Know more about Enqueue Deadlock Detection

今天在 ORACLE ALLSTAR群里讨论了一个关于队列锁死锁检测的问题， 原帖子在这里。 有同学指出对于enqueue lock的死锁检测应当是每3 秒钟检测一次，这样说的依据是通过一个简单可重复的实验可以证明在实际出现ora-00060 dead lock错前process等待了3s:  

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.5.0 - 64bi
PL/SQL Release 10.2.0.5.0 - Production
CORE    10.2.0.5.0      Production
TNS for Linux: Version 10.2.0.5.0 - Production
NLSRTL Version 10.2.0.5.0 - Production

SQL> select * from global_name;

GLOBAL_NAME
--------------------------------------------------------------------------------
www.oracledatabase12g.com

PROCESS A:

set timing on;
update maclean1 set t1=t1+1;

PROCESS B:

update maclean2 set t1=t1+1;

PROCESS A:
update maclean2 set t1=t1+1;

PROCESS B:

update maclean1 set t1=t1+1;

等待3s后 PROCESS A 会报

ERROR at line 1:
ORA-00060: deadlock detected while waiting for resource

Elapsed: 00:00:03.02

    可以看到Process A在检测到死锁之前确实等待了 3s，而且这是一个可以重复的实验，很具有说服力。 事实真的是这样的吗？   来看下面的演示：  

SQL> col name for a30
SQL> col value for a5
SQL> col DESCRIB for a50
SQL> set linesize 140 pagesize 1400
SQL> SELECT x.ksppinm NAME, y.ksppstvl VALUE, x.ksppdesc describ
  2   FROM SYS.x$ksppi x, SYS.x$ksppcv y
  3   WHERE x.inst_id = USERENV ('Instance')
  4   AND y.inst_id = USERENV ('Instance')
  5   AND x.indx = y.indx
  6  AND x.ksppinm='_enqueue_deadlock_scan_secs';

NAME                           VALUE DESCRIB
------------------------------ ----- --------------------------------------------------
_enqueue_deadlock_scan_secs    0    deadlock scan interval

SQL> alter system set "_enqueue_deadlock_scan_secs"=18 scope=spfile;

System altered.

Elapsed: 00:00:00.01

SQL> startup force;
ORACLE instance started.

Total System Global Area  851443712 bytes
Fixed Size                  2100040 bytes
Variable Size             738198712 bytes
Database Buffers          104857600 bytes
Redo Buffers                6287360 bytes
Database mounted.
Database opened.

PROCESS A:

SQL> set timing on;
SQL> update maclean1 set t1=t1+1;

1 row updated.

Elapsed: 00:00:00.06

Process B

SQL> update maclean2 set t1=t1+1;

1 row updated.

SQL>  update maclean1 set t1=t1+1;

Process A:
SQL>
SQL> alter session set events '10704 trace name context forever,level 10:10046 trace name context forever,level 8';

Session altered.

SQL> update maclean2 set t1=t1+1;
update maclean2 set t1=t1+1
       *
ERROR at line 1:

ORA-00060: deadlock detected while waiting for resource