Mysql force index和ignore index 使用实例

 前几天统计一个sql,是一个人提交了多少工单,顺便做了相关sql优化。数据大概2000多w。

select  CustName,count(1) c from WorkOrder  where CreateDate>‘2016-5-1‘ and CreateDate<‘2017-1-1‘group by CustName having c>100 order by c desc;

为了实验最少受其他因素干扰,将生产库的200多w数据导出来,用测试服务器进行测试。

导出来的数据是一个堆表,没有主键,没有索引。

mysql> show index from WorkOrder;   查询index方法1
Empty set (0.00 sec)

mysql> show keys from WorkOrder;    查询index方法2
Empty set (0.00 sec)

 1.堆表的情况

  这时候就在这时候,用执行计划分析下语句。

 
mysql>  explain select  CustName,count(1) c from WorkOrder 
 where CreateDate>‘2016-5-1‘ and CreateDate<‘2017-1-1‘ group by CustName having c>100 order by c desc;
+----+-------------+-----------+------+---------------+------+---------+------+---------+----------------------------------------------+
| id | select_type | table     | type | possible_keys | key  | key_len | ref  | rows    | Extra                                        |
+----+-------------+-----------+------+---------------+------+---------+------+---------+----------------------------------------------+
|  1 | SIMPLE      | WorkOrder | ALL  | NULL          | NULL | NULL    | NULL | 2528727 | Using where; Using temporary; Using filesort |
+----+-------------+-----------+------+---------------+------+---------+------+---------+----------------------------------------------+
1 row in set
 

select_type的值为SIMPLE,表示简单的select查询,不使用union或子查询。

type的值为ALL,表示要对表进行表扫描。

possible_keys 表示能使用哪个索引找到行记录。

key 表示Mysql决定使用的索引(键)。

       key_len 表示Mysql决定使用索引的长度。

ref  表示使用哪个列和key一起从表中选择行。

rows 表示Mysql认为它执行查询时必须检查的行数。

extra 表示查询的详情信息,用到where,临时表,排序。

   

执行下该语句三次,发现执行了16.30 sec、16.34 sec、16.24 sec。

  2.有索引的情况

建了四个索引,分别以custname,CreateDate建两个单列索引,另外两个是联合索引,只是最左边列不一样。

alter table WorkOrder add index ix_name(custname)  
alter table WorkOrder add index ix_date(CreateDate)  
alter table WorkOrder add index ix_namedate(custname,CreateDate)  
alter table WorkOrder add index ix_datename(CreateDate,custname)
 
mysql> show keys from WorkOrder;
+-----------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table     | Non_unique | Key_name    | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| WorkOrder |          1 | ix_name     |            1 | CustName    | A         |     1264363 |     NULL | NULL   | YES  | BTREE      |         |               |
| WorkOrder |          1 | ix_date     |            1 | CreateDate  | A         |     2528727 |     NULL | NULL   |      | BTREE      |         |               |
| WorkOrder |          1 | ix_namedate |            1 | CustName    | A         |     1264363 |     NULL | NULL   | YES  | BTREE      |         |               |
| WorkOrder |          1 | ix_namedate |            2 | CreateDate  | A         |     2528727 |     NULL | NULL   |      | BTREE      |         |               |
| WorkOrder |          1 | ix_datename |            1 | CreateDate  | A         |     2528727 |     NULL | NULL   |      | BTREE      |         |               |
| WorkOrder |          1 | ix_datename |            2 | CustName    | A         |     2528727 |     NULL | NULL   | YES  | BTREE      |         |               |
+-----------+------------+-------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
6 rows in set (0.00 sec)
 

