Improve threshold between rebuild and update MPTT

Indeed we should count the number of nodes updated instead of the number of
queries.

rebuild: 1 write and 1 select on each node
update: (1 write + 3 select + 2 write on average ½ all nodes + 1 write on child
of id) on each ids

So the threshold is:

2 * count < (4 + 2 * count / 2 + 1 * x) * len(ids)
2 * count < (4 + count + x) * len(ids)

which can be approximated to

2 * count < count * len(ids)
2 < len(ids)

issue5399
review20861002

trytond/model/modelsql.py

@@ -1291,7 +1291,6 @@ class ModelSQL(ModelStorage):
     @classmethod
     def _update_mptt(cls, field_names, list_ids, values=None):
         cursor = Transaction().connection.cursor()
-        count = None
         for field_name, ids in zip(field_names, list_ids):
             field = cls._fields[field_name]
             if (isinstance(field, fields.Many2One)
@@ -1315,11 +1314,7 @@ class ModelSQL(ModelStorage):
                         & (Column(parent, field.right) == 0)))
                 nested_create = cursor.fetchone()
 
-                if count is None:
-                    cursor.execute(*table.select(Count(Literal(1))))
-                    count, = cursor.fetchone()
-
-                if not nested_create and len(ids) < count / 4:
+                if not nested_create and len(ids) < 2:
                     for id_ in ids:
                         cls._update_tree(id_, field_name,
                             field.left, field.right)

trytond/tests/test_mptt.py

@@ -175,16 +175,18 @@ class MPTTTestCase(unittest.TestCase):
         records = Mptt.search([
                 ('parent', '=', None),
                 ])
-        with patch.object(Mptt, '_update_tree') as mock:
+        with patch.object(Mptt, '_update_tree') as update, \
+                patch.object(Mptt, '_rebuild_tree') as rebuild:
             Mptt.write(records, {'name': 'Parent Records'})
-            self.assertFalse(mock.called)
+            self.assertFalse(update.called)
+            self.assertFalse(rebuild.called)
 
             first_parent, second_parent = records[:2]
             Mptt.write(list(first_parent.childs), {
                     'parent': second_parent.id,
                     })
 
-            self.assertTrue(mock.called)
+            self.assertTrue(update.called or rebuild.called)
 
     @with_transaction()
     def test_nested_create(self):