在Oracle10g之前，優化SQL是個比較費力的技術活，不停的分析執行計劃，加hint，分析統計信息等等。在10g中，Oracle推出了自己的SQL優化輔助工具: SQL優化器(SQL Tuning Advisor ：STA)，它是新的DBMS_SQLTUNE包。使用STA一定要保證優化器是CBO模式下。
執行DBMS_SQLTUNE包進行sql優化需要有advisor的權限：
SQL> create user dave identified by dave;
用戶已創建。
SQL> grant connect,resource to dave;
授權成功。
SQL> grant advisor to dave;
授權成功。
下面簡單介紹一下如何優化一條找到的問題語句。
create table bigtab as select rownum as "id",a.* from sys.all_objects a;
create table smalltab as select rownum as "id", a.* FROM sys.all_tables a;
然後多運行幾次下面的腳本，增加表裏的數據：
insert into bigtab select rownum as "id",a.* from sys.all_objects a;
insert into smalltab select rownum as "id", a.* FROM sys.all_tables a;
這裏創建一張大表和一張小表，並且都沒有索引，下面執行一個查詢：
SQL> set timing on
SQL> set autot on
SQL> select count(*) from bigtab a, smalltab b where a.object_name=b.table_name;
COUNT(*)
----------
2141537
已用時間: 00: 00: 20.05
執行計劃
----------------------------------------------------------
Plan hash value: 3089226980
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 45 | 3146 (1)| 00:00:38 |
| 1 | SORT AGGREGATE | | 1 | 45 | | |
|* 2 | HASH JOIN | | 447K| 19M| 3146 (1)| 00:00:38 |
| 3 | TABLE ACCESS FULL| SMALLTAB | 27327 | 533K| 264 (1)| 00:00:04 |
| 4 | TABLE ACCESS FULL| BIGTAB | 712K| 16M| 2878 (1)| 00:00:35 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("A"."OBJECT_NAME"="B"."TABLE_NAME")
統計信息
----------------------------------------------------------
0 recursive calls
0 db block gets
31149 consistent gets
21058 physical reads
0 redo size
426 bytes sent via SQL*Net to client
416 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
第一步：創建優化任務
通過調用函數CREATE_TUNING_TASK來創建優化任務，調用存儲過程EXECUTE_TUNING_TASK執行該任務：
SQL> set autot off
SQL> set timing off
SQL> DECLARE
2 my_task_name VARCHAR2(30);
3 my_sqltext CLOB;
4 BEGIN
5 my_sqltext := 'select count(*) from bigtab a, smalltab b where a.object_name=b.table_name';
6 my_task_name := DBMS_SQLTUNE.CREATE_TUNING_TASK(
7 sql_text => my_sqltext,
8 user_name => 'DAVE', -- 注意是大寫，不然會報錯，用戶無效
9 scope => 'COMPREHENSIVE',
10 time_limit => 60,
11 task_name => 'tuning_sql_test',
12 description => 'Task to tune a query on a specified table');
13
14 DBMS_SQLTUNE.EXECUTE_TUNING_TASK( task_name => 'tuning_sql_test');
15 END;
16 /
PL/SQL procedure successfully completed.
在函數CREATE_TUNING_TASK，sql_text是需要優化的語句，user_name是該語句通過哪個用戶執行，scope是優化範圍（limited或comprehensive），time_limit優化過程的時間限制，task_name優化任務名稱，description優化任務描述。
第二步： 執行優化任務
通過調用dbms_sqltune.execute_tuning_task過程來執行前面創建好的優化任務。
SQL> exec dbms_sqltune.execute_tuning_task('tuning_sql_test');
PL/SQL 過程已成功完成。
第三步：檢查優化任務的狀態
通過查看user_advisor_tasks/dba_advisor_tasks視圖可以查看優化任務的當前狀態。
SQL> SELECT task_name,status FROM USER_ADVISOR_TASKS WHERE task_name ='tuning_sql_test';
TASK_NAME STATUS
------------------------------ -----------
tuning_sql_test COMPLETED
第四步：查看優化結果
通過dbms_sqltune.report_tning_task函數可以獲得優化任務的結果。
SQL> SET LONG 999999
SQL> set serveroutput on size 999999
SQL> SET LINESIZE 100
SQL> SELECT DBMS_SQLTUNE.REPORT_TUNING_TASK( 'tuning_sql_test') from DUAL;
DBMS_SQLTUNE.REPORT_TUNING_TASK('TUNING_SQL_TEST')
----------------------------------------------------------------------------------------------------
GENERAL INFORMATION SECTION
-------------------------------------------------------------------------------
Tuning Task Name : tuning_sql_test
Tuning Task Owner : DEMO
Scope : COMPREHENSIVE
Time Limit(seconds) : 60
Completion Status : COMPLETED
Started at : 5/28/2010 13:16:43
Completed at : 5/28/2010 13:16:44
Number of Index Findings : 1
Schema Name: DEMO
SQL ID : 6p64dnnsqf9pm
SQL Text : select count(*) from bigtab a, smalltab b where
a.object_name=b.table_name
-------------------------------------------------------------------------------
FINDINGS SECTION (1 finding)
-------------------------------------------------------------------------------
1- Index Finding (see explain plans section below)
The execution plan of this statement can be improved by creating one or more
indices.
