Oracle10gR2 autotrace function was changed and enhanced

作者:eygle
出处:http://www.eygle.com/blog
日期:September 15, 2005
本文链接:http://www.eygle.com/archives/2005/09/oracle10gr2_aut.html

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今天在测试过程中，偶然发现在Oracle10g Release 2中，Autotrace的功能已经被极大加强和改变。
这是一个很有意思的小的变化，关于autotrace的使用，请参考:启用AutoTrace
接下来让我们先来看一下什么地方发生了改变:

SQL> set linesize 120
SQL> set autotrace on
SQL> select count(*) from v$session
  2  /

  COUNT(*)
----------
        21


Execution Plan
----------------------------------------------------------
Plan hash value: 2376410614

---------------------------------------------------------------------------------------------
| Id  | Operation                 | Name            | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT          |                 |     1 |    65 |     0   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE           |                 |     1 |    65 |            |          |
|   2 |   NESTED LOOPS            |                 |     1 |    65 |     0   (0)| 00:00:01 |
|*  3 |    FIXED TABLE FULL       | X$KSUSE         |     1 |    52 |     0   (0)| 00:00:01 |
|*  4 |    FIXED TABLE FIXED INDEX| X$KSLED (ind:2) |     1 |    13 |     0   (0)| 00:00:01 |
---------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   3 - filter("S"."INST_ID"=USERENV('INSTANCE') AND BITAND("S"."KSSPAFLG",1)<>0 AND
              BITAND("S"."KSUSEFLG",1)<>0)
   4 - filter("S"."KSUSEOPC"="E"."INDX")


Statistics
----------------------------------------------------------
          1  recursive calls
          0  db block gets
          0  consistent gets
          0  physical reads
          0  redo size
        411  bytes sent via SQL*Net to client
        385  bytes received via SQL*Net from client
          2  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
          1  rows processed

注意，此时autotrace的输出被良好格式化，并给出关于执行计划部分的简要注释。
其实这里并没有带来新的东西，从Oracle9i开始，Oracle提供了一个新的工具dbms_xplan用以格式化和查看SQL的执行计划。其原理是通过对plan_table的查询和格式化提供更友好的用户输出。

dbms_xplan的调用的语法类似:
select * from table(dbms_xplan.display(format=>'BASIC'))
使用 TABLE() 操作符，或者 CAST 操作。

具体用法可以参考Oracle官方文档。

实际上从Oracle9i开始我们就经常使用如下方式调用dbms_xplan:

Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production

SQL> explain plan for
  2  select count(*) from dual;

Explained.

SQL> @?/rdbms/admin/utlxplp;

PLAN_TABLE_OUTPUT
----------------------------------------------------------------------------------

--------------------------------------------------------------------
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT     |             |       |       |       |
|   1 |  SORT AGGREGATE      |             |       |       |       |
|   2 |   TABLE ACCESS FULL  | DUAL        |       |       |       |
--------------------------------------------------------------------

Note: rule based optimization

10 rows selected.

utlxplp.sql脚本中正是调用了dbms_xplan:

SQL> get ?/rdbms/admin/utlxplp;
  1  Rem
  2  Rem $Header: utlxplp.sql 23-jan-2002.08:55:23 bdagevil Exp $
  3  Rem
  4  Rem utlxplp.sql
  5  Rem
  6  Rem Copyright (c) 1998, 2002, Oracle Corporation.  All rights reserved.
  7  Rem
  8  Rem    NAME
  9  Rem      utlxplp.sql - UTiLity eXPLain Parallel plans
 10  Rem
 11  Rem    DESCRIPTION
 12  Rem      script utility to display the explain plan of the last explain plan
 13  Rem         command. Display also Parallel Query information if the plan happens to
 14  Rem      run parallel
 15  Rem
 16  Rem    NOTES
 17  Rem      Assume that the table PLAN_TABLE has been created. The script
 18  Rem      utlxplan.sql should be used to create that table
 19  Rem
 20  Rem      With SQL*plus, it is recomended to set linesize and pagesize before
 21  Rem      running this script. For example:
 22  Rem            set linesize 130
 23  Rem            set pagesize 0
 24  Rem
 25  Rem    MODIFIED   (MM/DD/YY)
 26  Rem    bdagevil    01/23/02 - rewrite with new dbms_xplan package
 27  Rem    bdagevil    04/05/01 - include CPU cost
 28  Rem    bdagevil    02/27/01 - increase Name column
 29  Rem    jihuang     06/14/00 - change order by to order siblings by.
 30  Rem    jihuang     05/10/00 - include plan info for recursive SQL in LE row source
 31  Rem    bdagevil    01/05/00 - make deterministic with order-by
 32  Rem    bdagevil    05/07/98 - Explain plan script for parallel plans
 33  Rem    bdagevil    05/07/98 - Created
 34  Rem
 35  set markup html preformat on
 36  Rem
 37  Rem Use the display table function from the dbms_xplan package to display the last
 38  Rem explain plan. Use default mode which will display only relevant information
 39  Rem
 40* select * from table(dbms_xplan.display());
 41  
SQL> 

而在Oracle10gR2中，Oracle帮我们简化了这个过程，一个autotrace就完成了所有的输出，这也是易用性上的一个进步吧。在使用Oracle的过程中，我们经常能够感受到Oracle针对用户需求或易用性的改进，这也许是很多人喜爱Oracle的一个原因吧。

如果足够信息我们还会注意到，在Oracle10g中PLAN_TABLE不再需要创建，Oracle缺省增加了一个字典表PLAN_TABLE$，然后基于PLAN_TABLE$创建公用同义词供用户使用。

关于dbms_xplan工具的使用还可以参考Itpub上的讨论:使用dbms_xplan工具查看执行计划.