学习《MySQL高级》高阳老师解说索引课程的笔记，本篇偏重对where索引的剖析
建表
运行环境：MySQL 5.7.32
# 建表CREATE TABLE test03(id int primary key not null auto_increment,c1 char(10),c2 char(10),c3 char(10),c4 char(10),c5 char(10));insert into test03(c1,c2,c3,c4,c5) values('a1','a2','a3','a4','a5');insert into test03(c1,c2,c3,c4,c5) values('b1','b2','b3','b4','b5');insert into test03(c1,c2,c3,c4,c5) values('c1','c2','c3','c4','c5');insert into test03(c1,c2,c3,c4,c5) values('d1','d2','d3','d4','d5');insert into test03(c1,c2,c3,c4,c5) values('e1','e2','e3','e4','e5');select * from test03;# 建设 c1, c2, c3, c4 复合索引create index idx_test03_c1234 on test03(c1, c2, c3, c4);show index from test03;索引剖析
explain 跑在 MySQL 5.7.32 上
explain select * from test03 where c1 = 'a1';explain select * from test03 where c1 = 'a1' and c2 = 'a2';explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c3 = 'a3';explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c3 = 'a3' and c4 = 'a4';1. explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c3 = 'a3' and c4 = 'a4';
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------------------+------+----------+-------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------------------+------+----------+-------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 164 | const,const,const,const | 1 | 100.00 | NULL |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------------------+------+----------+-------+全值匹配，查问用到了全副4个字段的索引（通过 key 字段判断应用了复合索引，type 类型为 ref，通过 ref 字段判断应用4个索引）
2. explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c4 = 'a4' and c3 = 'a3';
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------------------+------+----------+-------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------------------+------+----------+-------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 164 | const,const,const,const | 1 | 100.00 | NULL |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------------------+------+----------+-------+全值匹配，查问用到了全副4个字段的索引，阐明与字段书写程序无关，只有两头不间断即可。
3. explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c3 > 'a3' and c4 = 'a4';
+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+-----------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+-----------------------+| 1 | SIMPLE | test03 | NULL | range | idx_test03_c1234 | idx_test03_c1234 | 123 | NULL | 1 | 20.00 | Using index condition |+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+-----------------------+查问应用到了c1、c2、c3三个字段的索引（通过 key 字段判断应用了复合索引，type 类型为 range，通过索引长度 key_len 判断应用了3个索引），因为c3是范畴，c4索引生效用不到（范畴之后全是生效），type 类型也从 ref 变为了 range。
4. explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c4 > 'a4' and c3 = 'a3';
+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+-----------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+-----------------------+| 1 | SIMPLE | test03 | NULL | range | idx_test03_c1234 | idx_test03_c1234 | 164 | NULL | 1 | 100.00 | Using index condition |+----+-------------+--------+------------+-------+------------------+------------------+---------+------+------+----------+-----------------------+查问应用了c1、c2、c3、c4四个索引（通过索引长度key_len 判断的），因为c4是范畴，type 类型也从 ref 变为了 range。
5. explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c4 = 'a4' order by c3;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 82 | const,const | 1 | 20.00 | Using index condition |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+查问应用到了c1、c2两个字段的索引；
排序时因为c1、c2、c3，c3索引能够用于排序，所以不会呈现文件排序（filesort）。
6. explain select * from test03 where c1 = 'a1' and c2 = 'a2' order by c3;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 82 | const,const | 1 | 100.00 | Using index condition |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+查问应用到了c1、c2两个字段的索引；
排序时因为c1、c2、c3，c3索引能够用于排序，所以不会呈现文件排序（filesort）。
7. explain select * from test03 where c1 = 'a1' and c2 = 'a2' order by c4;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+---------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+---------------------------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 82 | const,const | 1 | 100.00 | Using index condition; Using filesort |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+---------------------------------------+查问时应用了c1、c2两个字段的索引；
排序时，因为c4和c1、c2之间断掉了c3，所以无奈应用索引排序，会呈现文件排序。
8.1 explain select * from test03 where c1 = 'a1' and c5 = 'a5' order by c2, c3;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+------------------------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 41 | const | 1 | 20.00 | Using index condition; Using where |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+------------------------------------+查问时只用c1一个字段的索引；
