MySQL索引失效

2020-11-28 本文已影响0人 Doooook

哪些情况需要创建索引

主键自动建立唯一索引

频繁作为查询条件的字段应该创建索引

多表关联查询中，关联字段应该创建索引 on 两边都要创建索引

查询中排序的字段，应该创建索引

频繁查找字段覆盖索引

查询中统计或者分组字段，应该创建索引 group by

哪些情况不需要创建索引

表记录太少

经常进行增删改操作的表

频繁更新的字段

where条件里使用频率不高的字段

为什么使用组合索引

MySQL 创建组合索引的规则是首先会对组合索引的最左边的，也就是第一个name字段的数据进行排序，在第一个字段的排序基础上，然后再对后面第二个的cid字段进行排序。其实就相当于实现了类似 order by name cid 这样一种排序规则。
为了节省mysql索引存储空间以及提升搜索性能，可建立组合索引（能使用组合索引就不使用单列索引）

查看执行计划

介绍

MySQL 提供了一个 EXPLAIN 命令, 它可以对SELECT 语句的执行计划进行分析，并输出 SELECT 执行的详细信息，以供开发人员针对性优化。
使用explain这个命令来查看一个这些SQL语句的执行计划，查看该SQL语句有没有使用上了索引，有没有做全表扫描，这都可以通过explain命令来查看。
可以通过explain命令深入了解MySQL的基于开销的优化器，还可以获得很多可能被优化器考虑到的访问策略的细节，以及当运行SQL语句时哪种策略预计会被优化器采用。
EXPLAIN 命令用法十分简单，在 SELECT 语句前加上explain就可以了，例如：

mysql> explain select * from t_user;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    1 | NULL  |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

参数说明

expain出来的信息有10列，分别是：

id、select_type、table、type、possible_keys、key、key_len、ref、rows、Extras

案例表

-- 用户表
create table t_user(
    id int primary key,
    login_name varchar(100),
    name varchar(100),
    age int,
    sex char(1),
    dep int,
    address varchar(100)
);
-- 部门表
create table t_dep(
    id int primary key,
    name varchar(100)
);
-- 地址表
create table t_addr(
    id int primary key,
    addr varchar(100)
);
-- 创建普通索引
alter table t_user add index idx_dep(dep);
alter table t_dep add index idx_name(name);
-- 创建唯一索引
alter table t_user add unique index idx_login_name(login_name);
-- 创建组合索引
alter table t_user add index idx_name_age_sex(name, age, sex);
-- 创建全文索引
alter table t_addr add fulltext ft_addr(addr);

id

每个 SELECT 语句都会自动分配的一个唯一标识符。
表示查询中操作表的顺序，有三种情况：
- id相同：执行顺序由上到下。
- id不同：如果是子查询，id号会自增，id值越大优先级越高，越先被执行。
- id如果相同，可以认为是一组，从上往下顺序执行；在所有组中，id值越大，优先级越高，越先执行。
id列为null的就表示这是一个结果集，不需要使用它来进行查询。

select_type

查询类型，主要用于区别普通查询、联合查询(union、union all)、子查询等复杂查询。

SIMPLE

表示不需要union操作或者不包含子查询的简单select查询。有连接查询时，外层的查询为simple，且只有一个

mysql> explain select * from t_user;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    1 | NULL  |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

PRIMARY

一个需要union操作或者含有子查询的select，位于最外层的的select_type即为primary。且只有一个

mysql> explain select (select name from t_user) from t_user;
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key              | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
|  1 | PRIMARY     | t_user | index | NULL          | idx_dep          | 5       | NULL |    1 | Using index |
|  2 | SUBQUERY    | t_user | index | NULL          | idx_name_age_sex | 312     | NULL |    1 | Using index |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
2 rows in set (0.00 sec)

SUBQUERY

除了 from 子句中包含的子查询外，其他地方出现的子查询都可能是 subquery

mysql> explain select * from t_user where id = (select max(id) from t_user);
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | PRIMARY     | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
|  2 | SUBQUERY    | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | No matching min/max row                             |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
2 rows in set (0.01 sec)

