深入理解MYSQL索引优化：多列索引
作者：互联网
索引是什么
是存储引擎用于找到数据的一种数据结构。
MYSQL索引优化视频详解：「链接」
索引的性能
在数据量小的时候，一个坏的索引往往作用没有那么明显，但是在数据量比较大的时候一个坏的索引和好的索引有巨大的区别。
在查询优化的时候应该首先考虑索引优化。这个是最简单的，也是效果最好。
索引的执行流程
索引=> 索引值=> 数据行
mysql> explain select first_name from actor where actor_id=5;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | actor | NULL | const | PRIMARY | PRIMARY | 2 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
通过explian可以看到where条件后面是使用了主键索引。
多列索引
如果是多列组成的索引，那么在使用索引的时候要考虑索引的顺序。
多列索引有如下原则
最左匹配原则，如果缺失了索引的最左侧列，那么索引不生效中间不可中断原则，如果中间定义的索引没有在where条件里面体现，那么后面的字段过滤将不会用到索引范围中断原则，只要使用了范围查询，那么其右侧的索引都无法正常使用多列索引的验证建表语句
create table person
(
A INT(10) not null,
B INT(10) not null,
C INT(10) not null,
version VARCHAR(20) not null
);
create index A
on person (A, B, C);填充数据的存储过程
delimiter //
//
CREATE DEFINER=`dev`@`%` PROCEDURE `insert_person`(IN item int)
BEGIN
DECLARE counter INT;
SET counter=item;
WHILE counter >=1000 DO
INSERT INTO person VALUES(left(counter, 2),left(counter,3), counter,counter );
SET counter=counter - 1;
END WHILE;
END;
//
delimiter ;
call insert_person(100000);数据详情
mysql> select count(*) from person;
+----------+
| count(*) |
+----------+
| 100000 |
+----------+
1 row in set (0.04 sec)场景分析一、索引生效1. 符合多列索引生效的三个原则，全部用上了索引
(a) 只对A做等值查询
mysql> explain select * from person where A=12;
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-------+
| 1 | SIMPLE | person | NULL | ref | A | A | 4 | const | 1100 | 100.00 | NULL |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
可以看到是用到了A的索引的。
(b)只对A做范围查询,并且结果集覆盖索引的时候。
mysql> explain select A,B,C from person where A > 13;
+----+-------------+--------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
| 1 | SIMPLE | person | NULL | range | A | A | 4 | NULL | 49555 | 100.00 | Using where; Using index |
+----+-------------+--------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
（c）对A,B做等值查询
mysql> explain select A,B,C from person where A=13 and b=129;
+----+-------------+--------+------------+------+---------------+------+---------+-------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+-------------+------+----------+-------------+
| 1 | SIMPLE | person | NULL | ref | A | A | 8 | const,const | 1 | 100.00 | Using index |
+----+-------------+--------+------------+------+---------------+------+---------+-------------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
(d) 对A做等值，B做范围
mysql> explain select A,B,C from person where A=13 and b > 129;
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | person | NULL | range | A | A | 8 | NULL | 1100 | 100.00 | Using where; Using index |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
(e) ABC等值查询
mysql> explain select A,B,C from person where A=12 and b=129 and c=1294;
+----+-------------+--------+------------+------+---------------+------+---------+-------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+-------------------+------+----------+-------------+
| 1 | SIMPLE | person | NULL | ref | A | A | 12 | const,const,const | 1 | 100.00 | Using index |
+----+-------------+--------+------------+------+---------------+------+---------+-------------------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
(f) 对AB做等值，C范围查询
mysql> explain select A,B,C from person where A=12 and b=129 and c > 1294;
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | person | NULL | range | A | A | 12 | NULL | 105 | 100.00 | Using where; Using index |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
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二、索引部分生效
满足最左原则，但是不满足中间不中断或者范围中断。
(a) 对A做等值，C做等值或者是范围
mysql> explain select * from person where A=12 and c=1230;
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | person | NULL | ref | A | A | 4 | const | 1100 | 10.00 | Using index condition |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from person where A=12 and c >1230;
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | person | NULL | ref | A | A | 4 | const | 1100 | 33.33 | Using index condition |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
(b)对A做等值，B做范围，C做等值或者范围，B做了范围查询之后，无法对C正常使用索引
mysql> explain select * from person where A=12 and b > 123 and c >1230;
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | person | NULL | range | A | A | 8 | NULL | 660 | 33.33 | Using index condition |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from person where A=12 and b=123 and c >1230;
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | person | NULL | range | A | A | 12 | NULL | 109 | 100.00 | Using index condition |
+----+-------------+--------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
(c) 对A做范围查询，BC做等值或者范围，那么BC将不会使用索引
mysql> explain select A,B,C from person where A > 12 and b > 123 and c >1230;
