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SQL优化是Oracle数据库中比较难的部分，需要对Oracle数据库具备非常扎实的理论基础。但是在刚开始接触时，往往不能很好地将理论知识应用到实践，或者有了一定的思路，又不自信或不敢确定是不是正确的。那么如何入门将理论知识转化为实践经验？本文介绍一下基于ADDM与SQL tuning的SQL优化，希望入门学习者能够从中获取一定的收获。

• 使用ADDM定位SQL
  如果你没有从AWR中定位到需要优化的SQL，可以结合ADDM查看分析。示例如下：

Finding 1: Top SQL Statements
Impact is 17.86 active sessions, 61.29% of total activity.
----------------------------------------------------------
SQL statements consuming significant database time were found. These
statements offer a good opportunity for performance improvement.Recommendation 1: SQL TuningEstimated benefit is 4.76 active sessions, 16.35% of total activity.--------------------------------------------------------------------ActionRun SQL Tuning Advisor on the SELECT statement with SQL_ID"XXXXXXXXXXX".Related ObjectSQL statement with SQL_ID XXXXXXXXXXX.RationaleThe SQL spent 99% of its database time on CPU, I/O and Cluster waits.This part of database time may be improved by the SQL Tuning Advisor.RationaleDatabase time for this SQL was divided as follows: 100% for SQLexecution, 0% for parsing, 0% for PL/SQL execution and 0% for Javaexecution.RationaleSQL statement with SQL_ID "XXXXXXXXXXX" was executed 1094801 times andhad an average elapsed time of 0.015 seconds.RationaleI/O and Cluster wait for INDEX "XXXXXX.XXXXXXXX" withobject ID 2133671 consumed 47% of the database time spent on this SQLstatement.
XXXXXXXX为出于隐私进行准换。

以上信息描述SQL_ID XXXXXXXXXXX 99％用于CPU，I/O和群集等待已执行1094801次，并且平均执行时间为0.015秒。基于xxxx索引（object ID 2133671）的I/O和群集等待占用数据库时间的47％，建议使用SQL tuning进行优化分析。

• 使用SQL tuning进行分析

  基于快照之间sql_id优化。

--1、创建任务

set autot off
set timing off
DECLARE
my_task_name VARCHAR2(30);
BEGIN
my_task_name := dbms_sqltune.create_tuning_task(begin_snap => 22176, --开始快照号
end_snap => 22184, --结束快照号
sql_id => '2hrbkst309jyj', --sqlid
scope => 'COMPREHENSIVE', --优化范围（limited或comprehensive）
time_limit => 60, --优化过程的时间限制
task_name => 'tuning_sql_test', --优化任务名称
description => 'tuning'); --优化任务描述
DBMS_SQLTUNE.EXECUTE_TUNING_TASK( task_name => 'tuning_sql_test');
END;
/

--2、执行任务

exec dbms_sqltune.execute_tuning_task('tuning_sql_test');

--3、查询执行当前状态

SELECT task_name,status FROM USER_ADVISOR_TASKS WHERE task_name ='tuning_sql_test';

--4、 查看优化结果

set long 999999
set serveroutput on size 999999
set line 120
select DBMS_SQLTUNE.REPORT_TUNING_TASK( 'tuning_sql_test') from dual;

--5、删除已经存在的优化任务，释放资源

exec dbms_sqltune.drop_tuning_task('tuning_sql_test');

第4步中查询SQL tuning建议内容如下：

• 绑定sql profile
  SQL tuning的第一个建议是绑定推荐的profile，使用并行。但也提示使用parallel可能带来的高资源消耗。最后部分可以看到未使用parallel与使用parallel DB time对比。

1- SQL Profile Finding (see explain plans p below)--------------------------------------------------------  A potentially better execution plan was found for this statement.Recommendation (estimated benefit: 99.13%)  ------------------------------------------  - Consider accepting the recommended SQL profile to use parallel execution    for this statement.    execute dbms_sqltune.accept_sql_profile(task_name => 'tuning_sql_test',            task_owner => 'SYS', replace => TRUE, profile_type =>            DBMS_SQLTUNE.PX_PROFILE);Executing this query parallel with DOP 128 will improve its response time  99.13% over the original plan. However, there is some cost in enabling  parallel execution. It will increase the statement's resource consumption by  an estimated 11.03% which may result in a reduction of system throughput.  Also, because these resources are consumed over a much smaller duration, the  response time of concurrent statements might be negatively impacted if  sufficient hardware capacity is not available.The following data shows some sampled statistics for this SQL from the past  week and projected weekly values when parallel execution is enabled.Past week sampled statistics for this SQL                                 -----------------------------------------  Number of executions                                                2648  Percent of total activity                                           1.79  Percent of samples with #Active Sessions > 2*CPU                       0  Weekly DB time (in sec)                                        483633.69Projected statistics with Parallel Execution                              --------------------------------------------  Weekly DB time (in sec)