DEPENDENT SUBQUERY

与dependent union类似，表示这个subquery的查询要受到外部表查询的影响

mysql> explain select id, name, (select name from t_dep a where a.id = b.dep) from t_user b;
+----+--------------------+-------+--------+---------------+---------+---------+-------------------+------+-------+
| id | select_type        | table | type   | possible_keys | key     | key_len | ref               | rows | Extra |
+----+--------------------+-------+--------+---------------+---------+---------+-------------------+------+-------+
|  1 | PRIMARY            | b     | ALL    | NULL          | NULL    | NULL    | NULL              |    1 | NULL  |
|  2 | DEPENDENT SUBQUERY | a     | eq_ref | PRIMARY       | PRIMARY | 4       | review_test.b.dep |    1 | NULL  |
+----+--------------------+-------+--------+---------------+---------+---------+-------------------+------+-------+
2 rows in set (0.00 sec)

UNION

union连接的两个select查询，第一个查询是PRIMARY，除了第一个表外，第二个以后的表select_type都是union

mysql> explain select * from t_user where sex='1' union select * from t_user where sex='2';
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra           |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
|  1 | PRIMARY      | t_user     | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where     |
|  2 | UNION        | t_user     | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where     |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
3 rows in set (0.00 sec)

DEPENDENT UNION

与union一样，出现在union 或union all语句中，但是这个查询要受到外部查询的影响

mysql> explain select * from t_user where sex in (select sex from t_user where sex='1' union select sex from t_user where sex='2');
+----+--------------------+------------+-------+---------------+------------------+---------+------+------+--------------------------+
| id | select_type        | table      | type  | possible_keys | key              | key_len | ref  | rows | Extra                    |
+----+--------------------+------------+-------+---------------+------------------+---------+------+------+--------------------------+
|  1 | PRIMARY            | t_user     | ALL   | NULL          | NULL             | NULL    | NULL |    1 | Using where              |
|  2 | DEPENDENT SUBQUERY | t_user     | index | NULL          | idx_name_age_sex | 312     | NULL |    1 | Using where; Using index |
|  3 | DEPENDENT UNION    | t_user     | index | NULL          | idx_name_age_sex | 312     | NULL |    1 | Using where; Using index |
| NULL | UNION RESULT       | <union2,3> | ALL   | NULL          | NULL             | NULL    | NULL | NULL | Using temporary          |
+----+--------------------+------------+-------+---------------+------------------+---------+------+------+--------------------------+
4 rows in set (0.00 sec)

UNION RESULT

如上所示，包含union的结果集，在union和union all语句中，因为它不需要参与查询，所以id字段为null

DERIVED

from字句中出现的子查询，也叫做派生表，其他数据库中可能叫做内联视图或嵌套select

mysql> explain select * from (select * from t_user where sex='1') b;
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | <derived2> | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL        |
|  2 | DERIVED     | t_user     | ALL  | NULL          | NULL | NULL    | NULL |    1 | Using where |
+----+-------------+------------+------+---------------+------+---------+------+------+-------------+
2 rows in set (0.00 sec)

table

显示的查询表名，如果查询使用了别名，那么这里显示的是别名
如果不涉及对数据表的操作，那么这显示为null
如果显示为尖括号括起来的就表示这个是临时表，后边的N就是执行计划中的id，表示结果来自于这个查询产生。
如果是尖括号括起来的<union M,N>，与之类似，也是一个临时表，表示这个结果来自于union查询的id为M, N的结果集。

type

依次从好到差：

system，const，eq_ref，ref，fulltext，ref_or_null，unique_subquery，index_subquery，range， index_merge，index，ALL

