python: avoid useless JSON conversion to enhance performance

This patch removes a useless conversion to/from JSON in the
processing of any 'modify' operations inside the process_update2
method in Python IDL implementation.

Previous code will make resources creation take longer as the number
of elements in the row grows because of that JSON conversion. This
patch eliminates it and now the time remains consant regardless
of the database contents improving performance and scaling.

Reported-by: Daniel Alvarez <dalvarez@redhat.com>
Reported-at: https://mail.openvswitch.org/pipermail/ovs-discuss/2018-February/046263.html
Signed-off-by: Daniel Alvarez <dalvarez@redhat.com>
Acked-by: Terry Wilson <twilson@redhat.com>
Tested-By: Terry Wilson <twilson@redhat.com>
Acked-by: Han Zhou <hzhou8@ebay.com>
Signed-off-by: Ben Pfaff <blp@ovn.org>

python/ovs/db/idl.py

@@ -533,10 +533,8 @@ def __process_update2(self, table, uuid, row_update):
             if not row:
                 raise error.Error('Modify non-existing row')
 
-            old_row_diff_json = self.__apply_diff(table, row,
-                                                  row_update['modify'])
-            self.notify(ROW_UPDATE, row,
-                        Row.from_json(self, table, uuid, old_row_diff_json))
+            old_row = self.__apply_diff(table, row, row_update['modify'])
+            self.notify(ROW_UPDATE, row, Row(self, table, uuid, old_row))
             changed = True
         else:
             raise error.Error('<row-update> unknown operation',
@@ -605,7 +603,7 @@ def __add_default(self, table, row_update):
                         row_update[column.name] = self.__column_name(column)
 
     def __apply_diff(self, table, row, row_diff):
-        old_row_diff_json = {}
+        old_row = {}
         for column_name, datum_diff_json in six.iteritems(row_diff):
             column = table.columns.get(column_name)
             if not column:
@@ -622,12 +620,12 @@ def __apply_diff(self, table, row, row_diff):
                           % (column_name, table.name, e))
                 continue
 
-            old_row_diff_json[column_name] = row._data[column_name].to_json()
+            old_row[column_name] = row._data[column_name].copy()
             datum = row._data[column_name].diff(datum_diff)
             if datum != row._data[column_name]:
                 row._data[column_name] = datum
 
-        return old_row_diff_json
+        return old_row
 
     def __row_update(self, table, row, row_json):
         changed = False