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一、Oracle的锁

锁是Oracle管理共享数据库资源并发访问并防止并发数据库事务之间“相互干涉”的核心机制。

1、锁的类型

1DML锁:允许并发执行数据修改,特定数据行上的锁或表中的所有行的锁。

2DDL锁:保护对象结构定义。

3Latch锁:这是Oracle内部锁,用来协调对期共享数据结构的访问。

2LOCK锁的模式

1)空锁(NULL)

2)排它锁(X)

3)共享锁(S)

二、DML

1TX锁:修改数据的事务在执行期间会获得这种锁。

TX锁的模式:排它锁(X)

TX锁和行级锁:TX锁不是行锁,一个事务不管修改了多少行,都只会有一个TX锁。TX锁算是行锁的代表,行锁上发生了等待,会表    现为TX锁的等待。行锁是属于事务的,事务开始,行锁产生,事务结束,行锁也被释放。

两个会话时DML操作同一行数据,会产生阻塞,操作如下:

会话125号,更新id=1这行数据。

gyj@OCM> select distinct sid from v$mystat;

SID

----------

125

gyj@OCM> update t5 set name='aaaaaa' where id=1;

1 row updated.

事务未提交!!

会话145,同时也更新id=1这行数据。

gyj@OCM> select distinct sid from v$mystat;

SID

----------

145

gyj@OCM> update t5 set name='bbbbb' where id=1;

这里被阻塞了!

v$lock视图

sys@OCM>select TYPE,ID1,ID2,LMODE,REQUEST,CTIME,BLOCK from v$lock where sidin(125,145);

TY ID1 ID2LMODE REQUEST CTIMEBLOCK

-- ---------- ---------- ---------- ---------- ---------- ----------

AE 100 0 4 0 1647 0

AE 100 0 4 0 682 0

TX 65539 2978 0 6 770

TO 65908 1 3 0 676 0

TM 74709 0 3 0 77 0

TM 74709 0 3 0 206 0

TX 65539 2978 6 0206 1

TX锁的标识:TX-ID1-ID2 --ID1表示事务使用的回滚段编号以及在事务表中对应的记录编号,ID2表示该记录编号被重用的次数(wrap)

ID1拆解:ID1是由4个字节组成的,高位2个字节和低位2个字节分别表示v$trasaction中的xidusn,xidslot

selecttrunc(id1/power(2,16)) as undo_blk#,bitand(id1,to_number('ffff','xxxx')) + 0 asslot# from dual;

另一计算方法:selectto_char(id1,'xxxxxxx') from dual;

查询持有锁和请求锁的信息:v$lock

gyj@OCM> desc v$lock

名称是否为空?类型

----------------- -------- ------------

ADDR RAW(4):锁状态对象的地址V$transaction.addr V$session.taddr这个其实是事务地址

KADDR RAW(4):锁的地址。此列是真正的锁地址。V$session.lockwait

SID NUMBER:持有或获得锁的会话

TYPE VARCHAR2(2):用户或系统锁的类型

ID1 NUMBER:锁的标识

ID2 NUMBER:锁的标识

LMODE NUMBER:会话持有的锁的模式,0 - none1 - null(NULL)2 - row-S (SS)3 - row-X (SX)4 - share (S)5 - S/Row-X (SSX)6 - exclusive (X)

REQUEST NUMBER:进程正在请求锁的模式,模式类型同上。

CTIME NUMBER:当前模式持续的时间

BLOCK NUMBER:当前锁是否正在阻塞其他的锁(1:是,0:不是)

系统进程也可能持有锁,系统锁被持有很短的时间,系统锁有如下类型:

System Type

Description

System Type

Description

AT

Lock held for the ALTER TABLE statement

NA..NZ

Library cache pin instance (A..Z = namespace)

BL

Buffer hash table instance

PF

Password File

CF

Control file schema global enqueue

PI, PS

Parallel operation

CI

Cross-instance function invocation instance

PR

Process startup

CU

Cursor bind

QA..QZ

Row cache instance (A..Z = cache)

