MySQL之索引使用原则详解（验证索引效率，SQL提示等）

索引使用

  • 验证索引效率

在未建立索引之前，执行如下SQL语句，查看SQL的耗时

select *  from tb_user where id = 1;

针对字段创建索引

create index idx_sku_sn on tb_sku(sn) ;

针对于用户量大的表中，添加索引要比没有添加索引的字段查询速度快。

  • 最左前缀法则

如果索引了多列（联合索引），要遵守最左前缀法则。最左前缀法则指的是查询从索引的最左列开始， 并且不跳过索引中的列。如果跳跃某一列，索引将会部分失效(后面的字段索引失效)。

mysql> explain select * from tb_user where profession = '通讯工程' and age = 33 and status = '0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref               | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
#如图所示，如果删除了条件profession则不满足最左前缀法则，为nullmysql> explain select * from tb_user where  age = 33 and status = '0';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    6 |    16.67 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)#如果删除age条件，那么会执行profession字段，不会执行status字段
mysql> explain select * from tb_user where  profession = '通讯工程' and status = '0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | const |    1 |    16.67 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

  • 范围查询

联合索引中，出现范围查询（>,<）范围查询右侧的列索引失效

#范围查询右边的列将会失效
mysql> explain select * from tb_user where profession = '软件工程' and age > 30 and status = '0';
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_pro_age_sta | idx_user_pro_age_sta | 49      | NULL |    2 |    10.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)#如果将范围修改为>=那么索引长度为54
mysql> explain select * from tb_user where profession = '软件工程' and age >= 30 and status = '0';+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_pro_age_sta | idx_user_pro_age_sta | 54      | NULL |    2 |    10.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

  • 索引列运算

不要在索引列上进行运算操作，索引将失效，性能将会降低

#查看phone的单列索引
mysql> select * from tb_user where phone = '17799990018';
+----+-----------+-------------+----------------------+--------------+------+--------+--------+---------------------+
| id | name      | phone       | email                | profession   | age  | gender | status | createtime          |
+----+-----------+-------------+----------------------+--------------+------+--------+--------+---------------------+
| 18 | 狄仁杰    | 17799990018 | jujiamlm8166@163.com | 国际贸易     |   30 | 1      | 0      | 2007-03-12 00:00:00 |
+----+-----------+-------------+----------------------+--------------+------+--------+--------+---------------------+
1 row in set (0.00 sec)mysql> explain select * from tb_user where phone = '17799990018';
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type  | possible_keys  | key            | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | const | idx_user_phone | idx_user_phone | 46      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)#如果添加函数运算，索引将会失效
mysql> select * from tb_user where substring(phone,10,2) = '18';
+----+-----------+-------------+----------------------+--------------+------+--------+--------+---------------------+
| id | name      | phone       | email                | profession   | age  | gender | status | createtime          |
+----+-----------+-------------+----------------------+--------------+------+--------+--------+---------------------+
| 18 | 狄仁杰    | 17799990018 | jujiamlm8166@163.com | 国际贸易     |   30 | 1      | 0      | 2007-03-12 00:00:00 |
+----+-----------+-------------+----------------------+--------------+------+--------+--------+---------------------+
1 row in set (0.00 sec)mysql> explain select * from tb_user where substring(phone,10,2) = '18';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   18 |   100.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

  • 字符串不加引号

字符串类型字段使用时，不加引号，索引将失效

mysql> explain select * from tb_user where phone = '17799990018';
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type  | possible_keys  | key            | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | const | idx_user_phone | idx_user_phone | 46      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+-------+----------------+----------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)#如果去掉引号，就会失效
mysql> explain select * from tb_user where phone = 17799990018;
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys  | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | idx_user_phone | NULL | NULL    | NULL |   18 |    10.00 | Using where |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
1 row in set, 3 warnings (0.00 sec)

  • 模糊查询

如果仅仅是尾部模糊匹配，索引不会失效。如果是头部模糊匹配，索引失效

mysql> explain select * from tb_user where profession like '软件%';
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | NULL |    4 |   100.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------------+----------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
#如果前面加入百分号则查询不到
mysql> explain select * from tb_user where profession like '%工程';
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | NULL          | NULL | NULL    | NULL |   18 |    11.11 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

  • or连接的条件

用or分割开的条件，如果or前的条件中的列有索引，而后面的列中没有索引，那么涉及的索引都不会被用到

mysql> explain select * from tb_user where id = 10 or age = 23;
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | PRIMARY       | NULL | NULL    | NULL |   18 |    15.00 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

 由于age没有索引，所以即使id，phone有索引，索引也会失效。所以需要针对于age也要建立索引。

mysql> create index idx_user_age on tb_user(age);
Query OK, 0 rows affected (0.04 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> explain select * from tb_user where id = 10 or age = 23;
+----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table   | partitions | type        | possible_keys        | key                  | key_len | ref  | rows | filtered | Extra                                          |
+----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+
|  1 | SIMPLE      | tb_user | NULL       | index_merge | PRIMARY,idx_user_age | PRIMARY,idx_user_age | 4,2     | NULL |    3 |   100.00 | Using union(PRIMARY,idx_user_age); Using where |
+----+-------------+---------+------------+-------------+----------------------+----------------------+---------+------+------+----------+------------------------------------------------+
1 row in set, 1 warning (0.02 sec)

- 数据分布影响

如果MySQL评估使用索引比全表更慢，则不使用索引

#由于phone的号码尾数全部大于十，所以全文扫描更加方便。而不走索引扫描
mysql> explain select * from tb_user where phone  >= '17799990000';
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys  | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | idx_user_phone | NULL | NULL    | NULL |   18 |   100.00 | Using where |
+----+-------------+---------+------------+------+----------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)mysql> explain select * from tb_user where phone  >= '17799990013';
+----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys  | key            | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | range | idx_user_phone | idx_user_phone | 46      | NULL |    6 |   100.00 | Using index condition |
+----+-------------+---------+------------+-------+----------------+----------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

查询时MySQL会评估，走索引快，还是全表扫描快，如果全表 扫描更快，则放弃索引走全表扫描。 因此，is null 、is not null是否走索引，得具体情况具体 分析，并不是固定的。

mysql> explain select * from tb_user where profession is null;
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)mysql> explain select * from tb_user where profession is not null;
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys        | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ALL  | idx_user_pro_age_sta | NULL | NULL    | NULL |   18 |   100.00 | Using where |
+----+-------------+---------+------------+------+----------------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

 

  • SQL提示

SQL提示，是优化数据库的一个重要手段，简单来说，就是在SQL语句中加入一些人为的提示来达到优 化操作的目的。

A. 执行SQL : explain select * from tb_user where profession = '软件工程';

mysql> explain select * from tb_user where profession = '软件工程';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 47      | const |    4 |   100.00 | NULL  |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
查找走了联合索引

B. 执行SQL，创建profession的单列索引：create index idx_user_pro on tb_user(profession);

mysql> create index idx_user_pro on tb_user(profession);
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

 C. 创建单列索引后，再次执行A中的SQL语句，查看执行计划，看看到底走哪个索引。

mysql> explain select * from tb_user where profession = '软件工程';
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys                     | key                  | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta,idx_user_pro | idx_user_pro_age_sta | 47      | const |    4 |   100.00 | NULL  |
+----+-------------+---------+------------+------+-----------------------------------+----------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

测试结果，我们可以看到，possible_keys中 idx_user_pro_age_sta,idx_user_pro 这两个 索引都可能用到，最终MySQL选择了idx_user_pro_age_sta索引。这是MySQL自动选择的结果。

1). use index ： 建议MySQL使用哪一个索引完成此次查询（仅仅是建议，mysql内部还会再次进 行评估）。

explain select * from tb_user use index(idx_user_pro) where profession = '软件工
程';mysql> explain select * from tb_user use index(idx_user_pro) where profession = '软件工'> 程';
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys | key          | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro  | idx_user_pro | 47      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+---------------+--------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

 2). ignore index ： 忽略指定的索引。

explain select * from tb_user ignore index(idx_user_pro) where profession = '软件工
程';

 3). force index ： 强制使用索引。

explain select * from tb_user force index(idx_user_pro) where profession = '软件工
程';

 

