一、概述：

阻塞是DBA经常碰到的情形，尤其是不良的应用程序设计所造成的阻塞将导致数据库性能的严重下降，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。

二、演示阻塞：

--更新表，注，提示符scott@CNMMBO表明用户为scott的session，用户名不同，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)

--另起两个session更新同样的行，这两个session都会处于等待，直到第一个session提交或回滚

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;

--下面在第一个session 查询阻塞情况

scott@CNMMBO> @blocker

BLOCK_MSG                        BLOCK

-------------------------------------------------- ----------

pts/5 ('1073,4642') is blocking 1067,10438         1

pts/5 ('1073,4642') is blocking 1065,4464          1

--上面的结果表明session 1073,4642 阻塞了后面的2个

--即session 1073,4642是阻塞者，后面2个session是被阻塞者

--Author : Leshami

--Blog  : http://blog.csdn.net/leshami

--下面查询正在阻塞的session id，SQL语句以及被阻塞的时间

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

--下面的查询阻塞时锁的持有情况

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个用户都在请求524319/27412上的Exclusive锁，而此时已经被SCOTT加了Exclusive锁

--查询阻塞时锁的持有详细信息

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、文中涉及到的相关SQL脚本完整代码如下：

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;

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