DF

datafile instance

RT

Redo thread global enqueue

DL

Direct loader parallel index create

SC

System change number instance

DM

Mount/startup db primary/secondary instance

SM

SMON

DR

Distributed recovery process

SN

Sequence number instance

DX

Distributed transaction entry

SQ

Sequence number enqueue

FS

File set

SS

Sort segment

HW

Space management operations on a specific segment

ST

Space transaction enqueue

IN

Instance number

SV

Sequence number value

IR

Instance recovery serialization global enqueue

TA

Generic enqueue

IS

Instance state

TS

Temporary segment enqueue (ID2=0)

IV

Library cache invalidation instance

TS

New block allocation enqueue (ID2=1)

JQ

Job queue

TT

Temporary table enqueue

KK

Thread kick

UN

User name

LA .. LP

Library cache lock instance lock (A..P = namespace)

US

Undo segment DDL

MM

Mount definition global enqueue

WL

Being-written redo log instance

MR

Media recovery

2TM锁:表级锁

TM锁的主要作用,在DML期间,防止对表的DDL操作,可以确保对象的结构不被修改,例如,如果你已经更新了一个表,会得到这个表的一个TM锁。这会防止另一个用户在该表上执行DROPALTER命令。如果你有表的一个TM锁,而另一个用户试图在这个表上执行DDL,操作如下,他不会得到以下错误消息:

会话一

gyj@OCM> select distinct sid from v$mystat;

SID

----------

125

gyj@OCM> delete from t5 where id=1;

1 row deleted.

事务没提交!

会话二:

gyj@OCM> drop table t5;

drop table t5

*

ERROR at line 1:

ORA-00054: resource busy and acquire with NOWAIT specified ortimeout expired

v$lock视图

gyj@OCM> select TYPE,ID1,ID2,LMODE,REQUEST,CTIME,BLOCK fromv$lock where sid=125;

TY ID1 ID2LMODE REQUEST CTIMEBLOCK

-- ---------- ---------- ---------- ---------- ---------- ----------

AE 100 0 4 0 1171 0

TM 74709 0 3 0 241 0

TX 196641 3996 6 0 241 0

锁的标识:TM-ID1-ID2 --ID1表示被锁定的对象的对象IDID2始终为0

TM锁的模式:

代码

锁模式

0-None

没有锁

1-Null

空锁

2-SSRS

行级共享锁。Row Share

共享

3-SXRX

行级排它锁。Row Exclusive

排他

4-S

表共享锁Share

共享

5-SSXSRX

表共享行排它锁。Share Row Exclusive

共享

排他

6-X

表排它锁。Exclusive

排他

通过lock tablet10 in row share mode;命令添加RS

通过lock tablet10 in row exclusive mode;命令对表添加RX锁定;

通过select from forupdate命令添加RX

通过lock tablet10 in share mode;命令添加该S(wait for ITL release)

通过lock tablet10 in share row exclusive mode;命令添加SRX

通过lock tablet10 in exclusive mode命令添加X

语法:lock table in [row share][rowexclusive][share][share row exclusive][exclusive] mode;

各种TM锁的兼容性:

持有/得到

1-Null

2-SSRS

3-SXRX

4-S

5-SSXSRX

6-X

1-Null

2-SSRS

3-SXRX

4-S

5-SSXSRX

6-X

二、DDL锁

(省,单独讲)

三、Latch

(省,单独讲)

四、相关查询锁的语句

1、查询持有锁和请求锁的信息:v$lock

sys@OCM> selectsid,type,id1,id2,decode(lmode,0,'None',1,'Null',2,'Row share',3,'RowExclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive')lock_mode,decode(request,0,'None',1,'Null',2,'Row share',3,'RowExclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive') request_mode,blockfrom v$lock where sid in(125,145);

SID TY ID1 ID2 LOCK_MODE REQUEST_MODE BLOCK

---------- -- ---------- ----------------------------- ------------------- ----------

125 AE 100 0 Share None 0

145 AE 100 0 Share None 0

145 TX 65539 2978 None Exclusive 0

145 TO 659081 Row Exclusive None 0

145 TM 74709 0 Row Exclusive None 0

125 TM 74709 0 Row Exclusive None 0

125 TX 65539 2978 Exclusive None

2、查询请求锁的信息:v$enqueue_lock

sys@OCM>select sid,type,decode(request,0,'None',1,'Null',2,'Row share',3,'RowExclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive') request_mode fromv$enqueue_lock where sid in(125,145);

