Add an XXX about a non-scalable query with MySQL.

This query happens when a storage node joins the cluster, to delete all
rows belonging to partitions it is not assigned to (ex: storage was down
and partition table was reorganised before it rejoined cluster).

git-svn-id: https://svn.erp5.org/repos/neo/trunk@2540 71dcc9de-d417-0410-9af5-da40c76e7ee4

neo/storage/database/mysqldb.py

@@ -416,6 +416,9 @@ class MySQLDatabaseManager(DatabaseManager):
         offset_list = ', '.join((str(i) for i in offset_list))
         self.begin()
         try:
+            # XXX: these queries are inefficient (execution time increase with
+            # row count, although we use indexes) when there are rows to
+            # delete. It should be done as an idle task, by chunks.
             self.objQuery('DELETE FROM %%(table)s WHERE partition IN (%s)' %
                 (offset_list, ))
             q("""DELETE FROM trans WHERE partition IN (%s)""" %