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在11GR2上SQL Tuning Advisor已经变得非常好用，我非常喜欢将一些非常复杂的SQL语句交给SQL Tuning Advisor来调优，几乎每次都不让我失望，通常调优结束后，SQL Tuning Advisor都会给你一些建议，例如建议你创建索引或者收集统计信息，或者建议你接受SQL Profile并且给出了接受SQL Profile后性能将得到的提升。本节将会给出一个示例来演示如何通过SQL Tuning Advisor来使用SQL Profile。首先我们需要构建一下需要用到的测试表：
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| SQL>CREATE TABLE test 2 AS 3 SELECT ROWNUM id, 4 DBMS_RANDOM.STRING('A', 12) name, 5 DECODE(MOD(ROWNUM, 500), 0, 'Inactive', 'Active') status 6 FROM all_objects a,dba_objects b 7 WHERE ROWNUM <= 50000;
Table created.
SQL>create index t_ind on t(status);
Index created.
SQL>begin 2 dbms_stats.gather_table_stats(ownname =>'test', 3 tabname => 'test', 4 no_invalidate => FALSE, 5 estimate_percent => 100, 6 force => true, 7 degree => 5, 8 method_opt => 'for all columns size 1', 9 cascade => true); 10 end; 11 /
SQL>select status,count(*) from test group by status;
STATUS COUNT(*) ---------------- ---------- Active 49900 Inactive 100
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上面的代码做了下面几件事：
l 创建了一张测试表test,总记录数50000。
l 表上的字段status一共有2个唯一值：Active和Inactive，此字段有数据倾斜。
l 列status上值为Active的值有49900个，占了表里绝大多数的记录，为Inactive的记录非常少，只有100个。
l status字段上有索引，分析了表的统计信息，但是status字段没有收集直方图。
我们来对status为Inactive的值做查询，由于status为Inactive的值非常少，因此走索引扫描性能更好，但是由于列上缺少直方图，因此执行计划会走全表扫描：
| SQL>select count(name) from test where status='Inactive';
COUNT(NAME) ----------- 100
SQL>select * from table(dbms_xplan.display_cursor); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------ SQL_ID c37q7z5qjnwwf, child number 0 ------------------------------------- select count(name) from test where status='Inactive' Plan hash value: 1950795681 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 51 (100)| | | 1 | SORT AGGREGATE | | 1 | 21 | | | |* 2 | TABLE ACCESS FULL| TEST | 25000 | 512K| 51 (2)| 00:00:01 | --------------------------------------------------------------------------- |
上面执行计划里显示的经过谓词过滤后的全表扫描返回的基数为25000，因为缺少直方图，因此优化器就简单的通过 基数=表的总记录数/status字段的唯一值数量=50000/2=25000来得出基数。我们来通过SQL Tuning Advisor分析一下这个SQL，看看优化器能不能识别到这是一个低效的执行计划，能否给出我们一些建议：
| SQL>var c varchar2(100) SQL>exec :c := dbms_sqltune.CREATE_TUNING_TASK(SQL_ID=>'c37q7z5qjnwwf')
PL/SQL procedure successfully completed.
SQL>exec dbms_sqltune.execute_tuning_task(task_name => :c)
PL/SQL procedure successfully completed.
SQL>select dbms_sqltune.report_tuning_task(:c) from dual;
DBMS_SQLTUNE.REPORT_TUNING_TASK(:C) ------------------------------------------------------------------------------- GENERAL INFORMATION SECTION ------------------------------------------------------------------------------- Tuning Task Name : TASK_1112 Tuning Task Owner : TEST Workload Type : Single SQL Statement Scope : COMPREHENSIVE Time Limit(seconds): 1800 Completion Status : COMPLETED Started at : 08/01/2014 15:59:32 Completed at : 08/01/2014 15:59:33 ------------------------------------------------------------------------------- Schema Name: TEST SQL ID : c37q7z5qjnwwf SQL Text : select count(name) from test where status='Inactive' ------------------------------------------------------------------------------- FINDINGS SECTION (1 finding) ------------------------------------------------------------------------------- 1- SQL Profile Finding (see explain plans section below) -------------------------------------------------------- A potentially better execution plan was found for this statement. Recommendation (estimated benefit: 51.46%) ------------------------------------------ - Consider accepting the recommended SQL Profile. execute dbms_sqltune.accept_sql_profile(task_name => 'TASK_1112', task_owner => 'TEST', replace => TRUE);
Validation results ------------------ The SQL Profile was tested by executing both its plan and the original plan and measuring their respective execution statistics. A plan may have been only partially executed if the other could be run to completion in less time.
