全表扫描引发的db file sequential read

今天我要做一个SQL调优，监控该SQL， 利用ASH 监控 该SQL是在sid=4848 上面跑的
db file sequential read等待事件有3个参数：file＃，first block＃，和block数量。
这个等待事件有3个参数P1，P2，P3，其中P1代表Oracle要读取的文件的绝对文件号，P2代表Oracle从这个文件中开始读取的起始数据块号，P3代表读取的BLOCK数量，通常这个值为1，表明是道单个BLOCK被读取。
SQL>  select SESSION_ID,NAME,P1,P2,P3,WAIT_TIME,CURRENT_OBJ#,CURRENT_FILE#,CURRENT_BLOCK#
  2         from v$active_session_history ash, v$event_name enm
  3         where ash.event#=enm.event#
  4         and SESSION_ID=4848;
SESSION_ID NAME                                   P1         P2         P3  WAIT_TIME CURRENT_OBJ# CURRENT_FILE# CURRENT_BLOCK#
---------- ------------------------------ ---------- ---------- ---------- ---------- ------------ ------------- --------------
      4848 db file sequential read                87     409499          1          0     58744541            87         409499
      4848 db file sequential read               674     178953          1          0     58744541           674         178953
      4848 db file sequential read               409     491921          1          0     58744541           409         491921
      4848 db file sequential read               169     310289          1          0     58744541           169         310289
      4848 db file sequential read                21     811534          1          0     58744541            21         811534
      4848 db file sequential read                12     494321          1          0     58744541            12         494321
      4848 db file sequential read               571     359814          1          0     58744541           571         359814
      4848 db file sequential read                18     782340          1          0     58744541            18         782340
      4848 db file sequential read                87     409073          1          0     58744541            87         409073
      4848 db file sequential read               107     362350          1          0     58744545           107         362350
      4848 db file sequential read               674     179953          1          0     58744545           674         179953
      4848 db file sequential read                14     562832          1          0     58744545            14         562832
      4848 db file sequential read               674     170372          1          0     58744545           674         170372
      4848 db file sequential read                15     434867          1          0     58744545            15         434867
      4848 db file sequential read               674     180111          1          0     58744545           674         180111
      4848 db file sequential read               258     336128          1          0     58744545           258         336128
      4848 gc cr grant 2-way                     206    1696993          1          0     58744545           206        1696993
      4848 gc cr grant 2-way                      87     412582          1          0     58744545            87         412582
      4848 db file sequential read                15     434831          1          0     58744545            15         434831
      4848 db file sequential read               677     175793          1          0     58744545           677         175793
      4848 db file sequential read               571     360776          1          0     58744545           571         360776
      4848 db file sequential read               250     115741          1          0     58988767           250         115741
      4848 db file sequential read               247     337993          1          0     58744545           247         337993
      4848 db file sequential read               169     315606          1          0     58744549           169         315606
      4848 db file sequential read               246     411238          1          0     58744549           246         411238
      4848 db file sequential read               258     341515          1          0     58744549           258         341515
      4848 db file sequential read               258     341748          1          0     58744549           258         341748
      4848 db file sequential read                17     409377          1          0     58744549            17         409377
      4848 db file sequential read               674     171523          1          0     58744549           674         171523
      4848 db file sequential read               409     788005          1          0     58744549           409         788005
发现很多都是 db file sequential read等待事件
SQL> select owner,object_name,object_type from dba_objects where object_id=58744549;
OWNER                OBJECT_NAME                              OBJECT_TYPE