除了all之外，其他的type都可以使用到索引，除了index_merge之外，其他的type只可以用到一个索引
注意事项：最少要索引使用到range级别。

system

表中只有一行数据或者是空表（这里的“表”指的是子查询的“表”，而不是t_user表）

mysql> explain select * from (select * from t_user where id=1) a;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL  |    1 | NULL  |
|  2 | DERIVED     | t_user     | const  | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
2 rows in set (0.00 sec)

const（重要）

使用唯一索引或者主键，返回记录一定是1行记录的等值where条件时，通常type是const。其他数据库也叫做唯一索引扫描

-- 主键
mysql> explain select * from t_user where id=1;
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t_user | const | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
+----+-------------+--------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
-- 唯一索引
mysql> explain select * from t_user where login_name = 'Rose';
+----+-------------+--------+-------+----------------+----------------+---------+-------+------+-------+
| id | select_type | table  | type  | possible_keys  | key            | key_len | ref   | rows | Extra |
+----+-------------+--------+-------+----------------+----------------+---------+-------+------+-------+
|  1 | SIMPLE      | t_user | const | idx_login_name | idx_login_name | 303     | const |    1 | NULL  |
+----+-------------+--------+-------+----------------+----------------+---------+-------+------+-------+
1 row in set (0.00 sec)

eq_ref（重要）

关键字：连接字段主键或者唯一性索引。
此类型通常出现在多表的 join 查询，表示对于前表的每一个结果，都只能匹配到后表的一行结果，并且查询的比较操作通常是 '='，查询效率较高。

mysql> explain select a.id from t_user a left join t_dep b on a.dep=b.id;
+----+-------------+-------+--------+---------------+---------+---------+-------------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref               | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+-------------------+------+-------------+
|  1 | SIMPLE      | a     | index  | NULL          | idx_dep | 5       | NULL              |    5 | Using index |
|  1 | SIMPLE      | b     | eq_ref | PRIMARY       | PRIMARY | 4       | review_test.a.dep |    1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+-------------------+------+-------------+
2 rows in set (0.00 sec)

ref（重要）

针对非唯一索引，使用等值（=）查询非主键。或者是使用了最左前缀规则索引 [参看：MySQL索引最左前缀原则）]的查询。

-- 非唯一索引
mysql> explain select * from t_user where dep=1;
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------+
| id | select_type | table  | type | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | t_user | ref  | idx_dep       | idx_dep | 5       | const |    1 | NULL  |
+----+-------------+--------+------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
-- 等值非主键连接 
mysql> explain select a.id from t_user a left join t_dep b on a.name=b.name;
+----+-------------+-------+-------+---------------+------------------+---------+--------------------+------+-------------+
| id | select_type | table | type  | possible_keys | key              | key_len | ref                | rows | Extra       |
+----+-------------+-------+-------+---------------+------------------+---------+--------------------+------+-------------+
|  1 | SIMPLE      | a     | index | NULL          | idx_name_age_sex | 312     | NULL               |    5 | Using index |
|  1 | SIMPLE      | b     | ref   | idx_name      | idx_name         | 303     | review_test.a.name |    1 | Using index |
+----+-------------+-------+-------+---------------+------------------+---------+--------------------+------+-------------+
2 rows in set (0.00 sec)
-- 最左前缀
mysql> explain select * from t_user where name = 'zhaoyun';
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 303     | const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)

fulltext

全文索引检索，要注意，全文索引的优先级很高，若全文索引和普通索引同时存在时，mysql不管代价，优先选择使用全文索引

 mysql> explain select * from t_addr where match(addr) against('bei');
+----+-------------+--------+----------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type     | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+----------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t_addr | fulltext | ft_addr       | ft_addr | 0       | NULL |    1 | Using where |
+----+-------------+--------+----------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

ref_or_null

与ref方法类似，只是增加了null值的比较。实际用的不多。

unique_subquery

用于where中的in形式子查询，子查询返回不重复值唯一值

index_subquery

用于in形式子查询使用到了辅助索引或者in常数列表，子查询可能返回重复值，可以使用索引将子查询去重。

range（重要）

索引范围扫描，常见于使用>, <, is null, between, in, like等运算符的查询中。

-- 范围查询
mysql> explain select id from t_user where id>1;
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t_user | range | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using where; Using index |
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+
1 row in set (0.01 sec)
-- like 前缀索引
mysql> explain select * from t_user where name like 'z%';
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
| id | select_type | table  | type  | possible_keys    | key              | key_len | ref  | rows | Extra                 |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | t_user | range | idx_name_age_sex | idx_name_age_sex | 303     | NULL |    1 | Using index condition |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
1 row in set (0.00 sec)

