Online, Non-blocking Relational Schema Changes

A database schema should be able to evolve to reflect changes to the universe it represents. In existing systems, user transactions get blocked during complex schema transformations. Blocking user transactions is not an option in systems with very high availability requirements, like operational telecom databases. A non-blocking transformation framework is therefore needed. A method for performing non-blocking full outer join and split transformations, suitable for highly available databases, is presented in this paper. Only the log is used for change propagation, and this makes the method easy to integrate into existing DBMSs. Because the involved tables are not locked, the transformation may run as a low priority background process. As a result, the transformation has little impact on concurrent user transactions.