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.

该参数是oralce 11g中才有的；

实验：

--session 1

SQL> select * from v$version;
BANNER
--------------------------------------------------------------------------------
Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - Production
PL/SQL Release 11.2.0.1.0 - Production
CORE    11.2.0.1.0      Production
TNS for Linux: Version 11.2.0.1.0 - Production
NLSRTL Version 11.2.0.1.0 - Production

SQL> show parameter ddl
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
ddl_lock_timeout                     integer     0
enable_ddl_logging                   boolean     FALSE

--session 2

SQL> insert into t_1 values('b');
1 row created.

--session 3

SQL> alter table t_1 modify(a varchar2(50));
alter table t_1 modify(a varchar2(50))
            *
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired
该错误是执行之后没有等待直接出现；

--session 1

SQL> alter system set ddl_lock_timeout=60;
System altered.

SQL> show parameter ddl
NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
ddl_lock_timeout                     integer     60
enable_ddl_logging                   boolean     FALSE

--session 2

SQL> insert into t_1 values('a');
1 row created.

--session 3

SQL> alter table t_1 modify (a varchar2(30));
alter table t_1 modify (a varchar2(30))
            *
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified or timeout expired

该错误是在等待了60秒之后才出现；

--session 3

SQL> alter table t_1 add  c number;

则会一直等待下去，直到请求的锁资源被释放（该变量不起作用）；

在网上有些文章说，如果是添加列回立即提交，不敢苟同；
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作者：搞怪的索引
来源：CSDN
原文：https://blog.csdn.net/perfect\_db/article/details/9125625
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