阻塞是DBA經常碰到的情形，尤其是不良的應用程序設計的阻塞將導致性能嚴重下降直至數據庫崩潰。對DBA而言，有必要知道如何定位到當前系統有哪些阻塞，到底誰是阻塞者，誰是被阻塞者。本文對此給出了描述并做了相關演示。
a、INSERT阻塞主要是由于有一個帶主鍵的表，或者表上有惟一的約束，在兩個會話試圖用同樣的值插入一行時引發阻塞。多表通過引用完整性約束相互鏈接時，在其依賴的父表正在創建或刪除期間，對子表的插入可能會阻塞。對于該類情形建議使用序列來生成主鍵/惟一列值。
b、對于UPDATE、DELETE、MERGE 和SELECT FOR UPDATE阻塞，只要有任一session使用這些操作已經鎖定行，其余的必須處于等待狀態。直到當前鎖定行上的鎖(排他鎖)釋放。對于該類情形，建議盡可能快速提交事務，或采用批量SQL方式提交。
c、對于一個阻塞的SELECT FOR UPDATE，解決方案很簡單：只需增加NOWAIT 子句，它就不會阻塞了。
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;
