Fixed #31376 -- Optimized nulls ordering when possible on SQLite and MySQL.

Both backends order NULLs first on ascending ordering and last on
descending ordering which makes ORDER BY IS (NOT)? NULL wasteful when
asc(nulls_first) and desc(nulls_last) are used since it prevents indice
usage.

django/db/backends/base/features.py

@@ -91,6 +91,9 @@ class BaseDatabaseFeatures:
     # Does the backend support NULLS FIRST and NULLS LAST in ORDER BY?
     supports_order_by_nulls_modifier = True
 
+    # Does the backend orders NULLS FIRST by default?
+    order_by_nulls_first = False
+
     # The database's limit on the number of query parameters.
     max_query_params = None
 

django/db/backends/mysql/features.py

@@ -51,6 +51,7 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     # Neither MySQL nor MariaDB support partial indexes.
     supports_partial_indexes = False
     supports_order_by_nulls_modifier = False
+    order_by_nulls_first = True
 
     @cached_property
     def _mysql_storage_engine(self):

django/db/backends/sqlite3/features.py

@@ -45,3 +45,4 @@ class DatabaseFeatures(BaseDatabaseFeatures):
     supports_frame_range_fixed_distance = Database.sqlite_version_info >= (3, 28, 0)
     supports_aggregate_filter_clause = Database.sqlite_version_info >= (3, 30, 1)
     supports_order_by_nulls_modifier = Database.sqlite_version_info >= (3, 30, 0)
+    order_by_nulls_first = True

django/db/models/expressions.py

@@ -1120,9 +1120,13 @@ class OrderBy(BaseExpression):
             elif self.nulls_first:
                 template = '%s NULLS FIRST' % template
         else:
-            if self.nulls_last:
+            if self.nulls_last and not (
+                self.descending and connection.features.order_by_nulls_first
+            ):
                 template = '%%(expression)s IS NULL, %s' % template
-            elif self.nulls_first:
+            elif self.nulls_first and not (
+                not self.descending and connection.features.order_by_nulls_first
+            ):
                 template = '%%(expression)s IS NOT NULL, %s' % template
         connection.ops.check_expression_support(self)
         expression_sql, params = compiler.compile(self.expression)