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.

2 files changed, 150 insertions, 21 deletions

diff --git a/django/db/models/sql/compiler.py b/django/db/models/sql/compiler.py
index 1c0ab2d212..858142913b 100644
--- a/django/db/models/sql/compiler.py
+++ b/django/db/models/sql/compiler.py
@@ -9,6 +9,7 @@ from django.db import DatabaseError, NotSupportedError
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.expressions import F, OrderBy, RawSQL, Ref, Value
 from django.db.models.functions import Cast, Random
+from django.db.models.lookups import Lookup
 from django.db.models.query_utils import select_related_descend
 from django.db.models.sql.constants import (
     CURSOR,
@@ -73,7 +74,9 @@ class SQLCompiler:
         """
         self.setup_query(with_col_aliases=with_col_aliases)
         order_by = self.get_order_by()
-        self.where, self.having = self.query.where.split_having()
+        self.where, self.having, self.qualify = self.query.where.split_having_qualify(
+            must_group_by=self.query.group_by is not None
+        )
         extra_select = self.get_extra_select(order_by, self.select)
         self.has_extra_select = bool(extra_select)
         group_by = self.get_group_by(self.select + extra_select, order_by)
@@ -584,6 +587,74 @@ class SQLCompiler:
             params.extend(part)
         return result, params
 
+    def get_qualify_sql(self):
+        where_parts = []
+        if self.where:
+            where_parts.append(self.where)
+        if self.having:
+            where_parts.append(self.having)
+        inner_query = self.query.clone()
+        inner_query.subquery = True
+        inner_query.where = inner_query.where.__class__(where_parts)
+        # Augment the inner query with any window function references that
+        # might have been masked via values() and alias(). If any masked
+        # aliases are added they'll be masked again to avoid fetching
+        # the data in the `if qual_aliases` branch below.
+        select = {
+            expr: alias for expr, _, alias in self.get_select(with_col_aliases=True)[0]
+        }
+        qual_aliases = set()
+        replacements = {}
+        expressions = list(self.qualify.leaves())
+        while expressions:
+            expr = expressions.pop()
+            if select_alias := (select.get(expr) or replacements.get(expr)):
+                replacements[expr] = select_alias
+            elif isinstance(expr, Lookup):
+                expressions.extend(expr.get_source_expressions())
+            else:
+                num_qual_alias = len(qual_aliases)
+                select_alias = f"qual{num_qual_alias}"
+                qual_aliases.add(select_alias)
+                inner_query.add_annotation(expr, select_alias)
+                replacements[expr] = select_alias
+        self.qualify = self.qualify.replace_expressions(
+            {expr: Ref(alias, expr) for expr, alias in replacements.items()}
+        )
+        inner_query_compiler = inner_query.get_compiler(
+            self.using, elide_empty=self.elide_empty
+        )
+        inner_sql, inner_params = inner_query_compiler.as_sql(
+            # The limits must be applied to the outer query to avoid pruning
+            # results too eagerly.
+            with_limits=False,
+            # Force unique aliasing of selected columns to avoid collisions
+            # and make rhs predicates referencing easier.
+            with_col_aliases=True,
+        )
+        qualify_sql, qualify_params = self.compile(self.qualify)
+        result = [
+            "SELECT * FROM (",
+            inner_sql,
+            ")",
+            self.connection.ops.quote_name("qualify"),
+            "WHERE",
+            qualify_sql,
+        ]
+        if qual_aliases:
+            # If some select aliases were unmasked for filtering purposes they
+            # must be masked back.
+            cols = [self.connection.ops.quote_name(alias) for alias in select.values()]
+            result = [
+                "SELECT",
+                ", ".join(cols),
+                "FROM (",
+                *result,
+                ")",
+                self.connection.ops.quote_name("qualify_mask"),
+            ]
+        return result, list(inner_params) + qualify_params
+
     def as_sql(self, with_limits=True, with_col_aliases=False):
         """
         Create the SQL for this query. Return the SQL string and list of
@@ -614,6 +685,9 @@ class SQLCompiler:
                 result, params = self.get_combinator_sql(
                     combinator, self.query.combinator_all
                 )
+            elif self.qualify:
+                result, params = self.get_qualify_sql()
+                order_by = None
             else:
                 distinct_fields, distinct_params = self.get_distinct()
                 # This must come after 'select', 'ordering', and 'distinct'
diff --git a/django/db/models/sql/where.py b/django/db/models/sql/where.py
index 42a4b054a5..e2af46a309 100644
--- a/django/db/models/sql/where.py
+++ b/django/db/models/sql/where.py
@@ -35,48 +35,81 @@ class WhereNode(tree.Node):
     resolved = False
     conditional = True
 
