Refs #33992 -- Refactored subquery grouping logic.

This required moving the combined queries slicing logic to the compiler
in order to allow Query.exists() to be called at expression resolving
time.

It allowed for Query.exists() to be called at Exists() initialization
time and thus ensured that get_group_by_cols() was operating on the
terminal representation of the query that only has a single column
selected.

django/db/models/expressions.py

@@ -1470,11 +1470,10 @@ class Subquery(BaseExpression, Combinable):
     def get_external_cols(self):
         return self.query.get_external_cols()
 
-    def as_sql(self, compiler, connection, template=None, query=None, **extra_context):
+    def as_sql(self, compiler, connection, template=None, **extra_context):
         connection.ops.check_expression_support(self)
         template_params = {**self.extra, **extra_context}
-        query = query or self.query
-        subquery_sql, sql_params = query.as_sql(compiler, connection)
+        subquery_sql, sql_params = self.query.as_sql(compiler, connection)
         template_params["subquery"] = subquery_sql[1:-1]
 
         template = template or template_params.get("template", self.template)
@@ -1482,13 +1481,7 @@ class Subquery(BaseExpression, Combinable):
         return sql, sql_params
 
     def get_group_by_cols(self, alias=None):
-        # If this expression is referenced by an alias for an explicit GROUP BY
-        # through values() a reference to this expression and not the
-        # underlying .query must be returned to ensure external column
-        # references are not grouped against as well.
-        if alias:
-            return [Ref(alias, self)]
-        return self.query.get_group_by_cols()
+        return self.query.get_group_by_cols(alias=alias, wrapper=self)
 
 
 class Exists(Subquery):
@@ -1498,28 +1491,18 @@ class Exists(Subquery):
     def __init__(self, queryset, negated=False, **kwargs):
         self.negated = negated
         super().__init__(queryset, **kwargs)
+        self.query = self.query.exists()
 
     def __invert__(self):
         clone = self.copy()
         clone.negated = not self.negated
         return clone
 
-    def get_group_by_cols(self, alias=None):
-        # self.query only gets limited to a single row in the .exists() call
-        # from self.as_sql() so deferring to Query.get_group_by_cols() is
-        # inappropriate.
-        if alias is None:
-            return [self]
-        return super().get_group_by_cols(alias)
-
-    def as_sql(self, compiler, connection, template=None, **extra_context):
-        query = self.query.exists(using=connection.alias)
+    def as_sql(self, compiler, connection, **extra_context):
         try:
             sql, params = super().as_sql(
                 compiler,
                 connection,
-                template=template,
-                query=query,
                 **extra_context,
             )
         except EmptyResultSet:

django/db/models/sql/compiler.py

@@ -535,8 +535,8 @@ class SQLCompiler:
             if not query.is_empty()
         ]
         if not features.supports_slicing_ordering_in_compound:
-            for query, compiler in zip(self.query.combined_queries, compilers):
-                if query.low_mark or query.high_mark:
+            for compiler in compilers:
+                if compiler.query.is_sliced:
                     raise DatabaseError(
                         "LIMIT/OFFSET not allowed in subqueries of compound statements."
                     )
@@ -544,6 +544,11 @@ class SQLCompiler:
                     raise DatabaseError(
                         "ORDER BY not allowed in subqueries of compound statements."
                     )
+        elif self.query.is_sliced and combinator == "union":
+            limit = (self.query.low_mark, self.query.high_mark)
+            for compiler in compilers:
+                if not compiler.query.is_sliced:
+                    compiler.query.set_limits(*limit)
         parts = ()
         for compiler in compilers:
             try:

django/db/models/sql/query.py

@@ -556,7 +556,7 @@ class Query(BaseExpression):
     def has_filters(self):
         return self.where
 
-    def exists(self, using, limit=True):
+    def exists(self, limit=True):
         q = self.clone()
         if not (q.distinct and q.is_sliced):
             if q.group_by is True:
@@ -568,11 +568,8 @@ class Query(BaseExpression):
                 q.set_group_by(allow_aliases=False)
             q.clear_select_clause()
         if q.combined_queries and q.combinator == "union":
-            limit_combined = connections[
-                using
-            ].features.supports_slicing_ordering_in_compound
             q.combined_queries = tuple(
-                combined_query.exists(using, limit=limit_combined)
+                combined_query.exists(limit=False)
                 for combined_query in q.combined_queries
             )
         q.clear_ordering(force=True)
@@ -1150,12 +1147,16 @@ class Query(BaseExpression):
             if col.alias in self.external_aliases
         ]
 
-    def get_group_by_cols(self, alias=None):
+    def get_group_by_cols(self, alias=None, wrapper=None):
+        # If wrapper is referenced by an alias for an explicit GROUP BY through
+        # values() a reference to this expression and not the self must be
+        # returned to ensure external column references are not grouped against
+        # as well.
         if alias:
-            return [Ref(alias, self)]
+            return [Ref(alias, wrapper or self)]
         external_cols = self.get_external_cols()
         if any(col.possibly_multivalued for col in external_cols):
-            return [self]
+            return [wrapper or self]
         return external_cols
 
     def as_sql(self, compiler, connection):

tests/aggregation/tests.py

@@ -1440,9 +1440,7 @@ class AggregateTestCase(TestCase):
             .annotate(cnt=Count("isbn"))
             .filter(cnt__gt=1)
         )
-        query = publishers_having_more_than_one_book_qs.query.exists(
-            using=connection.alias
-        )
+        query = publishers_having_more_than_one_book_qs.query.exists()
         _, _, group_by = query.get_compiler(connection=connection).pre_sql_setup()
         self.assertEqual(len(group_by), 1)