-------------------- ---------------------------------------- ------------------------------
ADWU                 SHPMT_GDF_HIST_FCT_NEW                   TABLE SUBPARTITION
SQL> select owner,object_name,object_type from dba_objects where object_id=58744541;
OWNER                OBJECT_NAME                              OBJECT_TYPE
-------------------- ---------------------------------------- ------------------------------
ADWU                 SHPMT_GDF_HIST_FCT_NEW                   TABLE SUBPARTITION
但是这个等待事件不是 发生在index 上面，而是 发生在 table上！！！
也许你觉得 用object_id定位不准确，那么我用 file#,block#定位，同样发生在 table上
SQL> set lines 200
SQL> set pages 100
SQL> set timi on
SQL> col owner format a30
SQL> col segment_name format a40
SQL> col segment_type format a30
SQL> select owner,segment_name,segment_type from dba_extents
  2  where file_id=87 and 425778 between block_id and block_id+blocks-1;
OWNER                          SEGMENT_NAME                             SEGMENT_TYPE
------------------------------ ---------------------------------------- ------------------------------
ADWU                           SHPMT_GDF_HIST_FCT_NEW                   TABLE SUBPARTITION
为什么db file sequential read 发生在 table上呢？怀疑是行迁移/行连接(具体请看Metalink)
Full Table Scans On A Table Is Reading 1 Block At A Time. (Due To Chained / Migrated Rows) [ID 554366.1]
SQL> select c.sid,c.username,b.name,a.value,trunc(a.value/(select sum(value) from v$sesstat a,v$statname b
  2   where a.statistic#=b.statistic# and b.name='table fetch continued row'),4)*100 ||'%' pct_used
  3  from v$sesstat a, v$statname b,v$session c where a.statistic# =b.statistic# and b.name='table fetch continued row'
  4   and a.sid=c.sid and c.sid=4848;
       SID USERNAME                       NAME                                VALUE PCT_USED
---------- ------------------------------ ------------------------------ ---------- -----------------------------------------
      4848 BW9518                         table fetch continued row               8 .04%
更加让我怀疑是行迁移/行连接
那么现在Analyze表
 analyze table adwu.SHPMT_GDF_HIST_FCT_NEW compute statistics;
由于表很大(8亿多数据)。。。等Analyze完了再更新博客。。。
SQL> select s.inst_id, s.sid, s.serial#,  s.event,
  2         q.sql_text
  3  from   gv$session s, gv$sql q, gv$session_longops l
  4  where  s.inst_id = q.inst_id(+)
  5  and    s.sql_id  = q.sql_id(+)
  6  and    s.inst_id = l.inst_id(+)
  7  and    s.sid     = l.sid(+)
  8  and    s.serial# = l.serial#(+)
  9  and    s.username like upper('BW9518')
 10  group by s.inst_id, s.sid, s.serial#, s.username, s.logon_time, s.status, s.sql_id, s.machine, s.LAST_CALL_ET, s.event, q.sql_text, q.PLAN_HASH_VALUE
 11  ;
   INST_ID        SID    SERIAL# EVENT                                    SQL_TEXT
---------- ---------- ---------- ---------------------------------------- --------------------------------------------------------------------------------
         1       4669         59 PX Deq: Execution Msg                    select s.inst_id, s.sid, s.serial#,  s.event,        q.sql_text from   gv$sessio
         1       4791       2987 db file sequential read                  analyze table adwu.SHPMT_GDF_HIST_FCT_NEW compute statistics
         1       4817       3318 SQL*Net message from client              select s.inst_id, s.sid, s.serial#,  s.event,        q.sql_text from   gv$sessio
         1       4820       2427 SQL*Net message from client
         1       4826       2013 SQL*Net message from client
         1       4848       1325 gc cr request                            SELECT      i.initv_skid, f.prod_csu_type_code,                  i.time_perd_ski
         2       4809      49553 db file sequential read                   SELECT                 i.initv_skid, f.prod_csu_type_code,                 i.ti
         2       4859      10363 PX Deq: reap credit                      select s.inst_id, s.sid, s.serial#,  s.event,        q.sql_text from   gv$sessio
         3       4830       1778 PX Deq: reap credit                      select s.inst_id, s.sid, s.serial#,  s.event,        q.sql_text from   gv$sessio
         4       4827       4452 PX Deq: reap credit                      select s.inst_id, s.sid, s.serial#,  s.event,        q.sql_text from   gv$sessio
10 rows selected
更新---------
SQL> select table_name,pct_free from ALL_TAB_PARTITIONS where table_name='SHPMT_GDF_HIST_FCT_NEW';
TABLE_NAME                       PCT_FREE
------------------------------ ----------
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
SHPMT_GDF_HIST_FCT_NEW                  0
39 rows selected
这个表PCF_FREE=0，ANALYZE 已经没必要等了 
现在已经明确问题了，表上面肯定有很多行迁移/连接，analyze 都给我来db file sequential read
总结：对于 db file sequential read 表示单块读，出现该等待事件并不是 表示说 一定是读索引，也有可能是读表，如果有大量等待事件发生在表上面，那么很可能出现行迁移/连接。
单块读:要么是行迁移,要么是UNDO
pct_free=0  稍微update一下就产生hang迁移
												
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