In the tables operated with the help of MEDNTX and MEDCDX drivers, record numbers, deleting of records and expression indexes are implemented by adding special columns to the tables. The record number is implemented by the RECNO numeric column (not empty), and the values written there are given by Mediator server. Deleting of records is implemented by the IS_DELETED character column (not empty). A deleted record has a written value Y (the one which is not deleted has: N). Every expression index is implemented on an additional column in the source table. This column is fully operated by Mediator.