阻塞是DBA经常碰到的情形，尤其是不良的应用程序设计的阻塞将导致性能严重下降直至数据库崩溃。对DBA而言，有必要知道如何定位到当前系统有哪些阻塞，到底谁是阻塞者，谁是被阻塞者。本文对此给出了描述并做了相关演示。
a、INSERT阻塞主要是由于有一个带主键的表，或者表上有惟一的约束，在两个会话试图用同样的值插入一行时引发阻塞。多表通过引用完整性约束相互链接时，在其依赖的父表正在创建或删除期间，对子表的插入可能会阻塞。对于该类情形建议使用序列来生成主键/惟一列值。
b、对于UPDATE、DELETE、MERGE 和SELECT FOR UPDATE阻塞，只要有任一session使用这些操作已经锁定行，其余的必须处于等待状态。直到当前锁定行上的锁(排他锁)释放。对于该类情形，建议尽可能快速提交事务，或采用批量SQL方式提交。
c、对于一个阻塞的SELECT FOR UPDATE，解决方案很简单：只需增加NOWAIT 子句，它就不会阻塞了。
robin@SZDB:~/dba_scripts/custom/sql> more my_env.sql
SELECT spid, s.sid, s.serial#, p.username, p.program
FROM v$process p, v$session s
WHERE p.addr = s.paddr
AND s.sid = (SELECT sid
FROM v$mystat
WHERE rownum = 1);
robin@SZDB:~/dba_scripts/custom/sql> more blocker.sql
col block_msg format a50;
select c.terminal||' ('''||a.sid||','||c.serial#||''') is blocking '||b.sid||','||d.serial# block_msg, a.block
from v$lock a,v$lock b,v$session c,v$session d
where a.id1=b.id1
and a.id2=b.id2
and a.block>0
and a.sid >b.sid
and a.sid=c.sid
and b.sid=d.SID;
robin@SZDB:~/dba_scripts/custom/sql> more blocking_session_detail.sql
--To find the query for blocking session
--Access Privileges: SELECT on v$session, v$sqlarea
SELECT 'sid='
|| a.SID
|| ' Wait Class='
|| a.wait_class
|| ' Time='
|| a.seconds_in_wait
|| CHR (10)
|| ' Query='
|| b.sql_text
FROM v$session a, v$sqlarea b
WHERE a.blocking_session IS NOT NULL AND a.sql_address = b.address
ORDER BY a.blocking_session
/
robin@SZDB:~/dba_scripts/custom/sql> more request_lock_type.sql
--This script generates a report of users waiting for locks.
--Access Privileges: SELECT on v$session, v$lock
SELECT sn.username, m.sid, m.type,
DECODE(m.lmode, 0, 'None',
1, 'Null',
2, 'Row Share',
3, 'Row Excl.',
4, 'Share',
5, 'S/Row Excl.',
6, 'Exclusive',
lmode, ltrim(to_char(lmode,'990'))) lmode,
DECODE(m.request,0, 'None',
1, 'Null',
2, 'Row Share',
3, 'Row Excl.',
4, 'Share',
5, 'S/Row Excl.',
6, 'Exclusive',
request, ltrim(to_char(m.request,
'990'))) request, m.id1, m.id2
FROM v$session sn, v$lock m
WHERE (sn.sid = m.sid AND m.request != 0)
OR (sn.sid = m.sid
AND m.request = 0 AND lmode != 4
AND (id1, id2) IN (SELECT s.id1, s.id2
FROM v$lock s
WHERE request != 0
AND s.id1 = m.id1
AND s.id2 = m.id2)
)
ORDER BY id1, id2, m.request;
robin@SZDB:~/dba_scripts/custom/sql> more request_lock_detail.sql
set linesize 190
col osuser format a15
col username format a20 wrap
col object_name format a20 wrap
col terminal format a25 wrap
col Req_Mode format a20
select B.SID, C.USERNAME, C.OSUSER, C.TERMINAL,
DECODE(B.ID2, 0, A.OBJECT_NAME,
'Trans-'||to_char(B.ID1)) OBJECT_NAME,
B.TYPE,
DECODE(B.LMODE,0,'--Waiting--',
1,'Null',
2,'Row Share',
3,'Row Excl',
4,'Share',
5,'Sha Row Exc',
6,'Exclusive',
'Other') "Lock Mode",
DECODE(B.REQUEST,0,' ',
1,'Null',
2,'Row Share',
3,'Row Excl',
4,'Share',
5,'Sha Row Exc',
6,'Exclusive',
'Other') "Req_Mode"
from DBA_OBJECTS A, V$LOCK B, V$SESSION C
where A.OBJECT_ID(+) = B.ID1
and B.SID = C.SID
and C.USERNAME is not null
order by B.SID, B.ID2;