-- 注：like '%z' 不使用索引

index_merge

表示查询使用了两个以上的索引，最后取交集或者并集，常见and、or的条件使用了不同的索引，官方排序这个在ref_or_null之后，但是实际上由于要读取多个索引，性能可能大部分时间都不如range。

index（重要）

** 关键字：条件是出现在索引树中的节点的。可能没有完全匹配索引。**
索引全表扫描，把索引从头到尾扫一遍，常见于使用索引列就可以处理不需要读取数据文件的查询、可以使用索引排序或者分组的查询。

-- 单索引
mysql> explain select login_name from t_user;
+----+-------------+--------+-------+---------------+----------------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key            | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+----------------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | index | NULL          | idx_login_name | 303     | NULL |    5 | Using index |
+----+-------------+--------+-------+---------------+----------------+---------+------+------+-------------+
-- 组合索引
mysql> explain select age from t_user;
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key              | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | index | NULL          | idx_name_age_sex | 312     | NULL |    5 | Using index |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+

-- 思考：explain select login_name,age from t_user;
-- 覆盖索引

ALL（重要）

这个就是全表扫描数据文件，然后再在server层进行过滤返回符合要求的记录。

mysql> explain select * from t_user;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | NULL  |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
-- 回表查询

possible_keys

此次查询中可能选用的索引，一个或多个

key

查询真正使用到的索引，type为index_merge时，这里可能出现两个以上的索引，其他的type这里只会出现一个。

key_len

用于处理查询的索引长度，如果是单列索引，那就整个索引长度算进去，如果是多列索引，那么查询不一定都能使用到所有的列，具体使用到了多少个列的索引，这里就会计算进去，没有使用到的列，这里不会计算进去。
留意下这个列的值，算一下你的多列索引总长度就知道有没有使用到所有的列了。
另外，key_len只计算where条件用到的索引长度，而排序和分组就算用到了索引，不会计算到key_len中。

ref

如果是使用的常数等值查询，这里会显示const
如果是连接查询，被驱动表的执行计划这里会显示驱动表的关联字段
如果是条件使用了表达式或者函数，或者条件列发生了内部隐式转换，这里可能显示为func

extra（重要）

这个列包含不适合在其他列中显示但十分重要的额外的信息，这个列可以显示的信息非常多，有几十种，常用的有：distinct、no tables used、using filesort（重要）、using index（重要）、using join buffer（block nested loop）、using join buffer（batched key accss）、using sort_union、using_union、using intersect、using sort_intersection、using temporary、using where（重要）、firstmatch(tb_name)、loosescan(m..n)、filtered

distinct

在select部分使用了distinct关键字

no tables used

不带from字句的查询或者From dual查询

mysql> explain select 1 from dual;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+

使用not in()形式子查询或not exists运算符的连接查询，这种叫做反连接，即，一般连接查询是先查询内表，再查询外表，反连接就是先查询外表，再查询内表。

using filesort（重要）

排序时无法使用到索引时，就会出现这个。常见于order by和group by语句中。
说明MySQL会使用一个外部的索引排序，而不是按照索引顺序进行读取。
MySQL中无法利用索引完成的排序操作称为“文件排序”。

mysql> explain select * from t_user order by address;
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using filesort |
+----+-------------+--------+------+---------------+------+---------+------+------+----------------+

using index（重要）

查询时不需要回表查询，直接通过索引就可以获取查询的数据。

表示相应的SELECT查询中使用到了覆盖索引（Covering Index），避免访问表的数据行，效率不错！
如果同时出现Using Where ，说明索引被用来执行查找索引键值。
如果没有同时出现Using Where ，表明索引用来读取数据而非执行查找动作。