之后,用执行计划分析下sql查询语句。

 
mysql> explain select  CustName,count(1) c from WorkOrder 
     where CreateDate>‘2016-5-1‘ and CreateDate<‘2017-1-1‘ group by CustName having c>100 order by c desc;
+----+-------------+-----------+-------+-----------------------------------------+-------------+---------+------+--------+-----------------------------------------------------------+
| id | select_type | table     | type  | possible_keys                           | key         | key_len | ref  | rows   | Extra                                                     |
+----+-------------+-----------+-------+-----------------------------------------+-------------+---------+------+--------+-----------------------------------------------------------+
|  1 | SIMPLE      | WorkOrder | range | ix_name,ix_date,ix_namedate,ix_datename | ix_datename | 4       | NULL | 824372 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+-----------+-------+-----------------------------------------+-------------+---------+------+--------+-----------------------------------------------------------+
1 row in set (0.01 sec)
 

从执行计划可以看出,Mysql从四个索引中选取了ix_datename这个索引,type为range表示索引范围扫描。rows的数量值是没堆表的1/3。

执行语句三次,时间是 8.64 sec、8.61sec、8.55 sec。

我建了三个索引,那么我想用下另外三个索引怎么办?

   这里可以用force index(),这个指令可以指定本次查询强制使用哪个索引,因为Mysql优化器的选择并不是最优的索引。

 
mysql> explain select  CustName,count(1) c from WorkOrder force index(ix_namedate) where CreateDate>‘2016-5-1‘ and CreateDate<‘2017-1-1‘ group by CustName having c>100 order by c desc;
+----+-------------+-----------+-------+---------------------------------+-------------+---------+------+---------+-----------------------------------------------------------+
| id | select_type | table     | type  | possible_keys                   | key         | key_len | ref  | rows    | Extra                                                     |
+----+-------------+-----------+-------+---------------------------------+-------------+---------+------+---------+-----------------------------------------------------------+
|  1 | SIMPLE      | WorkOrder | index | ix_name,ix_namedate,ix_datename | ix_namedate | 307     | NULL | 2528727 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+-----------+-------+---------------------------------+-------------+---------+------+---------+-----------------------------------------------------------+
 

 选用另一个联合索引 ix_namedate,这次type变为index,可以这样理解,根据索引的顺序进行全表扫描,比ALL效率要高些,rows的值和堆表的值差不多。

 执行语句三次,时间是 7.84 sec、7.92 sec、7.84 sec。

 
mysql> explain select  CustName,count(1) c from WorkOrder force index(ix_name) 
where CreateDate>‘2016-5-1‘ and CreateDate<‘2017-1-1‘ group by CustName having c>100 order by c desc;
+----+-------------+-----------+-------+---------------------------------+---------+---------+------+---------+----------------------------------------------+
| id | select_type | table     | type  | possible_keys                   | key     | key_len | ref  | rows    | Extra                                        |
+----+-------------+-----------+-------+---------------------------------+---------+---------+------+---------+----------------------------------------------+
|  1 | SIMPLE      | WorkOrder | index | ix_name,ix_namedate,ix_datename | ix_name | 303     | NULL | 2528727 | Using where; Using temporary; Using filesort |
+----+-------------+-----------+-------+---------------------------------+---------+---------+------+---------+----------------------------------------------+
1 row in set
 

    选用另一个联合索引 ix_name,这次type是index,可以这样理解,根据索引的顺序进行全表扫描,比ALL效率要高些,rows的值和堆表的值差不多。

  执行语句三次,时间是 1 min 28.17 sec、1 min 27.64 sec、1 min 27.58 sec。

 
mysql> explain select  CustName,count(1) c from WorkOrder force index(ix_date) 
where CreateDate>‘2016-5-1‘ and CreateDate<‘2017-1-1‘ group by CustName having c>100 order by c desc;
+----+-------------+-----------+-------+-----------------------------------------+---------+---------+------+--------+-------------------------------------------------------------------+
| id | select_type | table     | type  | possible_keys                           | key     | key_len | ref  | rows   | Extra                                                             |
+----+-------------+-----------+-------+-----------------------------------------+---------+---------+------+--------+-------------------------------------------------------------------+
|  1 | SIMPLE      | WorkOrder | range | ix_name,ix_date,ix_namedate,ix_datename | ix_date | 4       | NULL | 921062 | Using index condition; Using MRR; Using temporary; Using filesort |
+----+-------------+-----------+-------+-----------------------------------------+---------+---------+------+--------+-------------------------------------------------------------------+
 

