DDL锁：保护数据结构，保护对象的完整性，也叫字典锁。
当我们想要向表中增加一列，要求我们先要锁定表的结构，然后增加一个新的列。
select table_name,table_lock from user_tables;TABLE_NAME TABLE_LO
------------------------------ --------
DEPT ENABLED
EMP ENABLED
BONUS ENABLED
SALGRADE ENABLED
Elapsed: 00:00:00.11创建一张临时表
create table e01 as select * from emp;select table_name,table_lock from user_tables;TABLE_NAME TABLE_LO
------------------------------ --------
DEPT ENABLED
EMP ENABLED
BONUS ENABLED
SALGRADE ENABLED
E01 ENABLED
Elapsed: 00:00:00.02alter table e01 disable table lock;select table_name,table_lock from user_tables;TABLE_NAME TABLE_LO
------------------------------ --------
DEPT ENABLED
EMP ENABLED
BONUS ENABLED
SALGRADE ENABLED
E01 DISABLED
Elapsed: 00:00:00.03现在表锁处于DISABLE状态
下面我们对表做一些操作
truncate table e01;truncate table e01
*
ERROR at line 1:
ORA-00069: cannot acquire lock -- table locks disabled for E01
Elapsed: 00:00:00.03drop table e01;drop table e01
*
ERROR at line 1:
ORA-00069: cannot acquire lock -- table locks disabled for E01
Elapsed: 00:00:00.12我们把锁打开
alter table e01 enable table lock;select table_name,table_lock from user_tables;TABLE_NAME TABLE_LO
------------------------------ --------
DEPT ENABLED
EMP ENABLED
BONUS ENABLED
SALGRADE ENABLED
E01 ENABLED
Elapsed: 00:00:00.02truncate table e01;Table truncated.
Elapsed: 00:00:00.09DML锁：在事务中产生的，为了保证并发数据一致性的锁，存在于行头，叫做行级锁
用sys用户授予scott用户查看视图权限
grant select on v_$mystat to scott;查看当前会话的sid
select sid from v$mystat where rownum=1; SID
----------
38
Elapsed: 00:00:00.00在sys用户下查看该会话有哪些锁
select * from v$lock where sid=38;ADDR KADDR SID TY ID1 ID2 LMODE REQUEST CTIME BLOCK
---------------- ---------------- ---------- -- ---------- ---------- ---------- ---------- ---------- ----------
000000009705A0A8 000000009705A100 38 AE 100 0 4 0 1822 0
000000009705A248 000000009705A2A0 38 TO 65927 1 3 0 425 0
Elapsed: 00:00:00.03SCOTT用户模拟会话产生锁
insert into e01 select * from emp;SYS用户查看锁的状态
select * from v$lock where sid=38;ADDR KADDR SID TY ID1 ID2 LMODE REQUEST CTIME BLOCK
---------------- ---------------- ---------- -- ---------- ---------- ---------- ---------- ---------- ----------
000000009705A0A8 000000009705A100 38 AE 100 0 4 0 2014 0
000000009705A248 000000009705A2A0 38 TO 65927 1 3 0 617 0
00007FAEC3237828 00007FAEC3237888 38 TM 74754 0 3 0 52 0
0000000096383280 00000000963832F8 38 TX 786441 6 6 0 48 0
Elapsed: 00:00:00.00我们队SCOTT用户事务进行提交
commit;再查看锁的状态
select * from v$lock where sid=38;ADDR KADDR SID TY ID1 ID2 LMODE REQUEST CTIME BLOCK
---------------- ---------------- ---------- -- ---------- ---------- ---------- ---------- ---------- ----------
000000009705A0A8 000000009705A100 38 AE 100 0 4 0 2112 0
000000009705A248 000000009705A2A0 38 TO 65927 1 3 0 715 0
Elapsed: 00:00:00.00Oracle是自动管理锁的资源的，在不同时间对不同对象操作就会产生不同的锁，如果两个会话同时修改一张表的同一行信息，会出现锁的征用问题，他们会以队列的形式进行排队处理
我们开启两个SCOTT用户的会话同时执行一个SQL
update e01 set sal=sal+1 where empno=7369;我们用SYS用户来看下锁队列的情况
@?/rdbms/admin/utllocktdrop table lock_holders
*
ERROR at line 1:
ORA-00942: table or view does not exist
Elapsed: 00:00:00.00
Table created.
Elapsed: 00:00:00.02
drop table dba_locks_temp
*
ERROR at line 1:
ORA-00942: table or view does not exist
Elapsed: 00:00:00.00
Table created.
Elapsed: 00:00:00.03
1 row created.
Elapsed: 00:00:00.00
Commit complete.
Elapsed: 00:00:00.01
Table dropped.
Elapsed: 00:00:00.03
1 row created.
Elapsed: 00:00:00.00
Commit complete.
Elapsed: 00:00:00.00
WAITING_SESSION LOCK_TYPE MODE_REQUESTED MODE_HELD LOCK_ID1 LOCK_ID2
----------------- ----------------- -------------- -------------- ----------------- -----------------
36 None
38 Transaction Exclusive Exclusive 1310747 6
Elapsed: 00:00:00.00
Table dropped.
Elapsed: 00:00:00.03如果出现两个会话交叉入队，都在等待另一方把锁放开，Oracle会中断其中的一个会话，这种情况叫死锁。
我们手工将处于资源等待的会话杀掉
select sid,serial# from v$session where sid=38; SID SERIAL#
---------- ----------
38 6
Elapsed: 00:00:00.00alter system kill session '38,6' immediate;