SQL优化单表案例

数据准备：

-- 创建数据库
mysql> create database db_index_case;
Query OK, 1 row affected (0.00 sec)

-- 查看数据库
mysql> show databases;
+--------------------+
| Database           |
+--------------------+
| information_schema |
| db02               |
| db_index           |
| db_index_case      |
| db_test            |
| mysql              |
| performance_schema |
| test               |
+--------------------+
8 rows in set (0.01 sec)

导入表数据结构：

/*
Navicat MySQL Data Transfer

Source Server         : localhost_3306
Source Server Version : 50203
Source Host           : localhost:3306
Source Database       : db_test

Target Server Type    : MYSQL
Target Server Version : 50203
File Encoding         : 65001

Date: 2017-06-29 09:19:23
*/

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for tb_emp
-- ----------------------------
DROP TABLE IF EXISTS `tb_emp`;
CREATE TABLE `tb_emp` (
  `id` int(11) NOT NULL AUTO_INCREMENT COMMENT '主键',
  `name` varchar(20) NOT NULL COMMENT '姓名',
  `sex` char(1) NOT NULL COMMENT '性别',
  `age` int(11) NOT NULL COMMENT '年龄',
  `jobLen` int(11) NOT NULL COMMENT '工作年限',
  PRIMARY KEY (`id`),
  KEY `idx_emp_sja` (`sex`,`jobLen`,`age`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of tb_emp
-- ----------------------------
INSERT INTO `tb_emp` VALUES ('', '张三', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '李四', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '王二', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '麻子', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小花', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小明', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小四', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小李', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小丹', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小花', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小冷', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小邳', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小影', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小兰', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小翟', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '大花', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '大胖', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '小胖', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '骚欣', '', '', '');
INSERT INTO `tb_emp` VALUES ('', '雷子', '', '', '');

业务要求：查找性别为男性(0:女，1：男)，工作年限大于3年，年龄最小的员工。

mysql> select * from tb_emp where sex = '' and jobLen > 3 order by age limit 1;
+----+--------+-----+-----+--------+
| id | name   | sex | age | jobLen |
+----+--------+-----+-----+--------+
| 11 | 小冷   | 1   |  24 |      5 |
+----+--------+-----+-----+--------+
1 row in set (0.00 sec)

查看执行计划：

mysql> explain select * from tb_emp where sex = '' and jobLen > 3 order by age limit 1;
+----+-------------+--------+------+---------------+------+---------+------+------+-----------------------------+
| id | select_type | table  | type | possible_keys | key  | key_len | ref  | rows | Extra                       |
+----+-------------+--------+------+---------------+------+---------+------+------+-----------------------------+
|  1 | SIMPLE      | tb_emp | ALL  | NULL          | NULL | NULL    | NULL |   20 | Using where; Using filesort |
+----+-------------+--------+------+---------------+------+---------+------+------+-----------------------------+
1 row in set (0.00 sec)

结论：type为ALL，即最坏的情况；key为null，没有用到索引；Extra中含有Using filesort，不是按照表内的索引进行排序。

优化开始：

•  创建索引：

• mysql> create index idx_emp_sex_jobLen_age on tb_emp(sex, jobLen, age);
  Query OK, 0 rows affected (1.59 sec)
  Records: 0  Duplicates: 0  Warnings: 0
  
  -- 查看索引
  mysql> show index from tb_emp;
  +--------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  | Table  | Non_unique | Key_name               | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
  +--------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  | tb_emp |          0 | PRIMARY                |            1 | id          | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
  | tb_emp |          1 | idx_emp_sex_jobLen_age |            1 | sex         | A         |           4 |     NULL | NULL   |      | BTREE      |         |               |
  | tb_emp |          1 | idx_emp_sex_jobLen_age |            2 | jobLen      | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
  | tb_emp |          1 | idx_emp_sex_jobLen_age |            3 | age         | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
  +--------+------------+------------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  4 rows in set (0.00 sec)

•  查看执行计划：

• mysql> explain select * from tb_emp where sex = '' and jobLen > 3 order by age limit 1;
  +----+-------------+--------+------+------------------------+------------------------+---------+-------+------+----------------------------------------------------+
  | id | select_type | table  | type | possible_keys          | key                    | key_len | ref   | rows | Extra                                              |
  +----+-------------+--------+------+------------------------+------------------------+---------+-------+------+----------------------------------------------------+
  |  1 | SIMPLE      | tb_emp | ref  | idx_emp_sex_jobLen_age | idx_emp_sex_jobLen_age | 3       | const |    8 | Using index condition; Using where; Using filesort |
  +----+-------------+--------+------+------------------------+------------------------+---------+-------+------+----------------------------------------------------+
  1 row in set (0.00 sec)
  
