通过dba_hist_*定位问题sql

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db服务器突然变慢，是不是执行计划出现的跳变导致sql性能下降呢.....

oracle的执行计划经常会遭遇绑定变量窥视问题，导致执行计划变更，引起数据库负载飙升，带来性能问题。

今天通过awr发现，等待事件第一位的是read by other session；SQL Statistics 中几乎都是同一条sql排在第一：56s18gn1k19yp 该sql一个小时内执行了3700多次。

从这种迹象来看，应该是执行计划发生了变更。

oracle 10G中可以通过下面的三个视图查询到语句的历史执行信息：

DBA_HIST_SQL_PLAN

DBA_HIST_SQLSTAT

DBA_HIST_SNAPSHOT

查看语句的历史执行信息，是否发生变化，何时发生了变化。如果发生了变化,找出以前的执行计划,与当前的执行计划进行对比,有什么不同。

通过下面的sql查询执行计划是否发生变化：

select a.INSTANCE_NUMBER,a.snap_id,a.sql_id,a.plan_hash_value,b.begin_interval_time

from dba_hist_sqlstat a, dba_hist_snapshot b

where sql_id ='56s18gn1k19yp'

and a.snap_id = b.snap_id

order by instance_number, snap_id;

SQL> select a.snap_id, a.sql_id, a.plan_hash_value,to_char(b.begin_interval_time,'yyyy-mm-dd hh24:mi:ss')

2 from dba_hist_sqlstat a, dba_hist_snapshot b

3 where sql_id ='56s18gn1k19yp'

