OBJ$ table is locked during query

Question

When we execute the following script on Oracle 10g R2 on Linux:
CREATE TABLE AUDIT_TMP TABLESPACE CIL_DATA AS
SELECT * FROM AUDIT
WHERE CREATION_TIME BETWEEN ADD_MONTHS(SYSDATE,-2) AND SYSDATE;Then OBJ$ table is locked.
SQL> select object_id, session_id, oracle_username, process, locked_mode from  sys.v_$locked_object;
OBJECT_ID SESSION_ID ORACLE_USERNAME                PROCESS      LOCKED_MODE
18        281 SYS                            11460                  3
SQL>
SQL>
SQL> select object_name from dba_objects where object_id = '18';
OBJECT_NAME
OBJ$Can anybody tell me as why OBJ$ table is locked. Is this normal behaviour?

Answer 1

Simplistically: A SQL uses one or more SQL objects (tables, indexes, etc). So a SQL needs to place a shared data dictionary lock on the object it uses, to prevent someone else from changing that object (removing it, updating its definition, changing it, etc).
Think of it as a protection mechanism - e.g. the SQL cannot execute on a table, when another session suddenly removes a column from that table that the SQL uses as a filter condition. Thus the shared lock - preventing the table from being changed while it is in use.
Likewise, a data dictionary change (create table, alter table, etc) also needs to lock when changing an object. However, this requires an exclusive lock in order to update that object in the data dictionary. Which means DDL needs to wait for a DML on the object to complete, before the DDL can update the object's definition. And a DML will need to wait for a DDL if that DDL is busy changing the object the DML want to use.