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
注重，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。
假如足够信息我们还会注重到，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.
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