SID TY REQUEST_MODE

---------- -- -------------------

125 AE None

145 AE None

145 TX Exclusive

145 TO None

3、查询请求锁时间过长的信息

sys@OCM> select a.sidblocker_sid,a.serial#,a.username asblocker_username,b.type,decode(b.lmode,0,'None',1,'Null',2,'Row share',3,'RowExclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive') lock_mode,b.ctimeas time_held,c.sid as waiter_sid,decode(c.request,0,'None',1,'Null',2,'Rowshare',3,'Row Exclusive',4,'Share',5,'Share Row Exclusive',6,'Exclusive')request_mode,c.ctime time_waited fromv$lock b, v$enqueue_lock c, v$session a

2 where a.sid = b.sid and b.id1= c.id1(+) and b.id2 = c.id2(+) andc.type(+) = 'TX' and b.type = 'TX'and b.block = 1

3 order by time_held,time_waited;

BLOCKER_SID SERIAL# BLOCKER_USERNAME TY LOCK_MODE TIME_HELD WAITER_SID REQUEST_MODE TIME_WAITED

----------- ---------------------------------------- -- ------------------- ---------- ----------------------------- -----------

125 9 GYJ TX Exclusive 605 145 Exclusive 476

125号会话给KILL掉,操作命令如下:

alter system kill session '125,9';

五、死锁

1、两个session(以AB来表示),如果A持有B正在申请的锁定,同时B也持有A正在申请的锁定时,这时发生死锁现象。

操作如下:

session A

update t1 set name='A1' where id=1;

session B

update t1 set name='B1' where id=2;

session A

update t1 set name='B2' where id=2;

session B

update t1 set name='A2' where id=1;

alter_.log

2、实际举个产生死锁的例子

步骤一:通过主外键创建班级代码表和学生基本信息表。

gyj@OCM> create table bjdmb(bjdm number(8) primary key,bjmc varchar2(20));

Table created.

gyj@OCM> create table xsjbxxb(xh number(10),xm varchar2(10),bjdm number(8),

2 foreign key(bjdm) references bjdmb(bjdm) on delete cascade,primary key(xh));

Table created.

步骤二:加载数据

gyj@OCM> insert into bjdmb values(01,'bj01');

1 row created.

gyj@OCM>insert into bjdmb values(02,'bj02');

1 row created.

gyj@OCM> insert into bjdmb values(03,'bj03');

1 row created.

gyj@OCM>insert into bjdmb values(04,'bj04');

1 row created.

gyj@OCM>commit;

Commit complete.

gyj@OCM> insert into xsjbxxb values(001,'tom',01);

1 row created.

gyj@OCM> insert into xsjbxxb values(002,'joe',02);

1 row created.

gyj@OCM> commit;

Commit complete.

步骤三:产生死锁

会话A,插入学生一条信息记录:

gyj@OCM>insert into xsjbxxb values(00,'joe',03);

1 row created.

会话B:删除4班信息

gyj@OCM> delete from bjdmb where bjdm=04;

会话B这时被阻塞了!!!!!

步骤四:回滚会话A和会话B

会话A回滚:

gyj@OCM> insert into xsjbxxbvalues(00,'joe',03);

1 row created.

gyj@OCM> rollback;

Rollback complete.

会话B回滚:

gyj@OCM> delete from bjdmb wherebjdm=04;

1 row deleted.

gyj@OCM> rollback;

Rollback complete.

步骤五:在外键上创建索引

gyj@OCM> create index idx_xsjbxxb_bjdm onxsjbxxb(bjdm);

Index created.

步骤六:然后重复执行步骤三的操作

会话A上操作:

gyj@OCM> insert into xsjbxxbvalues(00,'joe',03);

1 row created.

会话B上操作:

gyj@OCM> delete from bjdmb wherebjdm=04;

1 row deleted.

没有被阻塞!!!!

死锁99%是外键上没加索引原因!!!!!!!!!!!!!

最后总结:外键要不需要加索引的情况

 A)没有从交表删除行

 B)没有更新父表的惟一键/主键值

 C)没有从父表联结子表

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