11g的DDL_LOCK_TIMEOUT参数的作用，从字面上看，就是DDL获取锁的超时时间。
先看下官方文档的介绍：
DDL_LOCK_TIMEOUT
Property | Description |
Parameter type | Integer |
Default value | 0 |
Modifiable | ALTER SESSION |
Range of values | 0 to 1,000,000 (in seconds) |
Basic | No |
DDL_LOCK_TIMEOUT specifies a time limit for how long DDL statements will wait in a DML lock queue. The default value of zero indicates a status of NOWAIT. The maximum value of 1,000,000 seconds will result in the DDL statement waiting forever to acquire a DML lock.
If a lock is not acquired before the timeout period expires, then an error is returned.
从解释看，很好理解，该参数表示了执行DDL语句时需要等待DML锁的超时时间，默认0表示当无法立即获得DML锁时，会立即抛出NOWAIT，并且支持session级修改。
实验：
SQL> show parameter ddl_lock_timeout
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
ddl_lock_timeout integer 0
SQL> desc t;
Null? Type
----------------------------------------- -------- ----------------------------
TID NOT NULL NUMBER(38)
CLS_NAME
session1：
SQL> update t set cls_name='a' where tid=1;
0 rows updated.
session2：
SQL> drop table t;
drop table t
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
Elapsed: 00:00:00.00
session2：
SQL> alter session set ddl_lock_timeout = 10;
Session altered.
SQL> drop table t;
drop table t
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
Elapsed: 00:00:10.01
