ORA-12091: cannot online redefine table with materialized views

Problem Description
While I attempt to reorganize a table online whenever I do with DBMS_REDEFINITION.can_redef_table or DBMS_REDEFINITION.START_REDEF_TABLE procedure on the table it fails with error ORA-12091 as below.

SQL> exec dbms_redefinition.can_redef_table('CR_2', 'OUT_CDR');
BEGIN dbms_redefinition.can_redef_table('CR_2', 'OUT_CDR'); END;

*
ERROR at line 1:
ORA-12091: cannot online redefine table "CR_2"."OUT_CDR" with materialized views
ORA-06512: at "SYS.DBMS_REDEFINITION", line 137
ORA-06512: at "SYS.DBMS_REDEFINITION", line 1478
ORA-06512: at line 1

Cause of the Problem
If you unsuccessfully launched the DBMS_REDEFINITION.START_REDEF_TABLE procedure
once, the materialized view would be created on the interim table and still be remained there. So subsequent run of the DBMS_REDEFINITION.CAN_REDEF_TABLE BMS_REDEFINITION.START_REDEF_TABLE will return error until you remove the materalized view against the table.

Solution of the Problem
Solution 01:
Remove the snapshot log and materialized view created during the unsuccessful
first execution of the DBMS_REDEFINITION.START_REDEF_TABLE procedure.
You can define the snapshot log and materialized view log of current table by,
SQL> select log_table from user_snapshot_logs;

LOG_TABLE
------------------------------
MLOG$_IN_CDR
MLOG$_OUT_CDR

SQL> select master,log_table from user_mview_logs;

MASTER                         LOG_TABLE
------------------------------ ------------------------------
IN_CDR                         MLOG$_IN_CDR
OUT_CDR                        MLOG$_OUT_CDR

Remove it by,
SQL> DROP MATERIALIZED VIEW LOG ON IN_CDR;
Materialized view log dropped.

SQL> DROP MATERIALIZED VIEW LOG ON OUT_CDR;
Materialized view log dropped.

You can use the keyword SNAPSHOT in place of MATERIALIZED VIEW. The keyword SNAPSHOT is supported in place of MATERIALIZED VIEW for backward compatibility.

Now execute your statement like,
SQL> exec dbms_redefinition.can_redef_table('CR_2', 'OUT_CDR');
PL/SQL procedure successfully completed.
And it works.

Solution 02:
Run the dbms_redefinition.abort_redef_table procedure which will automatically do the clean up task. This procedure will remove the temporary objects that are created by the redefinition process such as materialized view logs.
SQL> exec dbms_redefinition.abort_redef_table('CR_2', 'IN_CDR', 'IN_CDR_');
PL/SQL procedure successfully completed.