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select...for update在mysql和oracle间锁行为的比较
环境：
[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$version where rownum=1;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - 64bi
SQL> !uname -a
Linux Think 2.6.32-220.el6.x86_64 #1 SMP Wed Nov 9 08:03:13 EST 2011 x86_64 x86_64 x86_64 GNU/Linux
对mysql而言，select for update必须在一个事务中，当事务commit，锁也就释放了。因此，在实验时，务必加个begin、start transaction 或者 set autocommit=0。
mysql：
[sql]
------------------sesson_A---------------:
mysql> begin;
Query OK, 0 rows affected (0.00 sec)
mysql> select * from t where i=2 for update;
+---+------+
| i | n |
+---+------+
| 2 | b
|
+---+------+
1 row in set (0.00 sec)
------------------session_B---------------:
mysql> select * from t;
+---+------+
| i | n |
+---+------+
| 2 | b
|
| 3 | c
|
+---+------+
2 rows in set (0.00 sec)
mysql> select * from t where i=2 for update;
被阻塞...
mysql> update t set n='f' where i=2;
被阻塞...
mysql> alter table t drop index t_idx;
被阻塞...
mysql> delete from t where i=2;
被阻塞...
oracle：
[sql]
-----------------------session_A---------------
SQL> select * from t where i=1 for update;
I N
---------- --------------------
1 think big
-----------------------session_B---------------
SQL> select * from t where i=1 for update;
被阻塞...
SQL> update t set n='think open' where i=1;
被阻塞...
SQL> create index t_idx on t(i);
create index t_idx on t(i)
*
ERROR at line 1:
ORA-00054: resource busy and acquire with NOWAIT specified
SQL> delete from t where i=1;
被阻塞...
于mysql，select ... for update 对行记录加个X锁。其他任何事务想在这些行上加任何锁都会被阻塞。这也符合InnoDB行级锁的概念。
在oracle中，我们再做下一个测试：
[sql]
-----------session_A-------------
SQL> select * from t for update;
A
-----
a
--------------session_B-------------
SQL> select sid,type,lmode from v$lock where sid=159;
SID TY LMODE
---------- -- ----------
159 TM 3
159 TX 6
对oracle，当发出select ... for update的时候、得到的是RX锁（lmode=3）,同时通过trc文件，我们还可以发现，Lck被置为1，也也就是同时被加上了行级锁。
trc部分摘录如下：
[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
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