Fixed #12079: Changed has_results to get a single result, thus preventing exists() from always returning True on backends that support chunked reads.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11654 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/sql/query.py

@@ -385,7 +385,8 @@ class BaseQuery(object):
         q.set_extra_mask(('a',))
         q.set_aggregate_mask(())
         q.clear_ordering()
-        return bool(q.execute_sql())
+        q.set_limits(high=1)
+        return bool(q.execute_sql(SINGLE))
 
     def as_sql(self, with_limits=True, with_col_aliases=False):
         """