The online redefinition function is implemented through materialized views. When calling the START_REDEF_TABLE stored procedure, a materialized view with the same name as the temporary table will be automatically created, and the records of the source table will be copied to the temporary table. At this time, the source table can be DML and queried, but not DDL.
The DML modification records generated by the source table during the online redefinition phase will be recorded in the materialized view log. In the SYNC_INTERIM_TABLE and final FINISH_REDEF_TABLE process, the temporary table will be updated according to the records in the materialized view log.
After calling the ABORT_REDEF_TABLE stored procedure, the established materialized view is cleared. If there is already copied data on the temporary table, the data on the temporary table is not cleared after ABORT, and the data on the temporary table should be cleared manually.
Table redefinition is started by the DBA creating an interim table based on the original table. The interim table can have a different structure than the original table, and will eventually take the original table's place in the database. While the table is redefined, DML operations on the original table are captured in a Materialized View Log table (MLOG$_%). These changes are eventually transformed and merged into the interim table. When done, the names of the original and the interim tables are swapped in the data dictionary. At this point all users will be working on the new table and the old table can be dropped.