Recommendation (estimated benefit: 100%)
----------------------------------------
- Consider running the Access Advisor to improve the physical schema design
or creating the recommended index.
create index DEMO.IDX$$_06C50001 on SYS.SMALLTAB('TABLE_NAME');
- Consider running the Access Advisor to improve the physical schema design
or creating the recommended index.
create index DEMO.IDX$$_06C50002 on SYS.BIGTAB('OBJECT_NAME');
Rationale
---------
Creating the recommended indices significantly improves the execution plan
of this statement. However, it might be preferable to run "Access Advisor"
using a representative SQL workload as opposed to a single statement. This
will allow to get comprehensive index recommendations which takes into
account index maintenance overhead and additional space consumption.
EXPLAIN PLANS SECTION
-------------------------------------------------------------------------------
1- Original
-----------
Plan hash value: 3089226980
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 36 | 3550 (2)| 00:00:43 |
| 1 | SORT AGGREGATE | | 1 | 36 | | |
|* 2 | HASH JOIN | | 155K| 5462K| 3550 (2)| 00:00:43 |
| 3 | TABLE ACCESS FULL| SMALLTAB | 1223 | 22014 | 11 (0)| 00:00:01 |
| 4 | TABLE ACCESS FULL| BIGTAB | 1205K| 20M| 3526 (1)| 00:00:43 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("A"."OBJECT_NAME"="B"."TABLE_NAME")
2- Using New Indices
--------------------
Plan hash value: 494801882
-----------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
-----------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 36 | 1108 (3)| 00:00:14 |
| 1 | SORT AGGREGATE | | 1 | 36 | | |
|* 2 | HASH JOIN | | 155K| 5462K| 1108 (3)| 00:00:14 |
| 3 | INDEX FAST FULL SCAN| IDX$$_06C50001 | 1223 | 22014 | 3 (0)| 00:00:01 |
| 4 | INDEX FAST FULL SCAN| IDX$$_06C50002 | 1205K| 20M| 1093 (2)| 00:00:14 |
-----------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("A"."OBJECT_NAME"="B"."TABLE_NAME")
------------------------------------------------------------------------------
看一下這個優化建議報告：
第一部分是關於這次優化任務的基本信息：如任務名稱、執行時間、範圍、涉及到的語句等等。
第二部分是關於這次優化任務的所找到的問題以及給出的優化建議。前面先給出了問題描述：可以通過建立更多的所引來提高性能；然後是建議的具體內容：在表smalltab的字段table_name上創建索引，在表bigtab的字段object_name上創建索引；最後是相關注意事項：此次優化雖然給出了創建索引的建議，但是最好通過SQL訪問建議器(SQL Access Advisor SAA)結合整個數據庫的工作量來深入分析，那樣就能給出考慮了索引維護和空間消耗等因素的更加合理的建議。
最後，報告還給出了原有的查詢計劃，以及採用優化建議以後的查詢計劃的對比。可以看出COST值大大下降。
五、刪除優化任務
通過調用dbms_sqltuen.drop_tuning_task可以刪除已經存在的優化任務
SQL>exec dbms_sqltune.drop_tuning_task('tuning_sql_test');
PL/SQL procedure successfully completed.