Inefficient query produced when using OR'ed Q objects

[Tom Forbes]

When using OR'ed Q objects in a .filter() call Django could be more intelligent about the query it produces.

For example, the following two queries both produce LEFT OUTER JOIN:

SomeModel.objects.filter(Q(related_objects__field=1) | Q(related_objects__other_field=1)

SomeModel.objects.filter(Q(related_objects__field=1) | Q())

In the case of the second query it could be reduced to a INNER JOIN as the OR is redundant and it is functionally the same as a .filter(related_objects__field=1).

It is also quite a common pattern to do:

filters = Q()
if condition:
   filters |= Q(x=1)
if other_condition:
   filters |= Q(y=2)

And with the current implementation it will always produce a query that assumes filters is a valid OR. Django should/could be more intelligent and detect if there is only one OR condition, and reduce it.

[Tim Graham]

Tentatively accepting, though I don't know if this is feasible or correct. If a patch is provided, hopefully the test suite will validate its correctness.

[Tom Forbes]

I added a patch here: ​https://github.com/django/django/pull/8517

I took the easy route and fixed what I think is an issue in the Q object. Combining empty Q objects should be a no-op. This effectively fixes this ticket without many changes.

[Simon Charette]

I think the provided patch solves this in an efficient way.

We could add folding logic in the query compiler as well but given it will be very hard to generate combined Q with a falsey sides after with this patch (one would have to call Q.add manually) I don't think it's required.

[Tim Graham <timograham@…>]

In 1c3a6ce:

Refs #28211 -- Added a test for ANDing empty Q()'s.

This test passes to due some logic in Node.add().

[Tim Graham <timograham@…>]

In bb0b6e52:

Fixed #28211 -- Prevented ORing an empty Q() from reducing query join efficiency.