    选用另一个联合索引 ix_date,这次type是range,表示索引范围扫描,rows的值是堆表的1/3多些 。

  执行语句三次,时间是 9.55 sec、9.52 sec、9.39 sec。

假如我不想用索引了怎么办?

   可以使用ignore index(),这个指令可以强制Mysql在查询时,不使用某索引。

 
mysql> explain select  CustName,count(1) c from WorkOrder  ignore index(ix_date) 
where CreateDate>‘2016-5-1‘ and CreateDate<‘2017-1-1‘ group by CustName having c>100 order by c desc;
+----+-------------+-----------+-------+---------------------------------+-------------+---------+------+--------+-----------------------------------------------------------+
| id | select_type | table     | type  | possible_keys                   | key         | key_len | ref  | rows   | Extra                                                     |
+----+-------------+-----------+-------+---------------------------------+-------------+---------+------+--------+-----------------------------------------------------------+
|  1 | SIMPLE      | WorkOrder | range | ix_name,ix_namedate,ix_datename | ix_datename | 4       | NULL | 824372 | Using where; Using index; Using temporary; Using filesort |
+----+-------------+-----------+-------+---------------------------------+-------------+---------+------+--------+-----------------------------------------------------------+

mysql> explain select  CustName,count(1) c from WorkOrder  ignore index(ix_date,ix_name,ix_namedate,ix_datename) 
where CreateDate>‘2016-5-1‘ and CreateDate<‘2017-1-1‘ group by CustName having c>100 order by c desc;
+----+-------------+-----------+------+---------------------------------+------+---------+------+---------+----------------------------------------------+
| id | select_type | table     | type | possible_keys                   | key  | key_len | ref  | rows    | Extra                                        |
+----+-------------+-----------+------+---------------------------------+------+---------+------+---------+----------------------------------------------+
|  1 | SIMPLE      | WorkOrder | ALL  | ix_name,ix_namedate,ix_datename | NULL | NULL    | NULL | 2528727 | Using where; Using temporary; Using filesort |
+----+-------------+-----------+------+---------------------------------+------+---------+------+---------+----------------------------------------------+
 

上面第一个强制不使用ix_date索引,那么就Mysql就从剩下的三个索引中,选取他认为是最优的索引。第二个时将四个索引都不使用,那么Mysql就进行全表扫描了。

  总结:

      1.Mysql的语句优化,没有绝对的正确,explain也只是给出个大致的方向,例如 key_len值小的,rows小的按理说,时间应该最短,效率最高。但是,实验中时间最少的却不是那个值最小的。

       2. 优化还需根据实际数据情况,例如,假如我where选取的时间范围变化,或者说CustName的分布有些变化,可能跟刚才的实验,又会产生一定偏差。

       3. 同样我还实验了,当给表加上主键时,整体的查询时间会缩短些。

------------------附相关index命令--------------

 
删除主键ALTER TABLE WorkOrder  MODIFY id int(11); --1.先删除auto_increment
ALTER TABLE  WorkOrder  DROP PRIMARY KEY;  --2.再删除主键

ALTER TABLE WorkOrder DROP index ix_datename;--删除索引
 

参考:mysql如何添加主键约束和唯一性约束,删除主键和唯一性约束

 mysql强制使用索引与不使用索引

 利用 force index优化sql语句性能

  mysql 存在索引但不能使用索引的典型场景

  mysql explain用法和结果的含义

  MySQL 优化之 index merge(索引合并)

  MySQL单列索引和组合索引(联合索引)的区别详解

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