本文的示例代码参考index_explain
目录
startup
explain
-
select_type
聚簇索引
覆盖索引
-
type
唯一索引
主键索引
-
key_len
联合索引
前缀索引
startup
vim user_info.sql
CREATE TABLE `user_info` (
`id` BIGINT(20) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(50) NOT NULL DEFAULT '',
`age` INT(11) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `name_index` (`name`)
) ENGINE = InnoDB DEFAULT CHARSET = utf8;
INSERT INTO user_info (name, age) VALUES ('xys', 20);
INSERT INTO user_info (name, age) VALUES ('a', 21);
INSERT INTO user_info (name, age) VALUES ('b', 23);
INSERT INTO user_info (name, age) VALUES ('c', 50);
INSERT INTO user_info (name, age) VALUES ('d', 15);
INSERT INTO user_info (name, age) VALUES ('e', 20);
INSERT INTO user_info (name, age) VALUES ('f', 21);
INSERT INTO user_info (name, age) VALUES ('g', 23);
INSERT INTO user_info (name, age) VALUES ('h', 50);
INSERT INTO user_info (name, age) VALUES ('i', 15);
vim order_info.sql
CREATE TABLE `order_info` (
`id` BIGINT(20) NOT NULL AUTO_INCREMENT,
`user_id` BIGINT(20) DEFAULT NULL,
`product_name` VARCHAR(50) NOT NULL DEFAULT '',
`productor` VARCHAR(30) DEFAULT NULL,
PRIMARY KEY (`id`),
KEY `user_product_detail_index` (`user_id`, `product_name`, `productor`)
) ENGINE = InnoDB DEFAULT CHARSET = utf8;
INSERT INTO order_info (user_id, product_name, productor) VALUES (1, 'p1', 'WHH');
INSERT INTO order_info (user_id, product_name, productor) VALUES (1, 'p2', 'WL');
INSERT INTO order_info (user_id, product_name, productor) VALUES (1, 'p1', 'DX');
INSERT INTO order_info (user_id, product_name, productor) VALUES (2, 'p1', 'WHH');
INSERT INTO order_info (user_id, product_name, productor) VALUES (2, 'p5', 'WL');
INSERT INTO order_info (user_id, product_name, productor) VALUES (3, 'p3', 'MA');
INSERT INTO order_info (user_id, product_name, productor) VALUES (4, 'p1', 'WHH');
INSERT INTO order_info (user_id, product_name, productor) VALUES (6, 'p1', 'WHH');
INSERT INTO order_info (user_id, product_name, productor) VALUES (9, 'p8', 'TE');
mycli -uroot -p123456
DROP DATABASE IF EXISTS `explain`;
CREATE DATABASE IF NOT EXISTS `explain`;
USE explain;
source ./user_info.sql;
source ./order_info.sql;
explain
EXPLAIN (SELECT * FROM user_info WHERE id = 2);
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+--------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+--------+
| 1 | SIMPLE | user_info | | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.0 | |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+--------+
select_type: SIMPLE表示此查询不包含 UNION 查询或子查询
type: const针对主键或唯一索引的等值查询扫描 最多只返回一行数据
possible_keys: PRIMARY表示可以使用Primary Key优化查询
key: PRIMARY表示使用Primary Key优化查询
key_len: 表示查询优化器使用了索引的字节数为8 (BIGINT8字节)
rows: 表示为了找到所需的行而要读取的行数为1
select_type
EXPLAIN (SELECT * FROM user_info WHERE id IN (1, 2, 3))
UNION
(SELECT * FROM user_info WHERE id IN (3, 4, 5));
+--------+--------------+------------+------------+-------+---------------+---------+---------+--------+--------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+--------+--------------+------------+------------+-------+---------------+---------+---------+--------+--------+----------+-----------------+
| 1 | PRIMARY | user_info | | range | PRIMARY | PRIMARY | 8 | | 3 | 100.0 | Using where |
| 2 | UNION | user_info | | range | PRIMARY | PRIMARY | 8 | | 3 | 100.0 | Using where |
| | UNION RESULT | | | ALL | | | | | | | Using temporary |
+--------+--------------+------------+------------+-------+---------------+---------+---------+--------+--------+----------+-----------------+
select_type: PRIMARY表示此查询是最外层的查询 / UNION表示此查询是 UNION 的第二或随后的查询 / UNION RESULT表示UNION的结果
type: range表示使用索引范围查询 / ALL表示全表扫描
Extra: Using where表示优化器通过索引回表查询数据 / Using temporary表示使用临时表来存储结果集