-    def split_having(self, negated=False):
+    def split_having_qualify(self, negated=False, must_group_by=False):
         """
-        Return two possibly None nodes: one for those parts of self that
-        should be included in the WHERE clause and one for those parts of
-        self that must be included in the HAVING clause.
+        Return three possibly None nodes: one for those parts of self that
+        should be included in the WHERE clause, one for those parts of self
+        that must be included in the HAVING clause, and one for those parts
+        that refer to window functions.
         """
-        if not self.contains_aggregate:
-            return self, None
+        if not self.contains_aggregate and not self.contains_over_clause:
+            return self, None, None
         in_negated = negated ^ self.negated
-        # If the effective connector is OR or XOR and this node contains an
-        # aggregate, then we need to push the whole branch to HAVING clause.
-        may_need_split = (
+        # Whether or not children must be connected in the same filtering
+        # clause (WHERE > HAVING > QUALIFY) to maintain logical semantic.
+        must_remain_connected = (
             (in_negated and self.connector == AND)
             or (not in_negated and self.connector == OR)
             or self.connector == XOR
         )
-        if may_need_split and self.contains_aggregate:
-            return None, self
+        if (
+            must_remain_connected
+            and self.contains_aggregate
+            and not self.contains_over_clause
+        ):
+            # It's must cheaper to short-circuit and stash everything in the
+            # HAVING clause than split children if possible.
+            return None, self, None
         where_parts = []
         having_parts = []
+        qualify_parts = []
         for c in self.children:
-            if hasattr(c, "split_having"):
-                where_part, having_part = c.split_having(in_negated)
+            if hasattr(c, "split_having_qualify"):
+                where_part, having_part, qualify_part = c.split_having_qualify(
+                    in_negated, must_group_by
+                )
                 if where_part is not None:
                     where_parts.append(where_part)
                 if having_part is not None:
                     having_parts.append(having_part)
+                if qualify_part is not None:
+                    qualify_parts.append(qualify_part)
+            elif c.contains_over_clause:
+                qualify_parts.append(c)
             elif c.contains_aggregate:
                 having_parts.append(c)
             else:
                 where_parts.append(c)
+        if must_remain_connected and qualify_parts:
+            # Disjunctive heterogeneous predicates can be pushed down to
+            # qualify as long as no conditional aggregation is involved.
+            if not where_parts or (where_parts and not must_group_by):
+                return None, None, self
+            elif where_parts:
+                # In theory this should only be enforced when dealing with
+                # where_parts containing predicates against multi-valued
+                # relationships that could affect aggregation results but this
+                # is complex to infer properly.
+                raise NotImplementedError(
+                    "Heterogeneous disjunctive predicates against window functions are "
+                    "not implemented when performing conditional aggregation."
+                )
+        where_node = (
+            self.create(where_parts, self.connector, self.negated)
+            if where_parts
+            else None
+        )
         having_node = (
             self.create(having_parts, self.connector, self.negated)
             if having_parts
             else None
         )
-        where_node = (
-            self.create(where_parts, self.connector, self.negated)
-            if where_parts
+        qualify_node = (
+            self.create(qualify_parts, self.connector, self.negated)
+            if qualify_parts
             else None
         )
-        return where_node, having_node
+        return where_node, having_node, qualify_node
 
     def as_sql(self, compiler, connection):
         """
@@ -183,6 +216,14 @@ class WhereNode(tree.Node):
         clone.relabel_aliases(change_map)
         return clone
 
+    def replace_expressions(self, replacements):
+        if replacement := replacements.get(self):
+            return replacement
+        clone = self.create(connector=self.connector, negated=self.negated)
+        for child in self.children:
+            clone.children.append(child.replace_expressions(replacements))
+        return clone
+
     @classmethod
     def _contains_aggregate(cls, obj):
         if isinstance(obj, tree.Node):
@@ -231,6 +272,10 @@ class WhereNode(tree.Node):
 
         return BooleanField()
 
+    @property
+    def _output_field_or_none(self):
+        return self.output_field
+
     def select_format(self, compiler, sql, params):
         # Wrap filters with a CASE WHEN expression if a database backend
         # (e.g. Oracle) doesn't support boolean expression in SELECT or GROUP
@@ -245,19 +290,28 @@ class WhereNode(tree.Node):
     def get_lookup(self, lookup):
         return self.output_field.get_lookup(lookup)
 
+    def leaves(self):
+        for child in self.children:
+            if isinstance(child, WhereNode):
+                yield from child.leaves()
+            else:
+                yield child
+
 
 class NothingNode:
     """A node that matches nothing."""
 
     contains_aggregate = False
+    contains_over_clause = False
 
     def as_sql(self, compiler=None, connection=None):
         raise EmptyResultSet
 
 
 class ExtraWhere:
-    # The contents are a black box - assume no aggregates are used.
+    # The contents are a black box - assume no aggregates or windows are used.
     contains_aggregate = False
+    contains_over_clause = False
 
     def __init__(self, sqls, params):
         self.sqls = sqls
@@ -269,9 +323,10 @@ class ExtraWhere:
 
 
 class SubqueryConstraint:
-    # Even if aggregates would be used in a subquery, the outer query isn't
-    # interested about those.
+    # Even if aggregates or windows would be used in a subquery,
+    # the outer query isn't interested about those.
     contains_aggregate = False
+    contains_over_clause = False
 
     def __init__(self, alias, columns, targets, query_object):
         self.alias = alias