一,语法
在企业的应用场景中,为了知道优化SQL语句的执行,需要查看SQL语句的具体执行过程,以加快SQL语句的执行效率。
官网地址: https://dev.mysql.com/doc/refman/5.5/en/explain-output.html
1,explain
可以使用explain+SQL语句来模拟优化器执行SQL查询语句,从而知道mysql是如何处理sql语句的。
EXPLAIN 是解释 SQL 语句的执行计划,即显示该 SQL 语句怎么执行的。
EXPLAIN语法如下:
{
EXPLAIN | DESCRIBE | DESC}
tbl_name [col_name | wild]
{
EXPLAIN | DESCRIBE | DESC}
[explain_type]
{explainable_stmt | FOR CONNECTION connection_id}
explain_type: {
FORMAT = format_name
}
format_name: {
TRADITIONAL
| JSON
| TREE
}
explainable_stmt: {
SELECT statement
| DELETE statement
| INSERT statement
| REPLACE statement
| UPDATE statement
}
DESCRIBE和EXPLAIN语句是同义词。但DESCRIBE关键字更常用来获取关于表的信息结构,而EXPLAIN用于获取查询执行计划。
mysql> explain select * from dept;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | dept | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
注意:EXPLAIN 查看的是执行计划,做 SQL 解析,不会去真的执行,且到 MySQL 5.7 以后子查询也不会去执行
2,format 参数
FORMAT 参数可以格式化 EXPLAIN 的输出,比如格式化成 JSON 或 TREE 格式(FORMAT = JSON | TREE)。当使用 JSON 个格式的时候,会显示一些其他有用的信息,比如 SQL 的执行成本。
mysql> explain format=json select * from dept;
| {
"query_block": {
"select_id": 1,
"cost_info": {
"query_cost": "0.65"
},
"table": {
"table_name": "dept",
"access_type": "ALL",
"rows_examined_per_scan": 4,
"rows_produced_per_join": 4,
"filtered": "100.00",
"cost_info": {
"read_cost": "0.25",
"eval_cost": "0.40",
"prefix_cost": "0.65",
"data_read_per_join": "384"
},
"used_columns": [
"DEPTNO",
"DNAME",
"LOC"
]
}
}
} |
1 row in set, 1 warning (0.00 sec)
二,执行计划包含的信息
mysql> explain select * from dept;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | dept | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
列 | 含义 |
---|---|
id | 执行计划的 id 标志 |
select_type | SELECT 的类型 |
table | 输出记录的表 |
partitions | 匹配的分区 |
type | JOIN 的类型 |
possible_keys | 优化器可能选择的索引 |
key | 优化器实际选择的索引 |
key_len | 使用索引的字节长度 |
ref | 进行比较的索引列 |
rows | 优化器预估的记录数量 |
filtered | 根据条件过滤得到的记录的百分比 |
extra | 额外的显示选项 |
准备测试数据
/*
Navicat MySQL Data Transfer
Source Server : mybatis
Source Server Version : 50722
Source Host : localhost:3306
Source Database : demp
Target Server Type : MYSQL
Target Server Version : 50722
File Encoding : 65001
Date: 2021-02-02 09:43:02
*/
SET FOREIGN_KEY_CHECKS=0;
-- ----------------------------
-- Table structure for dept
-- ----------------------------
DROP TABLE IF EXISTS `dept`;
CREATE TABLE `dept` (
`DEPTNO` int(4) NOT NULL,
`DNAME` varchar(14) DEFAULT NULL,
`LOC` varchar(13) DEFAULT NULL,
PRIMARY KEY (`DEPTNO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of dept
-- ----------------------------
INSERT INTO `dept` VALUES ('10', 'ACCOUNTING', 'NEW YORK');
INSERT INTO `dept` VALUES ('20', 'RESEARCH', 'DALLAS');
INSERT INTO `dept` VALUES ('30', 'SALES', 'CHICAGO');
INSERT INTO `dept` VALUES ('40', 'OPERATIONS', 'BOSTON');
-- ----------------------------
-- Table structure for emp
-- ----------------------------
DROP TABLE IF EXISTS `emp`;
CREATE TABLE `emp` (
`EMPNO` int(4) NOT NULL,
`ENAME` varchar(10) DEFAULT NULL,
`JOB` varchar(9) DEFAULT NULL,
`MGR` int(4) DEFAULT NULL,
`HIREDATE` date DEFAULT NULL,
`SAL` double(7,2) DEFAULT NULL,
`COMM` double(7,2) DEFAULT NULL,
`DEPTNO` int(4) DEFAULT NULL,
PRIMARY KEY (`EMPNO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of emp
-- ----------------------------
INSERT INTO `emp` VALUES ('7369', 'SMITH', 'CLERK', '7902', '1980-12-17', '800.00', null, '20');
INSERT INTO `emp` VALUES ('7499', 'ALLEN', 'SALESMAN', '7698', '1981-02-20', '1600.00', '300.00', '30');
INSERT INTO `emp` VALUES ('7521', 'WARD', 'SALESMAN', '7698', '1981-02-22', '1250.00', '500.00', '30');
INSERT INTO `emp` VALUES ('7566', 'JONES', 'MANAGER', '7839', '1981-02-02', '2975.00', null, '20');
INSERT INTO `emp` VALUES ('7654', 'MARTIN', 'SALESMAN', '7698', '1981-09-28', '1250.00', '1400.00', '30');
INSERT INTO `emp` VALUES ('7698', 'BLAKE', 'MANAGER', '7839', '1981-01-05', '2850.00', null, '30');
INSERT INTO `emp` VALUES ('7782', 'CLARK', 'MANAGER', '7839', '1981-09-06', '2450.00', null, '10');
INSERT INTO `emp` VALUES ('7839', 'KING', 'PRESIDENT', null, '1981-11-17', '5000.00', null, '10');
INSERT INTO `emp` VALUES ('7844', 'TURNER', 'SALESMAN', '7698', '1981-09-08', '1500.00', '0.00', '30');
INSERT INTO `emp` VALUES ('7900', 'JAMES', 'CLERK', '7698', '1981-12-03', '950.00', null, '30');
INSERT INTO `emp` VALUES ('7902', 'FORD', 'ANALYST', '7566', '1981-12-03', '3000.00', null, '20');
INSERT INTO `emp` VALUES ('7934', 'MILLER', 'CLERK', '7782', '1982-01-23', '1300.00', null, '10');