Original Plan With SQL Profile % Improved ------------- ---------------- ---------- Completion Status: COMPLETE COMPLETE Elapsed Time (s): .00212 .000221 89.57 % CPU Time (s): .002099 .0002 90.47 % User I/O Time (s): 0 0 Buffer Gets: 210 102 51.42 % Physical Read Requests: 0 0 Physical Write Requests: 0 0 Physical Read Bytes: 0 0 Physical Write Bytes: 0 0 Rows Processed: 1 1 Fetches: 1 1 Executions: 1 1
Notes ----- 1. Statistics for the original plan were averaged over 10 executions. 2. Statistics for the SQL Profile plan were averaged over 10 executions. ------------------------------------------------------------------------------- EXPLAIN PLANS SECTION ------------------------------------------------------------------------------- 1- Original With Adjusted Cost ------------------------------ Plan hash value: 1950795681 --------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | --------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 21 | 51 (2)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 21 | | | |* 2 | TABLE ACCESS FULL| TEST | 100 | 2100 | 51 (2)| 00:00:01 | --------------------------------------------------------------------------- 2- Using SQL Profile -------------------- Plan hash value: 4130896540 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 1 | 21 | 2 (0)| 00:00:01 | | 1 | SORT AGGREGATE | | 1 | 21 | | | | 2 | TABLE ACCESS BY INDEX ROWID| TEST | 100 | 2100 | 2 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | T_IND | 100 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------- |
Dbms_sqltune包被用来创建调优任务、执行调优任务、查看调优结果，通过dbms_sqltune包的CREATE_TUNING_TASK函数来为SQL_ID为c37q7z5qjnwwf的SQL创建了一个调优任务。然后通过execute_tuning_task过程来执行这个调优任务，任务运行后，优化器会利用动态采样等技术去验证评估内容与实际内容的差异，并且根据差异去调整执行计划。最后通过report_tuning_task来产生report查看调优的结果。调优结果里为我们提供了一个建议，建议我们采用一个SQL Profile，并且比对了采用SQL Profile后的性能提升，report的后面EXPLAIN PLANS SECTION部分展示了采用SQL Profile后，执行计划变为了索引扫描，而且基数的评估非常准确，从25000已经变为了100。
| n Note：当你运行SQLTuning Advisor后，建议你接受一个SQL Profile，如果你想在接受SQL Profile前知道它到底为你提供了些什么，可以运行以下查询获得： sys@DLSP>select 2 -- b.ATTR1 -- 10g 列 3 b.ATTR5 -- 11g 列 4 from 5 wri$_adv_tasks a, 6 wri$_adv_rationale b 7 where 8 a.name = 'TASK_1112' 9 and b.task_id = a.id 10 order by 11 b.rec_id, b.id 12 ;
ATTR5 --------------------------------------------------------------------------- OPT_ESTIMATE(@"SEL$1", TABLE, "TEST"@"SEL$1", SCALE_ROWS=0.004) OPT_ESTIMATE(@"SEL$1", INDEX_SCAN, "TEST"@"SEL$1", "T_IND", SCALE_ROWS=0.004) OPTIMIZER_FEATURES_ENABLE(default)
wri$_adv_tasks的name字段为任务名，在我们上面的例子里，可以通过print c在SQLPLUS环境下获得任务名，也可以在dbms_sqltune.report_tuning_task(:c)的输出里找到任务名。 |
我们接受这个SQL Profile来看看再次查询是否能用到刚创建的SQL Profile：
| SQL>execute dbms_sqltune.accept_sql_profile(task_name => 'TASK_1112',- > task_owner => 'TEST', replace => TRUE);
PL/SQL procedure successfully completed.