-- 全值匹配，覆盖索引
mysql> explain select name, age, sex from t_user ;
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key              | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | index | NULL          | idx_name_age_sex | 312     | NULL |    5 | Using index |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
-- 对比select * from ...
mysql> explain select * from t_user ;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | NULL  |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
-- 索引用来读取数据而非执行查找动作
mysql> explain select id from t_user where id>1;
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra                    |
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t_user | range | PRIMARY       | PRIMARY | 4       | NULL |    4 | Using where; Using index |
+----+-------------+--------+-------+---------------+---------+---------+------+------+--------------------------+

using join buffer（block nested loop），using join buffer（batched key accss）

5.6.x之后的版本优化关联查询的BNL、BKA特性。主要是减少内表的循环数量以及比较顺序的扫描查询。

mysql> explain select a.id, a.name from t_user a, t_dep b where a.dep=b.id;
+----+-------------+-------+-------+---------------+----------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+-------+---------------+----------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | b     | index | PRIMARY       | idx_name | 303     | NULL |    2 | Using index                                        |
|  1 | SIMPLE      | a     | ALL   | NULL          | NULL     | NULL    | NULL |    5 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+---------------+----------+---------+------+------+----------------------------------------------------+
-- 对应的表结构
-- t_user
CREATE TABLE `t_user` (
  `id` int(11) NOT NULL,
  `login_name` varchar(100) DEFAULT NULL,
  `name` varchar(100) DEFAULT NULL,
  `age` int(11) DEFAULT NULL,
  `sex` char(1) DEFAULT NULL,
  `dep` int(11) DEFAULT NULL,
  `address` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `idx_login_name` (`login_name`),
  KEY `idx_name_age_sex` (`name`,`age`,`sex`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- t_dep
CREATE TABLE `t_dep` (
  `id` int(11) NOT NULL,
  `name` varchar(100) DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `idx_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

using sort_union，using_union，using intersect，using sort_intersection

using intersect：表示使用and的各个索引的条件时，该信息表示是从处理结果获取交集。
using union：表示使用or连接各个使用索引的条件时，该信息表示从处理结果获取并集。
using sort_union和using sort_intersection：与前面两个对应的类似，只是他们是出现在用and和or查询信息量大时，先查询主键，然后进行排序合并后，才能读取记录并返回。

using temporary

表示使用了临时表存储中间结果。
MySQL在对查询结果order by和group by时使用临时表。
临时表可以是内存临时表和磁盘临时表，执行计划中看不出来，需要查看status变量，used_tmp_table，used_tmp_disk_table才能看出来。

mysql> explain select distinct a.id from t_user a, t_dep b where a.dep=b.id;
+----+-------------+-------+-------+-----------------------------------------+----------+---------+------+------+----------------------------------------------------+
| id | select_type | table | type  | possible_keys                           | key      | key_len | ref  | rows | Extra                                              |
+----+-------------+-------+-------+-----------------------------------------+----------+---------+------+------+----------------------------------------------------+
|  1 | SIMPLE      | b     | index | PRIMARY                                 | idx_name | 303     | NULL |    2 | Using index; Using temporary                       |
|  1 | SIMPLE      | a     | ALL   | PRIMARY,idx_login_name,idx_name_age_sex | NULL     | NULL    | NULL |    5 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+-------+-----------------------------------------+----------+---------+------+------+----------------------------------------------------+

using where（重要）

表示存储引擎返回的记录并不是所有的都满足查询条件，需要在server层进行过滤。

-- 查询条件无索引
mysql> explain select * from t_user where address='USA';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
-- 索引失效
mysql> explain select * from t_user where age=1;
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
mysql> explain select * from t_user where id in(1, 2);
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+--------+-------+---------------+---------+---------+------+------+-------------+

查询条件中分为限制条件和检查条件，5.6之前，存储引擎只能根据限制条件扫描数据并返回，然后server层根据检查条件进行过滤再返回真正符合查询的数据。5.6.x之后支持ICP特性，可以把检查条件也下推到存储引擎层，不符合检查条件和限制条件的数据，直接不读取，这样就大大减少了存储引擎扫描的记录数量。extra列显示using index condition

mysql> explain select * from t_user where name='Rose';
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 303     | const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+

firstmatch(tb_name)

5.6.x开始引入的优化子查询的新特性之一，常见于where字句含有in()类型的子查询。如果内表的数据量比较大，就可能出现这个。

loosescan(m..n)