4 and a.snap_id = b.snap_id

5 order by snap_id desc; www.2cto.com

SNAP_ID SQL_ID PLAN_HASH_VALUE TO_CHAR(B.BEGIN_INT

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

30569 56s18gn1k19yp 947531627 2012-03-02 14:00:30

30558 56s18gn1k19yp 947531627 2012-03-02 03:00:12

30552 56s18gn1k19yp 947531627 2012-03-01 21:00:36

30551 56s18gn1k19yp 2658265176 2012-03-01 20:00:27

30550 56s18gn1k19yp 947531627 2012-03-01 19:00:21

30535 56s18gn1k19yp 947531627 2012-03-01 04:00:31

30527 56s18gn1k19yp 947531627 2012-02-29 20:00:37

30524 56s18gn1k19yp 947531627 2012-02-29 17:00:24

30521 56s18gn1k19yp 947531627 2012-02-29 14:00:11

30519 56s18gn1k19yp 947531627 2012-02-29 12:00:04

30518 56s18gn1k19yp 947531627 2012-02-29 11:00:59

30511 56s18gn1k19yp 947531627 2012-02-29 04:00:11

30510 56s18gn1k19yp 947531627 2012-02-29 03:00:04

30502 56s18gn1k19yp 947531627 2012-02-28 19:00:27

30501 56s18gn1k19yp 947531627 2012-02-28 18:00:22

30500 56s18gn1k19yp 947531627 2012-02-28 17:00:18

30498 56s18gn1k19yp 947531627 2012-02-28 15:00:11

30497 56s18gn1k19yp 947531627 2012-02-28 14:00:07

30491 56s18gn1k19yp 947531627 2012-02-28 08:00:25

30475 56s18gn1k19yp 947531627 2012-02-27 16:00:03

30464 56s18gn1k19yp 947531627 2012-02-27 05:00:06

30463 56s18gn1k19yp 947531627 2012-02-27 04:00:00

30461 56s18gn1k19yp 947531627 2012-02-27 02:00:51

30456 56s18gn1k19yp 947531627 2012-02-26 21:00:31

30448 56s18gn1k19yp 2658265176 2012-02-26 13:00:55

30440 56s18gn1k19yp 947531627 2012-02-26 05:00:33

30439 56s18gn1k19yp 947531627 2012-02-26 04:00:25

30438 56s18gn1k19yp 947531627 2012-02-26 03:00:17

30437 56s18gn1k19yp 947531627 2012-02-26 02:00:09

30428 56s18gn1k19yp 947531627 2012-02-25 17:00:16

30423 56s18gn1k19yp 947531627 2012-02-25 12:00:51

30418 56s18gn1k19yp 947531627 2012-02-25 07:00:05

30416 56s18gn1k19yp 947531627 2012-02-25 05:00:55

30415 56s18gn1k19yp 947531627 2012-02-25 04:00:50

30406 56s18gn1k19yp 947531627 2012-02-24 19:00:52

30398 56s18gn1k19yp 947531627 2012-02-24 11:00:12

30396 56s18gn1k19yp 947531627 2012-02-24 09:00:56

30392 56s18gn1k19yp 947531627 2012-02-24 05:00:23

30391 56s18gn1k19yp 947531627 2012-02-24 04:00:11

30388 56s18gn1k19yp 947531627 2012-02-24 01:00:46

30383 56s18gn1k19yp 947531627 2012-02-23 20:00:15

30380 56s18gn1k19yp 947531627 2012-02-23 17:00:56

30377 56s18gn1k19yp 947531627 2012-02-23 14:00:37

30377 56s18gn1k19yp 2658265176 2012-02-23 14:00:37

我们注意到最近一次3月1号20点左右，执行计划发生了变化。

具体查看这两种执行计划有什么区别：

select sql_id,plan_hash_value,id,operation,options,object_owner,object_name,depth,cost,timestamp

from DBA_HIST_SQL_PLAN

where sql_id ='56s18gn1k19yp'

and plan_hash_value in (947531627,2658265176);

SQL> select plan_hash_value,id,operation,options,object_name,depth,cost,timestamp

2 from DBA_HIST_SQL_PLAN www.2cto.com

3 where sql_id ='56s18gn1k19yp'

4 and plan_hash_value in (947531627,2658265176);

PLAN_HASH_VALUE ID OPERATION OPTIONS OBJECT_NAME DEPTH COST TIMESTAMP

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

947531627 0 SELECT STATEMENT 0 84 2011-09-27 04:33:34

947531627 1 COUNT STOPKEY 1 2011-09-27 04:33:34

947531627 2 VIEW 2 84 2011-09-27 04:33:34

947531627 3 SORT ORDER BY STOPKEY 3 84 2011-09-27 04:33:34

947531627 4 TABLE ACCESS BY INDEX ROWID BLOG_USER 4 3 2011-09-27 04:33:34

947531627 5 NESTED LOOPS 5 83 2011-09-27 04:33:34

947531627 6 HASH JOIN OUTER 6 17 2011-09-27 04:33:34

947531627 7 TABLE ACCESS BY INDEX ROWID CIRCLE_PAPER_MAIN 7 13 2011-09-27 04:33:34

947531627 8 INDEX RANGE SCAN IDX_CIRCLE_PAPER_MAIN_CID 8 3 2011-09-27 04:33:34

947531627 9 TABLE ACCESS BY INDEX ROWID CIRCLE_DISCUSS_CLASS 7 3 2011-09-27 04:33:34 www.2cto.com

947531627 10 INDEX RANGE SCAN IDX_CIRCLE_DISCUSS_CLASS_CID 8 1 2011-09-27 04:33:34

947531627 11 INDEX RANGE SCAN IDX_BLOG_USER_BLOGID 6 2 2011-09-27 04:33:34

2658265176 0 SELECT STATEMENT 0 9516 2012-02-22 15:54:33

2658265176 1 COUNT STOPKEY 1 2012-02-22 15:54:33

2658265176 2 VIEW 2 9516 2012-02-22 15:54:33

2658265176 3 SORT ORDER BY STOPKEY 3 9516 2012-02-22 15:54:33

2658265176 4 HASH JOIN RIGHT OUTER 4 8181 2012-02-22 15:54:33

2658265176 5 TABLE ACCESS BY INDEX ROWID CIRCLE_DISCUSS_CLASS 5 3 2012-02-22 15:54:33

2658265176 6 INDEX RANGE SCAN IDX_CIRCLE_DISCUSS_CLASS_CID 6 1 2012-02-22 15:54:33

2658265176 7 HASH JOIN 5 8177 2012-02-22 15:54:33

2658265176 8 TABLE ACCESS FULL CIRCLE_PAPER_MAIN 6 953 2012-02-22 15:54:33

2658265176 9 TABLE ACCESS FULL BLOG_USER 6 4954 2012-02-22 15:54:33

我们从查询结果中可以看到不同：

plan_hash_value = 947531627 --执行计划走索引

plan_hash_value = 2658265176 --执行计划走全表扫描

使用coe_xfr_sql_profile.sql可以发现两种执行计划的效率(AVG_ET_SECS)：

SQL> @coe_xfr_sql_profile.sql

Parameter 1: www.2cto.com

SQL_ID (required)

Enter value for 1: 56s18gn1k19yp

PLAN_HASH_VALUE AVG_ET_SECS

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

947531627 .037

2658265176 24.646

Parameter 2:

PLAN_HASH_VALUE (required)

Enter value for 2: 947531627

如何固定执行计划：

10g推荐使用sql profile来固定执行计划，coe_xfr_sql_profile.sql的本质也是调用sql profile来固定执行计划的。