一、概述：
阻塞是dba經(jīng)常碰到的情形，尤其是不良的應(yīng)用程序設(shè)計(jì)所造成的阻塞將導(dǎo)致數(shù)據(jù)庫(kù)性能的嚴(yán)重下降，直至數(shù)據(jù)庫(kù)崩潰。對(duì)dba而言，有必要知道如何定位到當(dāng)前系統(tǒng)有哪些阻塞，到底誰(shuí)是阻塞者，誰(shuí)是被阻塞者。本文對(duì)此給出了描述并做了相關(guān)演示。
二、演示阻塞：
--更新表，注，提示符scott@cnmmbo表明用戶為scott的session，用戶名不同，session不同。
scott@cnmmbo> update emp set sal=sal*1.1 where empno=7788;
1 row updated.
scott@cnmmbo> @my_env
spid sid serial# username program
------------ ---------- ---------- --------------- ------------------------------------------------
11205 1073 4642 robin oracle@szdb (tns v1-v3)
--另起兩個(gè)session更新同樣的行，這兩個(gè)session都會(huì)處于等待，直到第一個(gè)session提交或回滾
leshami@cnmmbo> update scott.emp set sal=sal+100 where empno=7788;
goex_admin@cnmmbo> update scott.emp set sal=sal-50 where empno=7788;
--下面在第一個(gè)session 查詢阻塞情況
scott@cnmmbo> @blocker
block_msg block
-------------------------------------------------- ----------
pts/5 ('1073,4642') is blocking 1067,10438 1
pts/5 ('1073,4642') is blocking 1065,4464 1
--上面的結(jié)果表明session 1073,4642 阻塞了后面的2個(gè)
--即session 1073,4642是阻塞者，后面2個(gè)session是被阻塞者
--author : leshami
--blog : http://blog.csdn.net/leshami
--下面查詢正在阻塞的session id，sql語(yǔ)句以及被阻塞的時(shí)間
scott@cnmmbo> @blocking_session_detail.sql
'sid='||a.sid||'waitclass='||a.wait_class||'time='||a.seconds_in_wait||chr(10)||'query='||b.sql_text
------------------------------------------------------------------------
sid=1067 wait class=application time=5995
query=update scott.emp set sal=sal+100 where empno=7788
sid=1065 wait class=application time=225
query=update scott.emp set sal=sal-50 where empno=7788
--下面的查詢阻塞時(shí)鎖的持有情況
scott@cnmmbo> @request_lock_type
username sid ty lmode request id1 id2
------------------------------ ---------- -- ----------- ----------- ---------- ----------
scott 1073 tx exclusive none 524319 27412
leshami 1067 tx none exclusive 524319 27412
goex_admin 1065 tx none exclusive 524319 27412
--可以看到leshami，goex_admin 2個(gè)用戶都在請(qǐng)求524319/27412上的exclusive鎖，而此時(shí)已經(jīng)被scott加了exclusive鎖
--查詢阻塞時(shí)鎖的持有詳細(xì)信息
scott@cnmmbo> @request_lock_detail
sid username osuser terminal object_name ty lock mode req_mode
---------- -------------------- --------------- ------------------------- -------------------- -- ----------- --------------------
1065 goex_admin robin pts/1 emp tm row excl
1065 goex_admin robin pts/1 trans-524319 tx --waiting-- exclusive
1067 leshami robin pts/0 emp tm row excl
1067 leshami robin pts/0 trans-524319 tx --waiting-- exclusive
1073 scott robin pts/5 emp tm row excl
1073 scott robin pts/5 trans-524319 tx exclusive
三、文中涉及到的相關(guān)sql腳本完整代碼如下：
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;