Moved select_sql in SQLCompiler.get_extra_select() to improve performance.

django/db/models/sql/compiler.py

@@ -357,8 +357,8 @@ class SQLCompiler:
 
     def get_extra_select(self, order_by, select):
         extra_select = []
-        select_sql = [t[1] for t in select]
         if self.query.distinct and not self.query.distinct_fields:
+            select_sql = [t[1] for t in select]
             for expr, (sql, params, is_ref) in order_by:
                 without_ordering = self.ordering_parts.search(sql).group(1)
                 if not is_ref and (without_ordering, params) not in select_sql: