mysql where and 使用的索引顺序

东华帝君
• 阅读 3323 
mysql> select * from test;
+----+------+------+
| id | a    | b    |
+----+------+------+
|  1 |    1 |    1 |
|  2 |    1 |    2 |
|  3 |    1 |    2 |
|  4 |    2 |    2 |
|  5 |    2 |    2 |
|  6 |    3 |    3 |
|  7 |    4 |    4 |
|  8 |    4 |    4 |
+----+------+------+
69 rows in set (0.00 sec)
mysql> show index from test;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| test  |          0 | PRIMARY  |            1 | id          | A         |          69 |     NULL | NULL   |      | BTREE      |         |               |
| test  |          1 | a        |            1 | a           | A         |           2 |     NULL | NULL   | YES  | BTREE      |         |               |
| test  |          1 | b        |            1 | b           | A         |           2 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)

EXPLAIN select * from test where b=31 and a=31;
mysql> EXPLAIN select * from test where b=31 and a=31;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | test  | NULL       | ref  | a,b           | a    | 5       | const |    1 |     5.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

结果说明：a与b同时不满足条件，使用了最左索引 a

mysql> EXPLAIN select * from test where b=11 and a=1;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+

| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | test  | NULL       | ref  | a,b           | b    | 5       | const |    1 |     5.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)

结果说明：a满足条件,b不满足条件，使用了索引 b

mysql> EXPLAIN select * from test where b=1 and a=11;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | test  | NULL       | ref  | a,b           | a    | 5       | const |    1 |     5.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

结果说明：a不满足条件,b满足条件，使用了索引 a

mysql> EXPLAIN select * from test where b=1 and a=1;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | test  | NULL       | ref  | a,b           | b    | 5       | const |    1 |     5.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

结果说明：a满足条件,b满足条件，使用了索引符合该条件的行数少的索引b

explain select * from test where  b=2 and a=1;
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type        | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                          |
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
|  1 | SIMPLE      | test  | NULL       | index_merge | a,b           | a,b  | 5,5     | NULL |    2 |   100.00 | Using intersect(a,b); Using where; Using index |
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
1 row in set, 1 warning (0.00 sec)

结果说明：a满足条件,b满足条件，最左索引a符合条件少于b，使用了索引 a,b

mysql> explain select * from test where  b=3 and a=3;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | test  | NULL       | ref  | a,b           | a    | 5       | const |    1 |    16.67 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

结果说明：a满足条件,b满足条件，最左索引a符合条件等于于b，使用了最左索引 a

mysql> explain select * from test where  b=4 and a=4;
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type        | possible_keys | key  | key_len | ref  | rows | filtered | Extra                                          |
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
|  1 | SIMPLE      | test  | NULL       | index_merge | a,b           | a,b  | 5,5     | NULL |    1 |   100.00 | Using intersect(a,b); Using where; Using index |
+----+-------------+-------+------------+-------------+---------------+------+---------+------+------+----------+------------------------------------------------+
1 row in set, 1 warning (0.00 sec

结果说明：a满足条件,b满足条件，最左索引a符合条件等于b(大于2)，使用了索引 a,b

总结：根据实验大概得知结果

  1. and查询会根据关联性高(符合该条件的行数少)选择具体走哪个索引，
  2. 如果关联性相等(符合该条件的行数相同)，满足一条，会走最左索引，大于一条，会走两个索引
  3. and查询会根据关联性高(符合该条件的行数少)，最左索引最小，会选择两个索引，
  4. 两个都不满足，走最左索引
以上测试结果仅供参考，如有错误，欢迎指正
mysql索引
点赞
收藏
评论区
推荐文章
blmius blmius
•
3年前
MySQL:[Err] 1292 - Incorrect datetime value: ‘0000-00-00 00:00:00‘ for column ‘CREATE_TIME‘ at row 1
文章目录问题用navicat导入数据时，报错：原因这是因为当前的MySQL不支持datetime为0的情况。解决修改sql\mode:sql\mode:SQLMode定义了MySQL应支持的SQL语法、数据校验等，这样可以更容易地在不同的环境中使用MySQL。全局s
5407
0
0
美凌格栋栋酱 美凌格栋栋酱
•
6个月前
Oracle 分组与拼接字符串同时使用
SELECTT.,ROWNUMIDFROM(SELECTT.EMPLID,T.NAME,T.BU,T.REALDEPART,T.FORMATDATE,SUM(T.S0)S0,MAX(UPDATETIME)CREATETIME,LISTAGG(TOCHAR(
2055
0
0
Wesley13 Wesley13
•
3年前
java将前端的json数组字符串转换为列表
记录下在前端通过ajax提交了一个json数组的字符串，在后端如何转换为列表。前端数据转化与请求varcontracts{id:'1',name:'yanggb合同1'},{id:'2',name:'yanggb合同2'},{id:'3',name:'yang
1543
0
0
待兔 待兔
•
1年前
手写Java HashMap源码
HashMap的使用教程HashMap的使用教程HashMap的使用教程HashMap的使用教程HashMap的使用教程22
1493
0
0
Jacquelyn38 Jacquelyn38
•
4年前
2020年前端实用代码段，为你的工作保驾护航
有空的时候，自己总结了几个代码段，在开发中也经常使用，谢谢。1、使用解构获取json数据let jsonData  id: 1,status: "OK",data: 'a', 'b';let  id, status, data: number   jsonData;console.log(id, status, number )
2802
0
0
Stella981 Stella981
•
3年前
List的Select 和Select().tolist()
List<PersondelpnewList<Person{newPerson{Id1,Name"小明1",Age11,Sign0},newPerson{Id2,Name"小明2",Age12,
1448
0
0
Wesley13 Wesley13
•
3年前
MySQL千万级别优化·中
MySQL千万级别的查询优化手段·中单列索引（假设在v\_record表中存在id列的索引）1、WHERE条件使用​EXPLAINSELECT\FROMv\_recordWHEREid2​结论：利用索引进行回表查询2、SELECT字段使用
991
0
0
Wesley13 Wesley13
•
3年前
mysql设置时区
mysql设置时区mysql\_query("SETtime\_zone'8:00'")ordie('时区设置失败，请联系管理员！');中国在东8区所以加8方法二：selectcount(user\_id)asdevice,CONVERT\_TZ(FROM\_UNIXTIME(reg\_time),'08:00','0
1823
0
0
Stella981 Stella981
•
3年前
JS 对象数组Array 根据对象object key的值排序sort,很风骚哦
有个js对象数组varary\{id:1,name:"b"},{id:2,name:"b"}\需求是根据name或者id的值来排序,这里有个风骚的函数函数定义:function keysrt(key,desc) {  return function(a,b){    return desc ? ~~(ak
1244
0
0
Wesley13 Wesley13
•
3年前
PHP创建多级树型结构
<!lang:php<?php$areaarray(array('id'1,'pid'0,'name''中国'),array('id'5,'pid'0,'name''美国'),array('id'2,'pid'1,'name''吉林'),array('id'4,'pid'2,'n
1680
0
0
京东云开发者 京东云开发者
•
2年前
为什么mysql不推荐使用雪花ID作为主键
作者：毛辰飞背景在mysql中设计表的时候，mysql官方推荐不要使用uuid或者不连续不重复的雪花id(long形且唯一)，而是推荐连续自增的主键id，官方的推荐是auto_increment，那么为什么不建议采用uuid，使用uuid究
1392
0
0
东华帝君
东华帝君
Lv1
勇于面对让你感到焦虑的事情。
文章
5
粉丝
0
获赞
0
热门文章
Linux 和 Windows 之间共享文件之 samba
惟实励新，精进臻善！MIAOYUN人人是讲师（第二季）焕新重启
一个基于React整套技术栈+Node.js的前端页面制作工具
Faiss利用mkl加速,构建索引训练时出错。