System for generating a gapless series of identity values

摘要

A system for generating a gapless series of identity values in a history log maintained in a database by a database management system without adversely impacting database performance. The system comprises the steps of determining a last used identity value independent of an intervening disruptive event that is disruptive to the database management system, generating a next identity value based on the last used identity value, and inserting a record of an event into the history log wherein a change record event contains the next identity value. Determining the last used identity value in the history log depending on whether or not a disruptive event has occurred during normal transaction processing. Absent a disruptive event, the last used identity value is the identity value used in the most recent change record insertion. In the event of a disruptive event, the last used identity value is retrieved from the last change record of the last committed transaction in the history log as the database recovery phase traverses the history log to identify committed transactions for reapplication and incomplete transactions for rollback to bring the database to a consistent state.