  -- 将 jobLen > 3 改变成 jobLen = 3 比较两个查询执行计划
  mysql> explain select * from tb_emp where sex = '' and jobLen = 3 order by age limit 1;
  +----+-------------+--------+------+------------------------+------------------------+---------+-------------+------+------------------------------------+
  | id | select_type | table  | type | possible_keys          | key                    | key_len | ref         | rows | Extra                              |
  +----+-------------+--------+------+------------------------+------------------------+---------+-------------+------+------------------------------------+
  |  1 | SIMPLE      | tb_emp | ref  | idx_emp_sex_jobLen_age | idx_emp_sex_jobLen_age | 7       | const,const |    5 | Using index condition; Using where |
  +----+-------------+--------+------+------------------------+------------------------+---------+-------------+------+------------------------------------+
  1 row in set (0.00 sec)

 　　结果：Extra 中还是存在 Using filesort。

　　 分析：比对改变jobLen > 3 条件前后的查询执行计划，可以得出是因为 jobLen > 3 导致索引失效。因为按照 BTree 索引的工作原理，首先排序 sex，若是遇到相同的 sex 则再去排序 jobLen，若是遇到相同的 jobLen 则再去排序 age。按照联合索引顺序向下去排序，可是 where 后查询条件 jobLen > 3  是一个范围值( range )，导致索引失效，MySQL 无法对后面的 age 进行检索，即 range 类型查询字段后面的索引失效。

• 删除索引，重新创建：

• -- 删除索引
  mysql> drop index idx_emp_sex_jobLen_age on tb_emp;
  Query OK, 0 rows affected (0.02 sec)
  Records: 0  Duplicates: 0  Warnings: 0
  
  -- 创建索引
  mysql> create index idx_emp_sex_age on tb_emp(sex, age);
  Query OK, 0 rows affected (1.60 sec)
  Records: 0  Duplicates: 0  Warnings: 0
  
  -- 查看索引
  mysql> show index from tb_emp;
  +--------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  | Table  | Non_unique | Key_name        | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
  +--------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  | tb_emp |          0 | PRIMARY         |            1 | id          | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
  | tb_emp |          1 | idx_emp_sex_age |            1 | sex         | A         |           4 |     NULL | NULL   |      | BTREE      |         |               |
  | tb_emp |          1 | idx_emp_sex_age |            2 | age         | A         |          20 |     NULL | NULL   |      | BTREE      |         |               |
  +--------+------------+-----------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
  3 rows in set (0.00 sec)

• 查询执行计划：

• mysql> explain select * from tb_emp where sex = '' and jobLen > 3 order by age limit 1;
  +----+-------------+--------+------+-----------------+-----------------+---------+-------+------+------------------------------------+
  | id | select_type | table  | type | possible_keys   | key             | key_len | ref   | rows | Extra                              |
  +----+-------------+--------+------+-----------------+-----------------+---------+-------+------+------------------------------------+
  |  1 | SIMPLE      | tb_emp | ref  | idx_emp_sex_age | idx_emp_sex_age | 3       | const |   13 | Using index condition; Using where |
  +----+-------------+--------+------+-----------------+-----------------+---------+-------+------+------------------------------------+
  1 row in set (0.00 sec)

 　　结果：Extra 中的 Using filesort 优化之后消失了，完成任务。 

注意：

mysql> explain select * from tb_emp where sex = 1 and jobLen > 3 order by age limit 1;
+----+-------------+--------+------+-----------------+------+---------+------+------+-----------------------------+
| id | select_type | table  | type | possible_keys   | key  | key_len | ref  | rows | Extra                       |
+----+-------------+--------+------+-----------------+------+---------+------+------+-----------------------------+
|  1 | SIMPLE      | tb_emp | ALL  | idx_emp_sex_age | NULL | NULL    | NULL |   20 | Using where; Using filesort |
+----+-------------+--------+------+-----------------+------+---------+------+------+-----------------------------+
1 row in set (0.02 sec)

结果： type 为 ALL，key 为 NULL，rows 为 20，Extra 中包含 Using filesort。

查看表结构：

-- 查看表结构
mysql> desc tb_emp;
+--------+-------------+------+-----+---------+----------------+
| Field  | Type        | Null | Key | Default | Extra          |
+--------+-------------+------+-----+---------+----------------+
| id     | int(11)     | NO   | PRI | NULL    | auto_increment |
| name   | varchar(20) | NO   |     | NULL    |                |
| sex    | char(1)     | NO   | MUL | NULL    |                |
| age    | int(11)     | NO   |     | NULL    |                |
| jobLen | int(11)     | NO   |     | NULL    |                |
+--------+-------------+------+-----+---------+----------------+
5 rows in set (0.02 sec)

分析：因为 sex 为 char 类型的字段， sql 查询条件中 sex = 1 是数字类型。因为类型不匹配，MySQL 会自动解析字符类型，会浪费性能。