  • 覆盖索引

尽量使用覆盖索引，减少select *。 那么什么是覆盖索引呢？

覆盖索引是指 查询使用了索引，并 且需要返回的列，在该索引中已经全部能够找到 。

mysql> explain select id,profession,age,status from tb_user where profession = '软件工程' and age = 31 and status = '0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+--------------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref               | rows | filtered | Extra                    |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+--------------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using where; Using index |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)mysql> explain select id,profession,age,status,name from tb_user where profession = '软件工程' and age = 31 and status = '0';
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys        | key                  | key_len | ref               | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_user_pro_age_sta | idx_user_pro_age_sta | 54      | const,const,const |    1 |   100.00 | Using index condition |
+----+-------------+---------+------------+------+----------------------+----------------------+---------+-------------------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
extra含义
using where;using index查找使用了索引，但是需要的数据都在索引列中能找到，所以不需要查询数据
using index conition查找使用了索引，但是需要回表查询数据

思考题：

一张表, 有四个字段(id, username, password, status), 由于数据量大, 需要对 以下SQL语句进行优化, 该如何进行才是最优方案:

select id,username,password from tb_user where username = 'itcast';

答案: 针对于 username, password建立联合索引, sql为: create index idx_user_name_pass on tb_user(username,password); 这样可以避免上述的SQL语句，在查询的过程中，出现回表查询。

  • 前缀索引

当字段类型为字符串（varchar，text，longtext等）时，有时候需要索引很长的字符串，这会让 索引变得很大，查询时，浪费大量的磁盘IO， 影响查询效率。此时可以只将字符串的一部分前缀，建 立索引，这样可以大大节约索引空间，从而提高索引效率。

语法：

create index idx_xxxx on table_name(column(n)) ;

前缀长度

可以根据索引的选择性来决定，而选择性是指不重复的索引值（基数）和数据表的记录总数的比值， 索引选择性越高则查询效率越高， 唯一索引的选择性是1，这是最好的索引选择性，性能也是最好的。

select count(distinct email) / count(*) from tb_user ;
select count(distinct substring(email,1,5)) / count(*) from tb_user ;
mysql> select count(distinct substring(email,1,10))/count(*) from tb_user;
+------------------------------------------------+
| count(distinct substring(email,1,10))/count(*) |
+------------------------------------------------+
|                                         1.0000 |
+------------------------------------------------+
1 row in set (0.01 sec)mysql> select count(distinct substring(email,1,9))/count(*) from tb_user;
+-----------------------------------------------+
| count(distinct substring(email,1,9))/count(*) |
+-----------------------------------------------+
|                                        0.9444 |
+-----------------------------------------------+
1 row in set (0.00 sec)mysql> create index idx_email_5 on tb_user(email(5));
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> select * from tb_user where email = 'jingke123@163.com';
+----+--------+-------------+-------------------+------------+------+--------+--------+---------------------+
| id | name   | phone       | email             | profession | age  | gender | status | createtime          |
+----+--------+-------------+-------------------+------------+------+--------+--------+---------------------+
| 11 | 荆轲   | 17799990011 | jingke123@163.com | 会计       |   29 | 1      | 0      | 2001-05-11 00:00:00 |
+----+--------+-------------+-------------------+------------+------+--------+--------+---------------------+
1 row in set (0.00 sec)mysql> explain select * from tb_user where email = 'jingke123@163.com';
+----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key         | key_len | ref   | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | tb_user | NULL       | ref  | idx_email_5   | idx_email_5 | 23      | const |    1 |   100.00 | Using where |
+----+-------------+---------+------------+------+---------------+-------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

  • 单列索引和联合索引

单列索引：即一个索引只包含单个列。

联合索引：即一个索引包含了多个列。

 

在查询出来的索引中，既有单列索引，又有联合索引

 

 在and连接的两个字段 phone、name上都是有单列索引的，但是 最终mysql只会选择一个索引，也就是说，只能走一个字段的索引，此时是会回表查询的。然后我们创建一个phone和name字段的联合索引查询一下执行：

mysql>  create unique index idx_user_phone_name on tb_user(phone,name);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0mysql> explain select id,phone,name from tb_user where phone = '17799990002' and name = '赵云';
+----+-------------+---------+------------+-------+--------------------------------------------------+----------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type  | possible_keys                                    | key            | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+-------+--------------------------------------------------+----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | tb_user | NULL       | const | idx_user_phone,idx_user_phone_name,idx_user_name | idx_user_phone | 46      | const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+-------+--------------------------------------------------+----------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

此时，查询时，就走了联合索引，而在联合索引中包含 phone、name的信息，在叶子节点下挂的是对 应的主键id，所以查询是无需回表查询的。

在业务场景中，如果存在多个查询条件，考虑针对于查询字段建立索引时，建议建立联合索引， 而非单列索引。

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