死锁一例:自己锁自己(执行catproc后)

oracle中配置的streams工作不正常了，察看系统对象时，发现有一些对象处于无效状态。于是执行catproc,但在执行过程中出现了死锁。

原因是有一些系统级别的触发器，对任何一个对象执行编译，修改，删除动作时，就会触发这些触发器。执行catproc后，一些对象被replace掉，比如view,那么基于这些view的对象就会被invalidate，这时oracle会尝试重新编译这些无效对象，不幸的是，这些系统级别的触发器也处于无效状态。所以系统首先得编译这些无效的触发器，这让就形成了死锁了。

执行catproc时的错误信息如下：
ERROR at line 1:
ORA-04020: deadlock detected while trying to lock object SYS.AFTER_ALTER


CREATE SEQUENCE sys.aq$_iotenqtxid START WITH 1 CACHE 1000
*
ERROR at line 1:
ORA-04020: deadlock detected while trying to lock object SYS.AFTER_ALTER

以下是发现死锁后做的一些尝试：

SQL> alter trigger sys.alter_after compile;
alter trigger sys.alter_after compile
*
ERROR at line 1:
ORA-04045: errors during recompilation/revalidation of
SYS.CDC_ALTER_CTABLE_BEFORE
ORA-04098: trigger 'SYS.AFTER_ALTER' is invalid and failed re-validation

SQL> alter trigger sys.alter_after disable;
alter trigger sys.alter_after disable
*
ERROR at line 1:
ORA-04045: errors during recompilation/revalidation of
SYS.CDC_ALTER_CTABLE_BEFORE
ORA-04098: trigger 'SYS.AFTER_ALTER' is invalid and failed re-validation


SQL> drop trigger sys.alter_after;
drop trigger sys.alter_after
*
ERROR at line 1:
ORA-04045: errors during recompilation/revalidation of
SYS.CDC_DROP_CTABLE_BEFORE
ORA-04098: trigger 'SYS.AFTER_ALTER' is invalid and failed re-validation

SQL> drop trigger cdc_drop_ctable_before;
drop trigger cdc_drop_ctable_before
*
ERROR at line 1:
ORA-04045: errors during recompilation/revalidation of SYS.AW_DROP_TRG
ORA-04098: trigger 'SYS.AFTER_ALTER' is invalid and failed re-validation


触发器after_alter 的代码如下：

CREATE OR REPLACE TRIGGER after_alter AFTER ALTER on database
BEGIN
IF (ora_dict_obj_type='USER') THEN
insert into event_table
values (ora_sysevent,
ora_login_user,
ora_instance_num,
ora_database_name,
ora_dict_obj_name,
ora_dict_obj_type,
ora_dict_obj_owner,
ora_des_encrypted_password,
sysdate);
END IF;
END;
/

如何解决这个死锁呢？oracle有一个隐含参数 _SYSTEM_TRIG_ENABLED，将该参数设置为false:
_SYSTEM_TRIG_ENABLED = FALSE
这样在运行catproc时，如果系统尝试编译无效对象，就不会触发上述的系统级别处发起了。

教训：执行catproc很安全吧，oracle support都经常如此建议。但在没有搞清楚系统之前，还是小心的好。不然出现这种类型的死锁，就必须宕机。

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转载于:http://blog.itpub.net/30426/viewspace-203995/