+----+-------------+--------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
| 1 | SIMPLE | person | NULL | range | A | A | 4 | NULL | 49555 | 11.11 | Using where; Using index |
+----+-------------+--------+------------+-------+---------------+------+---------+------+-------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)
需要注意的是这个地方ABC只是普通的索引如果是select * from,那么将不会使用索引。因为还有一个字段不再索引持有的数据上。
三、多列索引失效：
（a）查询条件中不包含最左列
mysql> explain select * from person where b=123 and c >1230;
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person | NULL | ALL | NULL | NULL | NULL | NULL | 99110 | 3.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from person where b=123
-> ;
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person | NULL | ALL | NULL | NULL | NULL | NULL | 99110 | 10.00 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from person where b >123;
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person | NULL | ALL | NULL | NULL | NULL | NULL | 99110 | 33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from person where c=123;
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person | NULL | ALL | NULL | NULL | NULL | NULL | 99110 | 10.00 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from person where c >123;
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person | NULL | ALL | NULL | NULL | NULL | NULL | 99110 | 33.33 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
(b) 含有最左列，但是索引的列带条件
mysql> explain select * from person where a + 1=12;
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person | NULL | ALL | NULL | NULL | NULL | NULL | 99110 | 100.00 | Using where |
+----+-------------+--------+------------+------+---------------+------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
## 从哪里带条件哪里就不开始使用索引
mysql> explain select * from person where a=12 and b + 1=123;
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | person | NULL | ref | A | A | 4 | const | 1100 | 100.00 | Using index condition |
+----+-------------+--------+------------+------+---------------+------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)四、order by使用索引
(a) order by 字段的顺序和table中多列索引定义的顺序一样。
mysql> explain select * from person1 order by a,b, c;
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+----------------+
| 1 | SIMPLE | person1 | NULL | ALL | NULL | NULL | NULL | NULL | 98999 | 100.00 | Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select a,b,c from person1 order by a,b, c;
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person1 | NULL | index | NULL | index_A_B_C | 21 | NULL | 98999 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
order by 不使用索引
(a) order by多个字段，每个字段都是独立的索引
mysql> create table person2 select * from person1;
Query OK, 99001 rows affected (0.68 sec)
Records: 99001 Duplicates: 0 Warnings: 0
mysql> alter table add key(a);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'add key(a)' at line 1
mysql> alter table add index_a key(a);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'add index_a key(a)' at line 1
mysql> alter table add index key(a);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'add index key(a)' at line 1
mysql> alter table add index index_a key(a);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'add index index_a key(a)' at line 1
mysql> alter table add index index_a(a);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'add index index_a(a)' at line 1
mysql> alter table person2 add index index_a(a);
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> alter table person2 add index index_b(b);
Query OK, 0 rows affected (0.15 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select * from person2 order by a,b;
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+----------------+
| 1 | SIMPLE | person2 | NULL | ALL | NULL | NULL | NULL | NULL | 99110 | 100.00 | Using filesort |
+----+-------------+---------+------------+------+---------------+------+---------+------+-------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
(b) order by语句中同时含有desc和asc
mysql> explain select a,b,c from person1 order by a desc ;
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person1 | NULL | index | NULL | index_A_B_C | 21 | NULL | 98999 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
mysql> explain select a,b,c from person1 order by a desc ,b asc;
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
| 1 | SIMPLE | person1 | NULL | index | NULL | index_A_B_C | 21 | NULL | 98999 | 100.00 | Using index; Using filesort |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)
(c) order by 里面有计算字段
mysql> explain select a,b,c from person1 order by a desc ,b + 1
-> ;
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
