最左前缀法则

概念

最左前缀法则是针对于复合索引而言的，也就是说一个索引有多个字段
那么索引的查询从最左列开始，并且不跳过索引的列，如果跳过索引中的某一列，那么，会导致索引部分失效(跳过列之后的索引失效)
如果出现了范围查询(>，<)，那么会导致索引列后的索引失效

栗子

建表

CREATE TABLE IF NOT EXISTS `article` (
`id` INT(10) UNSIGNED NOT NULL PRIMARY KEY AUTO_INCREMENT,
`author_id` INT(10) UNSIGNED NOT NULL,
`category_id` INT(10) UNSIGNED NOT NULL,
`views` INT(10) UNSIGNED NOT NULL,
`comments` INT(10) UNSIGNED NOT NULL,
`title` VARBINARY(255) NOT NULL,
`content` TEXT NOT NULL
);
 
INSERT INTO `article`(`author_id`, `category_id`, `views`, `comments`, `title`, `content`) VALUES
(1, 1, 1, 1, '1', '1'),
(2, 2, 2, 2, '2', '2'),
(1, 1, 3, 3, '3', '3');
 
SELECT * FROM article;

例题一

查询 category_id 为1 且 comments 等于 1 的情况下,views 等于1 article_id

没有索引

EXPLAIN SELECT id,author_id FROM article WHERE category_id = 1 AND comments =1  AND views = 1;

查询的执行结果为

+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | article | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+

我们可以看到他的执行计划，type为all，也就是全表扫描

复合索引

这个时候我们新建一个索引

CREATE INDEX idx_article_ccv ON article(category_id,comments,views);

全值匹配

再次执行

EXPLAIN SELECT id,author_id FROM article WHERE category_id = 1 AND comments =1  AND views = 1;

执行计划为

+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys   | key             | key_len | ref               | rows | filtered | Extra |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | article | NULL       | ref  | idx_article_ccv | idx_article_ccv | 12      | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------------+------+----------+-------+

我们可以看到这次的type是ref，走的索引为idx_article_ccv，也就是我们刚刚新建的索引，索引的长度为 12

and顺序

我们把执行的sql改为

EXPLAIN SELECT id,author_id FROM article WHERE category_id = 1 AND views = 1 AND comments =1  ;

我们可以发现他的执行计划一点没有变，也就是说与and的顺序无关

去掉views

我们去掉查询字段views，执行sql语句

EXPLAIN SELECT id,author_id FROM article WHERE category_id = 1 AND comments =1;

执行计划如下

+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys   | key             | key_len | ref         | rows | filtered | Extra |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | article | NULL       | ref  | idx_article_ccv | idx_article_ccv | 8       | const,const |    1 |   100.00 | NULL  |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------------+------+----------+-------+

我们可以看到结果key_len变成了8，没有全部走索引的列，能得出view的key_len为4

去掉views和comments

EXPLAIN SELECT id,author_id FROM article WHERE category_id = 1 ;

执行计划为

+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
| id | select_type | table   | partitions | type | possible_keys   | key             | key_len | ref   | rows | filtered | Extra |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | article | NULL       | ref  | idx_article_ccv | idx_article_ccv | 4       | const |    2 |   100.00 | NULL  |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-------+

我们会发现他的key_len变成了4，也就是说能得出view的key_len为4，comments的key_len为4，category_id key_len也为4

上面的查询完全根据最左前缀，没有跳过索引中的列

例题二

以例题一为基础，我们跳过一个索引列，

中间列

跳过comments的这个列进行查询

EXPLAIN SELECT id,author_id FROM article WHERE category_id = 1 AND views = 1;

执行计划如下

+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-----------------------+
| id | select_type | table   | partitions | type | possible_keys   | key             | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | article | NULL       | ref  | idx_article_ccv | idx_article_ccv | 4       | const |    2 |    33.33 | Using index condition |
+----+-------------+---------+------------+------+-----------------+-----------------+---------+-------+------+----------+-----------------------+

我们会发现他的key_len为4，也就是说只走了最左列的category_id 这个索引列

跳过最左列

跳过category_id 查询，我们运行下面的sql

 EXPLAIN SELECT id,author_id FROM article WHERE comments =1  AND views = 1;

执行计划为

+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table   | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | article | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+---------+------------+------+---------------+------+---------+------+------+----------+-------------+

我们会发现他完全再走全表扫描

例题三

范围查询，以例题一为基础，查询comments大于1的

EXPLAIN SELECT id,author_id FROM article WHERE category_id = 1 AND views = 1 AND comments > 1  ;

执行计划如下

+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys   | key             | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | article | NULL       | range | idx_article_ccv | idx_article_ccv | 8       | NULL |    1 |    33.33 | Using index condition |
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+

我们会发现他的key_len为8，views的索引列失效了，这个范围查询导致了索引的失效，但是我们只要把>加上一个等号，就会发现views索引可以使用了

 EXPLAIN SELECT id,author_id FROM article WHERE category_id = 1 AND views = 1 AND comments >= 1

执行计划如下

+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+
| id | select_type | table   | partitions | type  | possible_keys   | key             | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | article | NULL       | range | idx_article_ccv | idx_article_ccv | 12      | NULL |    2 |    33.33 | Using index condition |
+----+-------------+---------+------------+-------+-----------------+-----------------+---------+------+------+----------+-----------------------+