Refs #28333 -- Added partial support for filtering against window functions.

Adds support for joint predicates against window annotations through
subquery wrapping while maintaining errors for disjointed filter
attempts.

The "qualify" wording was used to refer to predicates against window
annotations as it's the name of a specialized Snowflake extension to
SQL that is to window functions what HAVING is to aggregates.

While not complete the implementation should cover most of the common
use cases for filtering against window functions without requiring
the complex subquery pushdown and predicate re-aliasing machinery to
deal with disjointed predicates against columns, aggregates, and window
functions.

A complete disjointed filtering implementation should likely be
deferred until proper QUALIFY support lands or the ORM gains a proper
subquery pushdown interface.

1 files changed, 25 insertions, 6 deletions

diff --git a/docs/ref/models/expressions.txt b/docs/ref/models/expressions.txt
index 25edd1f3e8..95f093e2a3 100644
--- a/docs/ref/models/expressions.txt
+++ b/docs/ref/models/expressions.txt
@@ -741,12 +741,6 @@ instead they are part of the selected columns.
 
 .. class:: Window(expression, partition_by=None, order_by=None, frame=None, output_field=None)
 
-    .. attribute:: filterable
-
-        Defaults to ``False``. The SQL standard disallows referencing window
-        functions in the ``WHERE`` clause and Django raises an exception when
-        constructing a ``QuerySet`` that would do that.
-
     .. attribute:: template
 
         Defaults to ``%(expression)s OVER (%(window)s)'``. If only the
@@ -819,6 +813,31 @@ to reduce repetition::
     >>>     ),
     >>> )
 
+Filtering against window functions is supported as long as lookups are not
+disjunctive (not using ``OR`` or ``XOR`` as a connector) and against a queryset
+performing aggregation.
+
+For example, a query that relies on aggregation and has an ``OR``-ed filter
+against a window function and a field is not supported. Applying combined
+predicates post-aggregation could cause rows that would normally be excluded
+from groups to be included::
+
+    >>> qs = Movie.objects.annotate(
+    >>>     category_rank=Window(
+    >>>         Rank(), partition_by='category', order_by='-rating'
+    >>>     ),
+    >>>     scenes_count=Count('actors'),
+    >>> ).filter(
+    >>>     Q(category_rank__lte=3) | Q(title__contains='Batman')
+    >>> )
+    >>> list(qs)
+    NotImplementedError: Heterogeneous disjunctive predicates against window functions
+    are not implemented when performing conditional aggregation.
+
+.. versionchanged:: 4.2
+
+    Support for filtering against window functions was added.
+
 Among Django's built-in database backends, MySQL 8.0.2+, PostgreSQL, and Oracle
 support window expressions. Support for different window expression features
 varies among the different databases. For example, the options in