5.6.x之后引入的优化子查询的新特性之一，在in()类型的子查询中，子查询返回的可能有重复记录时，就可能出现这个。
除了这些之外，还有很多查询数据字典库，执行计划过程中就发现不可能存在结果的一些提示信息。

filtered

使用explain extended时会出现这个列，5.7之后的版本默认就有这个字段，不需要使用explain extended了。这个字段表示存储引擎返回的数据在server层过滤后，剩下多少满足查询的记录数量的比例，注意是百分比，不是具体记录数。

索引失效分析

1、全值匹配我最爱
2、最佳左前缀法则
3、不在索引列上做任何操作（计算、函数、自动or手动类型转换），会导致索引失效而转向全表扫描
4、存储引擎不能使用索引中范围条件右边的列
5、尽量使用覆盖索引（只访问索引的查询（索引列和查询列一致）），减少select *
6、MySQL在使用不等于（！=或者<>)的时候无法使用索引会导致全表扫描
7、is null, is not null也无法使用索引
8、like以通配符开头（'%abc')mysql索引失效，变成全表扫描的操作（注：以通配符结尾('abc%')不会失效）
9、字符串不加引号索引失效
10、少用or，用它来连接时会索引失效

1. 全值匹配我最爱

-- 条件与索引一一对应
mysql> explain select * from t_user where name='Rose' and age=20 and sex='f';
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref               | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 312     | const,const,const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+

2. 最佳左前缀法则

组要针对的是组合索引而言，如果索引了多个列，要遵守最佳左前缀法则，指的是查询从索引的最左前列开始并且不跳过索引中的列。
1、where条件列的，从索引的最左前列开始，且不跳过索引中的列
2、违背原则：未以最左前列开始，索引失效
3、违背原则：跳过索引中间列（age），只引用了部分索引
结论： where条件要满足最佳左前缀法则。口诀：带头大哥不能死，中间兄弟不能断

-- 反例：带头大哥不死（where条件中没有组合索引的带头大哥name）
mysql> explain select * from t_user where age=23;
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
-- 中间索引断（where条件中没有age，带头索引生效，其他索引失效）
mysql> explain select * from t_user where name='aa' and sex='1';
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 303     | const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
-- 比较
mysql> explain select * from t_user where name='aa' and sex='f' and age=23;
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref               | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 312     | const,const,const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
-- 对比
mysql> explain select * from t_user where name='aa' and age=23 and sex='1';
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref               | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 312     | const,const,const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------------------+------+-----------------------+

3. 不要在索引上做计算

不要进行这些操作：计算、函数、自动/手动类型转换，不然会导致索引失效而转向全表扫描，即使满足最左前缀原则，但where条件中使用了函数后，索引失效。

mysql> explain select * from t_user where login_name='Bob';
+----+-------------+--------+-------+----------------+----------------+---------+-------+------+-------+
| id | select_type | table  | type  | possible_keys  | key            | key_len | ref   | rows | Extra |
+----+-------------+--------+-------+----------------+----------------+---------+-------+------+-------+
|  1 | SIMPLE      | t_user | const | idx_login_name | idx_login_name | 303     | const |    1 | NULL  |
+----+-------------+--------+-------+----------------+----------------+---------+-------+------+-------+

mysql> explain select * from t_user where left(login_name, 1)='B';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+

4. 范围条件右边的列失效

不能继续使用索引中范围条件（bettween、<、>、in等）右边的列

mysql> explain select * from t_user where name='Jack' and age>20 and sex='f';
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
| id | select_type | table  | type  | possible_keys    | key              | key_len | ref  | rows | Extra                 |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | t_user | range | idx_name_age_sex | idx_name_age_sex | 308     | NULL |    1 | Using index condition |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
-- 对比
mysql> explain select name from t_user where name='Jack' and age>20 and sex='f';
+----+-------------+--------+------+------------------+------------------+---------+-------+------+--------------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                    |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 303     | const |    1 | Using where; Using index |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+--------------------------+

5. 尽量使用覆盖索引

尽量使用覆盖索引（只查询索引的列），也就是索引列和查询列一致，减少select *，按需要查字段，从索引中检索数据。

mysql> explain select * from t_user ;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | NULL  |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+

mysql> explain select name, login_name from t_user ;
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    5 | NULL  |
+----+-------------+--------+------+---------------+------+---------+------+------+-------+

