Nested subqueries have conflicting alias prefixes

[Anssi Kääriäinen]

Currently the ORM has a way to bump table alias prefixes for subqueries. This means that for example alias T0 would be turned to U0. Unfortunately the current way doesn't work when nesting multiple queries, that is in cases like:

qs1 = MyModel.objects.all()
qs2 = MyModel.objects.filter(pk__in=qs1)
qs3 = MyModel.objects.filter(pk__in=qs2)

for qs2 the qs1 gets bumped to U-prefixes. But qs2 gets bumped *again* to U-prefixes in qs3. Luckily this doesn't usually matter that much in current Django, as for example "u0.id IN (SELECT u0.id FROM sometable u0) does work. Later on, if EXISTS queries are going to be more common this will become more important as "WHERE EXISTS (SELECT ... WHERE u0.id = u0.id) doesn't work at all.

[Anssi Kääriäinen]

Patch available at ​https://github.com/akaariai/django/tree/ticket_20874

I think this one is ready for checkin, but as the fix is nontrivial I will leave this open for reviews.

[Anssi Kääriäinen <akaariai@…>]

In dcdc579d162b750ee3449e34efd772703592faca:

Fixed #20874 -- bump_prefix() in nested subqueries

Also made some cleanup to build_filter() code by introducing submethods
solve_lookup_type() and prepare_lookup_value().