Question

oracle select 为什么要加锁

Answer 1

一般select会加一个共享锁的,怎么可能drop 得掉表呢.
除非一种可能
select语句早就运行完了,只是在前端显示的时候要花很长时间！

Answer 2

添设团闭嫁祸于人讶底

Answer 3

一、Oracle对select加锁方法
create table test(a number,b number);
insert into test values(1,2);
insert into test values(3,4);
insert into test values(8,9);
commit;
---session 1 模拟选中一个号码
SQL> select * from test where a =1 for update skip locked;
A B
---------- ----------
1 2
---session 2 对a=1再进行select
SQL> select * from test where a = 1 for update skip locked;
未选定行
-- session 3 全表select
SQL> select * from test for update skip locked;
A B
---------- ----------
3 4
8 9
SQL>
二、查询那些用户，操纵了那些表造成了锁机
SELECT s.username,
decode(l.type,'TM','TABLE LOCK',
'TX','ROW LOCK',
NULL) LOCK_LEVEL,
o.owner,o.object_name,o.object_type,
s.sid,s.serial#,s.terminal,s.machine,s.program,s.osuser
FROM v$session s,v$lock l,all_objects o
WHERE l.sid = s.sid
AND l.id1 = o.object_id(+)
AND s.username is NOT Null
三、查出被锁的表，和锁住这个表的会话ID
select a.session_id ,b.* from v$locked_object a,all_objects b
where a.object_id=b.object_id
四、 查出对应的SQL语句
select vs.SQL_TEXT,vsess.sid,vsess.SERIAL#,vsess.MACHINE,vsess.OSUSER
,vsess.TERMINAL,vsess.PROGRAM,vs.CPU_TIME,vs.DISK_READS
from v$sql vs,v$session vsess
where vs.ADDRESS=vsess.SQL_ADDRESS
and vsess.sid=(上面查出来的会话ID)
五、
1.查哪个过程被锁
查V$DB_OBJECT_CACHE视图:
SELECT * FROM V$DB_OBJECT_CACHE WHERE OWNER='过程的所属用户' AND LOCKS!='0';
2. 查是哪一个SID,通过SID可知道是哪个SESSION.
查V$ACCESS视图:
SELECT * FROM V$ACCESS WHERE OWNER='过程的所属用户' AND NAME='刚才查到的过程名';
3. 查出SID和SERIAL#
查V$SESSION视图:
SELECT SID,SERIAL#,PADDR FROM V$SESSION WHERE SID='刚才查到的SID'
查V$PROCESS视图:
SELECT SPID FROM V$PROCESS WHERE ADDR='刚才查到的PADDR';
4. 杀进程
(1).先杀ORACLE进程:
ALTER SYSTEM KILL SESSION '查出的SID,查出的SERIAL#';
(2).再杀操作系统进程:
KILL -9 刚才查出的SPID
或
ORAKILL 刚才查出的SID 刚才查出的SPID
六、查找最耗费系统资源的SQL
--CPU
select b.sql_text,
a.buffer_gets,
a.executions,
a.buffer_gets/decode(a.executions , 0 , 1 , a.executions),
c.username
from V$sqlarea a,
v$sqltext_with_newlines b,
dba_users c
where a.parsing_user_id = c.user_id
and a.address = b.address
order by a.buffer_gets desc , b.piece
;
--IO
select b.sql_text,
a.disk_reads,
a.executions,
a.disk_reads/decode(a.executions , 0 , 1 , a.executions),
c.username
from v$sqlarea a,
v$sqltext_with_newlines b,
dba_users c
where a.parsing_user_id = c.user_id
and a.address = b.address
order by a.disk_reads desc , b.piece
;
select s.sid,s.value "CPU Used"
from v$sesstat s,v$statname n
where s.statistic#=n.statistic# and n.name='CPU used by this session'
and s.value>0
order by 2 desc;