-- 覆盖索引
mysql> explain select name, age, sex from t_user;
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key              | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | index | NULL          | idx_name_age_sex | 312     | NULL |    5 | Using index |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+-------------+
mysql> explain select login_name from t_user;
+----+-------------+--------+-------+---------------+----------------+---------+------+------+-------------+
| id | select_type | table  | type  | possible_keys | key            | key_len | ref  | rows | Extra       |
+----+-------------+--------+-------+---------------+----------------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | index | NULL          | idx_login_name | 303     | NULL |    5 | Using index |
+----+-------------+--------+-------+---------------+----------------+---------+------+------+-------------+

6. 索引字段上不要使用不等

索引字段上使用（！= 或者 < >）判断时，会导致索引失效而转向全表扫描

7. 主键索引字段上不可以判断null

主键字段上不可以使用 null
索引字段上使用 is null / is not null 判断时，可使用索引
对于已经有 NOT NULL 修饰的字段来说，不要再使用 IS NULL 或者 IS NOT NULL 来作为查询条件，没有意义。

mysql> explain select * from t_user where name is null;
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 303     | const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+

8. 索引字段使用like不以通配符开头

索引字段使用like以通配符开头（‘%字符串’）时，会导致索引失效而转向全表扫描

mysql> explain select * from t_user where name like 'a%';
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
| id | select_type | table  | type  | possible_keys    | key              | key_len | ref  | rows | Extra                 |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+
|  1 | SIMPLE      | t_user | range | idx_name_age_sex | idx_name_age_sex | 303     | NULL |    1 | Using index condition |
+----+-------------+--------+-------+------------------+------------------+---------+------+------+-----------------------+

mysql> explain select * from t_user where name like '%a';
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | ALL  | NULL          | NULL | NULL    | NULL |    6 | Using where |
+----+-------------+--------+------+---------------+------+---------+------+------+-------------+

由结果可知，like以通配符结束相当于范围查找，索引不会失效。与范围条件（bettween、<、>、in等）不同的是：不会导致右边的索引失效。
问题：解决like ‘%字符串%’时，索引失效问题的方法？使用覆盖索引可以解决。

mysql> explain select name, age, sex from t_user where name like '%a%';
+----+-------------+--------+-------+---------------+------------------+---------+------+------+--------------------------+
| id | select_type | table  | type  | possible_keys | key              | key_len | ref  | rows | Extra                    |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t_user | index | NULL          | idx_name_age_sex | 312     | NULL |    6 | Using where; Using index |
+----+-------------+--------+-------+---------------+------------------+---------+------+------+--------------------------+

9. 索引字段字符串要加单引号

索引字段是字符串，但查询时不加单引号，会导致索引失效而转向全表扫描

mysql> explain select * from t_user where name=123;
+----+-------------+--------+------+------------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys    | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | ALL  | idx_name_age_sex | NULL | NULL    | NULL |    6 | Using where |
+----+-------------+--------+------+------------------+------+---------+------+------+-------------+

mysql> explain select * from t_user where name='123';
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref   | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 303     | const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------+------+-----------------------+

10. 索引字段不要使用or

索引字段使用 or 时，会导致索引失效而转向全表扫描

mysql> explain select * from t_user where name='asd' or age=23;
+----+-------------+--------+------+------------------+------+---------+------+------+-------------+
| id | select_type | table  | type | possible_keys    | key  | key_len | ref  | rows | Extra       |
+----+-------------+--------+------+------------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t_user | ALL  | idx_name_age_sex | NULL | NULL    | NULL |    6 | Using where |
+----+-------------+--------+------+------------------+------+---------+------+------+-------------+

mysql> explain select * from t_user where name='asd' and age=23;
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+-----------------------+
| id | select_type | table  | type | possible_keys    | key              | key_len | ref         | rows | Extra                 |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+-----------------------+
|  1 | SIMPLE      | t_user | ref  | idx_name_age_sex | idx_name_age_sex | 308     | const,const |    1 | Using index condition |
+----+-------------+--------+------+------------------+------------------+---------+-------------+------+-----------------------+

总结

组合索引索引失效总结

索引失效口诀