| 1 | SIMPLE | person1 | NULL | index | NULL | index_A_B_C | 21 | NULL | 98999 | 100.00 | Using index; Using filesort |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)
(d) orderby 里面顺序和索引的顺序不同
mysql> explain select a,b,c from person1 order by b,a;
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
| 1 | SIMPLE | person1 | NULL | index | NULL | index_A_B_C | 21 | NULL | 98999 | 100.00 | Using index; Using filesort |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)
(e) group by 和orderby 的顺序不同
mysql> explain select a,b,c from person1 group by a,b,c order by a,b;
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | person1 | NULL | index | index_A_B_C | index_A_B_C | 21 | NULL | 98999 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select a,b,c from person1 group by a,b,c order by b,a;
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------------------+
| 1 | SIMPLE | person1 | NULL | index | index_A_B_C | index_A_B_C | 21 | NULL | 98999 | 100.00 | Using index; Using temporary; Using filesort |
+----+-------------+---------+------------+-------+---------------+-------------+---------+------+-------+----------+----------------------------------------------+
1 row in set, 1 warning (0.00 sec)
上面的查询顺序一样，没有filesort，下面的不一样，就用了
(f) join查询的时候order只能按照主键的来排序，否则就会失效
mysql> explain select * from person3 t1 left join person4 t2 on t1.version=t2.version where t1.version=1200 order by t1.version;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
| 1 | SIMPLE | t1 | NULL | index | PRIMARY | PRIMARY | 22 | NULL | 101 | 10.00 | Using where |
| 1 | SIMPLE | t2 | NULL | eq_ref | PRIMARY | PRIMARY | 22 | dev.t1.version | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+
2 rows in set, 3 warnings (0.00 sec)
mysql> explain select * from person3 t1 left join person4 t2 on t1.version=t2.version where t1.version=1200 order by t2.version;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+----------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+----------------------------------------------+
| 1 | SIMPLE | t1 | NULL | ALL | PRIMARY | NULL | NULL | NULL | 101 | 10.00 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | t2 | NULL | eq_ref | PRIMARY | PRIMARY | 22 | dev.t1.version | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+----------------------------------------------+
2 rows in set, 3 warnings (0.00 sec)
mysql> show create table person3;
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| person3 | CREATE TABLE `person3` (
`A` varchar(10) DEFAULT NULL,
`B` int(50) NOT NULL,
`C` int(50) NOT NULL,
`version` varchar(20) NOT NULL,
PRIMARY KEY (`version`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> show create table person4;
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| person4 | CREATE TABLE `person4` (
`A` varchar(10) DEFAULT NULL,
`B` int(50) NOT NULL,
`C` int(50) NOT NULL,
`version` varchar(20) NOT NULL,
PRIMARY KEY (`version`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql> explain select * from person3 t1 left join person4 t2 on t1.version=t2.version order by t1.version;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+
| 1 | SIMPLE | t1 | NULL | index | NULL | PRIMARY | 22 | NULL | 101 | 100.00 | NULL |
| 1 | SIMPLE | t2 | NULL | eq_ref | PRIMARY | PRIMARY | 22 | dev.t1.version | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
mysql> explain select * from person3 t1 left join person4 t2 on t1.version=t2.version order by t2.version;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+---------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+---------------------------------+
| 1 | SIMPLE | t1 | NULL | ALL | NULL | NULL | NULL | NULL | 101 | 100.00 | Using temporary; Using filesort |
| 1 | SIMPLE | t2 | NULL | eq_ref | PRIMARY | PRIMARY | 22 | dev.t1.version | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------+------+----------+---------------------------------+
2 rows in set, 1 warning (0.00 sec)
## 不加where也是一样的。
优化此种情况
mysql> explain select * from person3 t1 left join person4 t2 on t1.version=t2.version join(select version from person3 order by a desc) a_order on t1.version=a_order.version;
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+
| 1 | SIMPLE | t1 | NULL | ALL | PRIMARY | NULL | NULL | NULL | 101 | 100.00 | NULL |
| 1 | SIMPLE | person3 | NULL | eq_ref | PRIMARY | PRIMARY | 22 | dev.t1.version | 1 | 100.00 | NULL |
| 1 | SIMPLE | t2 | NULL | eq_ref | PRIMARY | PRIMARY | 22 | dev.t1.version | 1 | 100.00 | NULL |
+----+-------------+---------+------------+--------+---------------+---------+---------+----------------+------+----------+-------+
3 rows in set, 1 warning (0.00 sec)
标签：index,NULL,ref,索引,key,MYSQL,多列,type,select 来源： https://blog.csdn.net/iqifenxia/article/details/122269332