the child model. However, the field in this particular case is known to be one-to-one, not many-to-one. So the reverse relation will only match one item. Thus, we might as well do filter(pk=some_value), which is much faster.

Detecting this case can probably be handled in UpdateQuery.add_update_fields() in some fashion, I suspect.

(This is another one of those long-term SQL optimisation projects that I'll get to at some point. Making a tracking bug for it, since it's been floating around for a while.)

[Chris Beaven]

This is a pretty old ticket... is it still just as relevant today with the current state of the ORM?

[Aymeric Augustin]

Change UI/UX from NULL to False.

[Aymeric Augustin]

Change Easy pickings from NULL to False.

[Anssi Kääriäinen]

I believe this one is now fixed for normal querying. The changes done to join path generation likely fix this one for update queries, too.

Anybody willing to write a test case for this ticket?

[Simon Charette]

I'll write that testcase.

[Simon Charette]

Testcase

[Simon Charette]

Added a patch with a testcase that passes on (py273, py323) X (SQLite3, Postgresql).

[Claude Paroz]

CaptureQueriesContext is a nice addition. From the bikeshedding department:

• separate commit for adding CaptureQueriesContext
• specific tests for CaptureQueriesContext in test_utils
• when __getitem__ is applied to a list, I'd rather use an argument named i or index

I'd suggest you go ahead with the commits when above points are addressed (unless someone else has other concerns, of course).

[Simon Charette]

Thanks for your review, was wondering if I should commit CaptureQueriesContext separately.

[Simon Charette]

Added CaptureQueriesContext with tests

[Simon Charette]

Testcase for this ticket using CaptureQueriesContext

[Simon Charette]

Added tests for CaptureQueriesContext, included claudep's suggestions and split the patch in two.

[Simon Charette <charette.s@…>]

In 952ba5237ea62e7647cdd5214b1df79c0e7cea38:

Added a context manager to capture queries while testing.

Also made some import cleanups while I was there.

Refs #10399.

[Simon Charette <charette.s@…>]

In fb3d85bd14f1f7a8969ef8c54f2f0603e161f428:

Fixed #10399 -- Tested that o2o field updates are not constrained by an inner query.