[FIX] mail: when checking for possible messages to fetch, perform

a search_count on mail_message when the actual number of messages or their
ids is not necessary, to speedup the computation.

addons/mail/mail_message.py

@@ -450,8 +450,8 @@ def _get_expandable(domain, message_nb, parent_id, max_limit):
             exp_domain = domain + [('id', '<', min(message_unload_ids + message_ids))]
         else:
             exp_domain = domain + ['!', ('id', 'child_of', message_unload_ids + parent_tree.keys())]
-        ids = self.search(cr, uid, exp_domain, context=context, limit=1)
-        if ids:
+        more_count = self.search_count(cr, uid, exp_domain, context=context)
+        if more_count:
             # inside a thread: prepend
             if parent_id:
                 messages.insert(0, _get_expandable(exp_domain, -1, parent_id, True))