Mysql中的force index和ignore index

　　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