select...for update在mysql和oracle間鎖行為的比較
環境:
[sql]
mysql> show variables like '%storage_engine%';
+----------------+--------+
| Variable_name | Value |
+----------------+--------+
| storage_engine | InnoDB |
+----------------+--------+
1 row in set (0.00 sec)
mysql> select version();
+-----------+
| version() |
+-----------+
| 5.1.52 |
+-----------+
1 row in set (0.06 sec)
[sql]
SQL> select * from v$version where rownum=1;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi
SQL> !uname -a
Linux Think 2.6.32-220.el6.x86_64 #1 SMP Wed Nov 9 08:03:13 EST 2011 x86_64 x86_64 x86_64 GNU/Linux
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對mysql而言,select for update必須在一個事務中,當事務commit,鎖也就釋放了。因此,在實驗時,務必加個begin、start transaction 或者 set autocommit=0。
mysql:
[sql]
------------------sesson_A---------------:
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from t where i=2 for update;
+---+------+
| i | n |
+---+------+
| 2 | b
|
+---+------+
1 row in set (0.00 sec)
------------------session_B---------------:
mysql> select * from t;
+---+------+
| i | n |
+---+------+
| 2 | b
|
| 3 | c
|
+---+------+
2 rows in set (0.00 sec)
mysql> select * from t where i=2 for update;
被阻塞...
mysql> update t set n='f' where i=2;
被阻塞...
mysql> alter table t drop index t_idx;
被阻塞...
mysql> delete from t where i=2;
被阻塞...
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oracle:
[sql]
-----------------------session_A---------------
SQL> select * from t where i=1 for update;
I N
---------- --------------------
1 think big
-----------------------session_B---------------
SQL> select * from t where i=1 for update;
被阻塞...
SQL> update t set n='think open' where i=1;
被阻塞...
SQL> create index t_idx on t(i);
create index t_idx on t(i)
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified
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SQL> delete from t where i=1;
被阻塞...
於mysql,select ... for update 對行記錄加個X鎖。其他任何事務想在這些行上加任何鎖都會被阻塞。這也符合InnoDB行級鎖的概念。
在oracle中,我們再做下一個測試:
[sql]
-----------session_A-------------
SQL> select * from t for update;
A
-----
a
--------------session_B-------------
SQL> select sid,type,lmode from v$lock where sid=159;
SID TY LMODE
---------- -- ----------
159 TM 3
159 TX 6
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對oracle,當發出select ... for update的時候、得到的是RX鎖(lmode=3),同時通過trc文件,我們還可以發現,Lck被置為1,也也就是同時被加上了行級鎖。
trc部分摘錄如下:
[sql]
Itl Xid Uba Flag Lck Scn/Fsc
0x01 0x000a.029.0000013b 0x008000dd.00c8.2b C--- 0 scn 0x0000.000911f4
0x02 0x0004.026.00000142 0x008000a3.00c7.04 --U- 1 fsc 0x0000.00091339
.....
tl: 5 fb: --H-FL-- lb: 0x2 cc: 1
col 0: [ 1] 61