• 建立索引
  第二个建议是建立索引，可以看到不同的执行计划：
  Plan hash value: 612724806，现使用执行计划，Time为00:36:55；
  Plan hash value: 2621731162,使用新的索引后，Time从00:36:55提升为00:05:53；
  Plan hash value: 3522323416,使用并行后，Time从00:36:55提升为00:00:20。

2- Index Finding (see explain plans p below)--------------------------------------------------  The execution plan of this statement can be improved by creating one or more  indices.Recommendation (estimated benefit: 84.07%)  ------------------------------------------  - Consider running the Access Advisor to improve the physical schema design    or creating the recommended index.    create index XXXXX.IDX$$_5191F0001 on    XxXX.XXXXXXxx(SUBSTR("ESN",-1),"STAT");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: 612724806
------------------------------------------------------------------------------------------------| Id  | Operation          | Name                      | Rows  | Bytes | Cost (%CPU)| Time     |------------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT   |                           |  8052 |  2665K|   184K  (1)| 00:36:55 ||*  1 |  COUNT STOPKEY     |                           |       |       |            |          ||*  2 |   TABLE ACCESS FULL| xxxxxxxxxx                |  8052 |  2665K|   184K  (1)| 00:36:55 |------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):---------------------------------------------------1 - filter(ROWNUM<=:1)   2 - filter("STAT"='0' AND SUBSTR("ESN",-1)='6' AND "INFO_TYPE"<>'4')
2- Using New Indices--------------------Plan hash value: 2621731162
----------------------------------------------------------------------------------------------------------| Id  | Operation                    | Name                      | Rows  | Bytes | Cost (%CPU)| Time     |----------------------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT             |                           | 10000 |  3310K| 29383   (1)| 00:05:53 ||*  1 |  COUNT STOPKEY               |                           |       |       |            |          ||*  2 |   TABLE ACCESS BY INDEX ROWID|xxxxxxxxxxxxxxxxxxxxxxxxxxx| 50325 |    16M| 29383   (1)| 00:05:53 ||*  3 |    INDEX RANGE SCAN          | IDX$$_5191F0001           | 46977 |       |   115   (0)| 00:00:02 |----------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):---------------------------------------------------1 - filter(ROWNUM<=:1)   2 - filter("INFO_TYPE"<>'4')   3 - access("DM_DATAREG_USER_INFO_ZL_T"."qsmmix_VCol_5001"='6' AND "STAT"='0')3- Using Parallel Execution---------------------------Plan hash value: 3522323416
---------------------------------------------------------------------------------------------------------------------------------| Id  | Operation              | Name                      | Rows  | Bytes | Cost (%CPU)| Time     | TQ  |IN-OUT| PQ Distrib |---------------------------------------------------------------------------------------------------------------------------------|   0 | SELECT STATEMENT       |                           |  8052 |  2665K|  1601   (0)| 00:00:20 |        |      |            ||*  1 |  COUNT STOPKEY         |                           |       |       |            |          |        |  |            ||   2 |   PX COORDINATOR       |                           |       |       |            |          |     |      |            ||   3 |    PX SEND QC (RANDOM) | :TQ10000                  |  8052 |  2665K|  1601   (0)| 00:00:20 |  Q1,00 | P->S | QC (RAND)  ||*  4 |     COUNT STOPKEY      |                           |       |       |     |          |  Q1,00 | PCWC |            ||   5 |      PX BLOCK ITERATOR |                           |  8052 |  2665K|  1601   (0)| 00:00:20 |  Q1,00 | PCWC |            ||*  6 |       TABLE ACCESS FULL| xxxxxxxxxxxxxxx           |  8052 |  2665K|  1601   (0)| 00:00:20 |  Q1,00 | PCWP |            |---------------------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):---------------------------------------------------1 - filter(ROWNUM<=:1)   4 - filter(ROWNUM<=:1)   6 - filter("STAT"='0' AND SUBSTR("ESN",-1)='6' AND "INFO_TYPE"<>'4')

通过以上信息，可以对SQL的优化方向以及优化后的带来的效益和资源有了一定的了解，并根据优化思路反推思考为什么如此做。日积月累之下，相信大家都能够对SQL优化有自己的理解。

关于作者

王茂材，云和恩墨北区交付团队技术顾问。从事Oracle DBA工作5年，维护过200+ Oracle数据库，涉及能源、医疗、体彩、银行、运营商等行业数据库的维护和操作。对Oracle数据库具备扎实的理论基础与丰富的实践经验，擅长故障处理、迁移、备份恢复、SQL优化等。

墨天轮原文链接：https://www.modb.pro/db/65575（复制到浏览器或者点击“阅读原文”立即查看）

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