排序时，c1、c2、c3索引用于排序，所以无filesort（文件排序）。
8.2 explain select * from test03 where c1 = 'a1' and c5 = 'a5' order by c3, c2;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+----------------------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+----------------------------------------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 41 | const | 1 | 20.00 | Using index condition; Using where; Using filesort |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+----------------------------------------------------+查问时用c1的索引；
排序时c3，c2因为不合乎复合索引的程序，所以无奈利用索引排序，会呈现filesort。
9. explain select * from test03 where c1 = 'a1' and c2 = 'a2' order by c2, c3;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 82 | const,const | 1 | 100.00 | Using index condition |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+-----------------------+查问时应用c1、c2字段的索引；
排序时，c1、c2、c3字段索引用于排序，所以无filesort。
10.1 explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c5 = 'a5' order by c2, c3;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+------------------------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 82 | const,const | 1 | 20.00 | Using index condition; Using where |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+------------------------------------+查问时应用c1、c2字段的索引；
排序时，c1、c2、c3字段索引用于排序，所以无filesort。
10.2 explain select * from test03 where c1 = 'a1' and c2 = 'a2' and c5 = 'a5' order by c3, c2;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+------------------------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 82 | const,const | 1 | 20.00 | Using index condition; Using where |+----+-------------+--------+------------+------+------------------+------------------+---------+-------------+------+----------+------------------------------------+查问时应用c1、c2字段的索引；
排序时，c3字段索引用于排序，c2字段已为定值无需排序，所以无filesort。
10.3 explain select * from test03 where c1 = 'a1' and c5 = 'a5' order by c3, c2;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+----------------------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+----------------------------------------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 41 | const | 1 | 20.00 | Using index condition; Using where; Using filesort |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+----------------------------------------------------+查问时用到了c1索引；
排序时，c1、c3、c2不是复合索引的程序，所以排序时会呈现filesort。
11. explain select * from test03 where c1 = 'a1' and c4 = 'a4' group by c2, c3;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+-----------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+-----------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 41 | const | 1 | 20.00 | Using index condition |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+-----------------------+查问时用到了c1索引，c4 和 c1 之间断掉了，所以查问仅应用c1索引；
group by 分组基本上都是须要排序的，可按 order by 剖析。
排序时，c1、c2、c3合乎复合索引的程序，所以排序时不会呈现filesort，分组也不会呈现长期表。
12. explain select * from test03 where c1 = 'a1' and c4 = 'a4' group by c3, c2;
+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+--------------------------------------------------------+| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+--------------------------------------------------------+| 1 | SIMPLE | test03 | NULL | ref | idx_test03_c1234 | idx_test03_c1234 | 41 | const | 1 | 20.00 | Using index condition; Using temporary; Using filesort |+----+-------------+--------+------------+------+------------------+------------------+---------+-------+------+----------+--------------------------------------------------------+查问时用到了c1索引，c4 和 c1 之间断掉了，所以查问仅应用c1索引；
排序时，c1、c3、c2不是复合索引的程序，所以排序时会呈现filesort，分组也会呈现长期表。
复合索引索引应用状况总结
| Where 语句 | 索引是否被应用到 |
|---|---|
| where a = 3 | Y，应用到 a |
| where a = 3 and b = 5 | Y，应用到 a，b |
| where a = 3 and b = 5 and c = 4 | Y，应用到 a，b，c |
| where b = 3 或者 where b = 3 and c = 4 或者 where c = 4 | N |
| where a = 3 and c = 5 | 应用到a，然而c不能够，因为b中间断了 |
| where a = 3 and b > 5 and c = 5 | 应用到a和b，因为c不能用在范畴之后，b断了（范畴前面全生效） |
| where a = 3 and b like 'kk%' and c = 4 | Y，应用到了a，b，c （与大于小于号的范畴不同，这里能够应用到索引） |
| where a = 3 and b like '%kk' and c = 4 | Y，只用到了a |
| where a = 3 and b like '%kk%' and c = 4 | Y，应用到了a |
| where a = 3 and b like 'k%kk%' and c = 4 | Y，应用到了a，b，c |
优化口诀总结
全值匹配我最爱，最左前缀要恪守;
带头大哥不能死，两头兄弟不能断;
索引列上少计算，范畴之后全生效;
LIKE百分写最右，笼罩索引不写星;
不等空值还有or，索引生效要少用;
[参考]
B站《MySQL高级》44.索引面试题剖析
B站《MySQL高级》45.索引优化答疑补充和总结口诀
MySQL高级篇（高阳）建表sql语句大全