SQL>select count(name) from test where status='Inactive';
COUNT(NAME) ----------- 100
SQL>select * from table(dbms_xplan.display_cursor);
PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------ SQL_ID c37q7z5qjnwwf, child number 0 ------------------------------------- select count(name) from test where status='Inactive'
Plan hash value: 4130896540 -------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | -------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | | | 2 (100)| | | 1 | SORT AGGREGATE | | 1 | 21 | | | | 2 | TABLE ACCESS BY INDEX ROWID| TEST | 100 | 2100 | 2 (0)| 00:00:01 | |* 3 | INDEX RANGE SCAN | T_IND | 100 | | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------------- Note ----- - SQL Profile SYS_SQLPROF_01479094feeb0003 used for this statement
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接受SQL Tuning Advisor提供的SQL Profile后，执行计划Note部分：- SQL Profile SYS_SQLPROF_01479094feeb0003 used for this statement，显示已经使用到了SQL Profile，注意由SQL Tuning Advisor产生的SQL Profile名称都是SYS_SQLPROF作为前缀，使用到SQL Profile后执行计划也已经从全表扫描变为了索引扫描。dbms_sqltune的accept_sql_profile过程有多个参数可用，task_name、task_owner指创建SQL调优任务的任务名和所属用户，参数name和DESCRIPTION指SQL Profile的名字和对SQL Profile的描述，参数CATEGORY来指定创建的SQL Profile所属的类，默认的类为default。参数replace代表是否取代已有的SQL Profile，由于一个SQL只能有一个SQL Profile，不像Baseline，一个SQL可用有多个Baseline，因此如果一个SQL已经存在了SQL Profile，那么重新创建时，必须指定replace参数，设置为true，参数force_match指明了文本标准化的方式，默认为false。一旦接受SQL Profile，就可以通过视图dba_sql_profiles视图来查看SQL Profile的相关信息。因为SQL Profile并不属于某个用户，因此all_sql_profiles和user_sql_profiles视图都不可用。
如果一个SQL使用了SQL Profile，那么这个SQL的v$sql的sql_profile字段会显示使用到的SQL Profile的名字。下面的查询显示了系统中存在的SQL Profile和当前共享池中正在使用的SQL Profile的SQL。
| SQL>select name, category, status, substr(sql_text,1,25) sql_text, force_matching 2 from dba_sql_profiles 3 where sql_text like nvl('&sql_text','%') 4 and name like nvl('&name',name) 5 order by last_modified 6 ; Enter value for sql_text: Enter value for name:
NAME CATEGORY STATUS SQL_TEXT FORCE_ ------------------------------ --------- -------- ------------------------- ------ profile_c99yw1xkb4f1u_dwrose DEFAULT ENABLED select * from test NO profile_bhm28h6575bjy_dwrose DEFAULT ENABLED select test2.object_name, NO profile_51k1ug4rwah3c_dwrose DEFAULT ENABLED select distinct substr(ma NO profile_cm6stbx539mcz_dwrose DEFAULT ENABLED select count(*) from tt NO profile_c37q7z5qjnwwf_dwrose DEFAULT ENABLED select count(name) from t NO
SQL>select sql_id, 2 child_number cn, 3 plan_hash_value plan_hash, 4 sql_profile, 5 executions execs, 6 buffer_gets / decode(nvl(executions, 0), 0, 1, executions) avg_lio 7 from v$sql s 8 where upper(sql_text) like upper(nvl('&sql_text', sql_text)) 9 and sql_text not like '%from v$sql where sql_text like nvl(%' 10 and sql_id like nvl('&sql_id', sql_id) 11 and sql_profile is not null 12 order by 1, 2, 3 ; Enter value for sql_text:
SQL_ID CN PLAN_HASH SQL_PROFILE EXECS AVG_LIO -------------- --- ---------- ------------------------------ ----- ---------- c37q7z5qjnwwf 0 4130896540 profile_c37q7z5qjnwwf_dwrose 1 108 c37q7z5qjnwwf 1 4130896540 profile_c37q7z5qjnwwf_dwrose 2 105 |
我们根据SQL_PROFILE的命名知道，这些SQL Profile都不是SQL Tuning Advisor创建的，是我们手工创建的，因为SQL Tuning Advisor创建的SQL Profile都是以SYS_SQLPROF作为前缀的。