The reason for "ORA-12052: cannot fast refresh materialized view" during Online Redefinition

Online Redefinition of an object relies on Material View (MView) logs to keep track of all data changes in the table under redefinition *while* the redefintion is going on.

If there is any change in the definition (DDL), then there is no guarantee that the Mview logs can capture all changes (they can only track DML )

The internal process works like this:

- Set up Mview log to track any changes on the table and consider the temp table as an MView of the original table
- Transfer all the data in the original table to the temporary table
- WHILE this is going on, any DATA changes to the table will be logged in the MView log
- If you perform a SYNC, Oracle essentially performs a "MVIEW FAST REFRESH" using the MView log to replicating the data changes. The MView log is then cleared.
- Once the data transfer is complete, the MView log (if not empty) is processed - the "MView" / temp table is now up-to-date.
- Upon executing dbms_redefinition.finish_redef_table, the MView log is dropped, the MView is converted back to simple table.
- After that, the names of the original and temp table are switched automatically.

If at any point during this process, the structure of the table is changed or DDL in general is executed (say, TRUNCATE, adding a column, dropping a partition, exchanging a partition), then the MView log cannot in itself guarantee it has all the information needed to "replicate" this change of the original table.

Hence, the MView log is regarded as "unusable" and the error you observed occurs:


ORA-42009: error occurred while synchronizing the redefinition
ORA-12052: cannot fast refresh materialized view BWALM.TOMBA
ORA-06512: at "SYS.DBMS_REDEFINITION", line 119
ORA-06512: at "SYS.DBMS_REDEFINITION", line 1740
ORA-06512: at line 2


Oracle therefore recommend to avoid all DDL on the original table while ONLINE REDEFINITION is ongoing.