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)庫性能的嚴(yán)重下降，直至數(shù)據(jù)庫崩潰。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，到底誰是阻塞者，誰是被阻塞者。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;