什么叫索引回表查询数据? 即先找到索引 再通过索引找到数据所在位置
聚簇索引
聚簇索引是指数据存储的方式
聚簇索引的索引和数据存储在一起
一个表只能有一个聚簇索引
聚簇索引不能手动设置 它通常即是主键或非空唯一索引
优点
数据访问速度更快 因为索引和数据存储在一起
- 缺点
插入更新代价更高 因为索引和数据存储在一起
覆盖索引
覆盖索引是指查询的数据列只用从索引中就能够取得 而不必读取数据行
覆盖索引可以进一步提高查询效率
EXPLAIN (SELECT id FROM user_info WHERE id IN (1, 2, 3));
+----+-------------+-----------+------------+-------+---------------+------------+---------+--------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+------------+---------+--------+------+----------+--------------------------+
| 1 | SIMPLE | user_info | | index | PRIMARY | name_index | 152 | | 10 | 30.0 | Using where; Using index |
+----+-------------+-----------+------------+-------+---------------+------------+---------+--------+------+----------+--------------------------+
- Extra: Using index表示直接访问索引就能获取到所需要的数据 不需要通过索引回表查询数据
type
EXPLAIN (SELECT * FROM user_info, order_info WHERE user_info.id = order_info.user_id);
+----+-------------+------------+------------+--------+---------------------------+---------------------------+---------+----------------------------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------------------+---------------------------+---------+----------------------------+------+----------+--------------------------+
| 1 | SIMPLE | order_info | | index | user_product_detail_index | user_product_detail_index | 254 | | 9 | 100.0 | Using where; Using index |
| 1 | SIMPLE | user_info | | eq_ref | PRIMARY | PRIMARY | 8 | explain.order_info.user_id | 1 | 100.0 | |
+----+-------------+------------+------------+--------+---------------------------+---------------------------+---------+----------------------------+------+----------+--------------------------+
- type: index表示全索引扫描 注意ALL是全表扫描 即index则仅仅扫描所有的索引而不扫描数据 / eq_ref表示唯一性索引扫描 即表连接时 前表的每一个结果 都只能匹配到后表的一行结果
唯一索引
- 唯一索引指表中没有两个数据行具有完全相同的键值
主键索引
主键索引是一种特殊的唯一索引
主健不可为空 而唯一索引可以
主键只有一个 而唯一索引可多个
主健可作外健 而唯一索引不可以
主键即是聚簇索引 没有主键的非空唯一索引是聚簇索引
key_len
EXPLAIN (SELECT * FROM order_info WHERE user_id < 3 AND product_name = 'p1' AND productor = 'WHH');
+----+-------------+------------+------------+-------+---------------------------+---------------------------+---------+--------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------------------+---------------------------+---------+--------+------+----------+--------------------------+
| 1 | SIMPLE | order_info | | range | user_product_detail_index | user_product_detail_index | 9 | | 5 | 11.11 | Using where; Using index |
+----+-------------+------------+------------+-------+---------------------------+---------------------------+---------+--------+------+----------+--------------------------+
key: user_product_detail_index表示表中的联合索引
key_len: 9 = 8 (user_id BIGINT) + 1 (product_name NULL)
根据最左前缀匹配原则 查询仅仅使用到了联合索引的user_id字段 因此效率并不高
联合索引
联合索引又称复合索引或多列索引
联合索引遵守最左前缀匹配原则
EXPLAIN (SELECT * FROM order_info WHERE user_id = 1 AND product_name = 'p1');
+----+-------------+------------+------------+------+---------------------------+---------------------------+---------+-------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+---------------------------+---------------------------+---------+-------------+------+----------+-------------+
| 1 | SIMPLE | order_info | | ref | user_product_detail_index | user_product_detail_index | 161 | const,const | 2 | 100.0 | Using index |
+----+-------------+------------+------------+------+---------------------------+---------------------------+---------+-------------+------+----------+-------------+
前缀索引
前缀索引又称压缩索引或前缀压缩索引
前缀索引是一种优化索引大小的解决方案
前缀索引对文本的前几个字符建立索引 这样索引更小因此查询更快
优点
索引更小查询更快
- 缺点
不能在ORDER BY或GROUP BY中使用前缀索引
不能把前缀索引用作覆盖索引
参考
高性能MySQL
MySQL 性能优化神器 Explain 使用分析
MySQL 执行计划(Using where,Using index 和 Using index condition)
前缀索引，一种优化索引大小的解决方案