wrong alembic when alter enum in postgre

      File "/home/luogangyi/tmp/gnocchi/gnocchi/indexer/alembic/env.py", line 83, in run_migrations_online
        context.run_migrations()
      File "<string>", line 8, in run_migrations
      File "/usr/lib/python2.7/site-packages/alembic/runtime/environment.py", line 797, in run_migrations
        self.get_context().run_migrations(**kw)
      File "/usr/lib/python2.7/site-packages/alembic/runtime/migration.py", line 312, in run_migrations
        step.migration_fn(**kw)
      File "/home/luogangyi/tmp/gnocchi/gnocchi/indexer/alembic/versions/3901f5ea2b8e_create_instance_disk_and_instance_.py", line 53, in upgrade
        op.execute("ALTER TYPE resource_type_enum ADD VALUE '%s'" % value)
      File "<string>", line 8, in execute
      File "<string>", line 3, in execute
      File "/usr/lib/python2.7/site-packages/alembic/operations/ops.py", line 1826, in execute
        return operations.invoke(op)
      File "/usr/lib/python2.7/site-packages/alembic/operations/base.py", line 318, in invoke
        return fn(self, operation)
      File "/usr/lib/python2.7/site-packages/alembic/operations/toimpl.py", line 161, in execute_sql
        execution_options=operation.execution_options
      File "/usr/lib/python2.7/site-packages/alembic/ddl/impl.py", line 121, in execute
        self._exec(sql, execution_options)
      File "/usr/lib/python2.7/site-packages/alembic/ddl/impl.py", line 118, in _exec
        return conn.execute(construct, *multiparams, **params)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/base.py", line 914, in execute
        return meth(self, multiparams, params)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/sql/elements.py", line 323, in _execute_on_connection
        return connection._execute_clauseelement(self, multiparams, params)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/base.py", line 1010, in _execute_clauseelement
        compiled_sql, distilled_params
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/base.py", line 1146, in _execute_context
        context)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/base.py", line 1337, in _handle_dbapi_exception
        util.raise_from_cause(newraise, exc_info)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/util/compat.py", line 199, in raise_from_cause
        reraise(type(exception), exception, tb=exc_tb)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/base.py", line 1139, in _execute_context
        context)
      File "/usr/lib64/python2.7/site-packages/sqlalchemy/engine/default.py", line 450, in do_execute
        cursor.execute(statement, parameters)
    oslo_db.exception.DBError: (psycopg2.InternalError) ALTER TYPE ... ADD cannot run inside a transaction block
     [SQL: "ALTER TYPE resource_type_enum ADD VALUE 'instance_network_interface'"]

it is the problem in postgre, see http://stackoverflow.com/questions/1771543/postgresql-updating-an-enum-type/10404041#10404041