-- ----------------------------
-- Table structure for salgrade
-- ----------------------------
DROP TABLE IF EXISTS `salgrade`;
CREATE TABLE `salgrade` (
`GRADE` int(11) NOT NULL,
`LOSAL` double DEFAULT NULL,
`HISAL` double DEFAULT NULL,
PRIMARY KEY (`GRADE`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
-- ----------------------------
-- Records of salgrade
-- ----------------------------
INSERT INTO `salgrade` VALUES ('1', '700', '1200');
INSERT INTO `salgrade` VALUES ('2', '1201', '1400');
INSERT INTO `salgrade` VALUES ('3', '1401', '2000');
INSERT INTO `salgrade` VALUES ('4', '2001', '3000');
INSERT INTO `salgrade` VALUES ('5', '3001', '9999');
1,id
select 查询的序列号,标识执行的顺序
- id 相同,执行顺序由上至下
- id 不同,如果是子查询,id 的序号会递增,id 值越大优先级越高,越先被执行
2,select_type
主要用来分辨查询的类型,是普通查询还是联合查询还是子查询
-
SIMPLE:简单的 select 查询,查询中不包含子查询或者 union
-
PRIMARY:查询中包含子部分,最外层查询则被标记为 primary
-
UNION:表示 union 中的第二个或后面的 select 语句
-
DEPENDENT UNION:union 中的第二个或后面的 select 语句,依赖于外面的查询
-
UNION RESULT:union 的结果
-
SUBQUERY:子查询中的第一个 select
-
DEPENDENT SUBQUERY:子查询中的第一个 select,依赖于外面的查询
-
DERIVED:派生表的 select(from 子句的子查询)
-
MATERIALIZED:物化子查询
-
- 产生中间临时表(实体)
- 临时表自动创建索引并和其他表进行关联,提高性能
- 和子查询的区别是,优化器将可以进行 MATERIALIZED 的语句自动改写成 join,并自动创建索引
-
UNCACHEABLE SUBQUERY:不会被缓存的并且对于外部查询的每行都要重新计算的子查询
-
UNCACHEABLE UNION:属于不能被缓存的 union 中的第二个或后面的 select 语句
2.1,simple
简单的 select 查询,查询中不包含子查询或者 union
mysql> explain select * from emp;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
2.2,primary
查询中包含子部分,最外层查询则被标记为 primary
mysql> explain select * from emp where deptno = 10 union select * from emp where sal >2000;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 10.00 | Using where |
| 2 | UNION | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 33.33 | Using where |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
2.3,union
若第二个select出现在union之后,则被标记为union
mysql> explain select * from emp where deptno = 10 union select * from emp where sal >2000;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 10.00 | Using where |
| 2 | UNION | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 33.33 | Using where |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
2.4,dependent union
跟union类似,此处的depentent表示union或union all联合而成的结果会受外部表影响
mysql> explain select * from emp e where e.empno in ( select empno from emp where deptno = 10 union select empno from emp where sal >2000);
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | emp | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 10.00 | Using where |
| 3 | DEPENDENT UNION | emp | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 33.33 | Using where |
| NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.01 sec)
2.5,union result
union 的结果
mysql> explain select * from emp where deptno = 10 union select * from emp where sal >2000;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 10.00 | Using where |
| 2 | UNION | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 33.33 | Using where |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.01 sec)
2.6,subquery
子查询中的第一个 select
mysql> explain select * from emp where sal > (select avg(sal) from emp) ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 33.33 | Using where |
| 2 | SUBQUERY | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
2.7,dependent subquery
子查询中的第一个 select,依赖于外面的查询
mysql> explain select * from emp e where e.empno in ( select empno from emp where deptno = 10 union select empno from emp where sal >2000);
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | emp | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 10.00 | Using where |
| 3 | DEPENDENT UNION | emp | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 33.33 | Using where |
| NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.01 sec)
2.8,derived
from子句中出现的子查询,也叫做派生类
explain select staname,ename supname from (select ename staname,mgr from emp) t join emp on t.mgr=emp.empno ;
2.9,uncacheable subquery
表示使用子查询的结果不能被缓存
mysql> explain select * from emp where empno = (select empno from emp where deptno=@@sort_buffer_size);
+----+----------------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+----------------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | no matching row in const table |
| 2 | UNCACHEABLE SUBQUERY | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 10.00 | Using where |
+----+----------------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------+
2 rows in set, 1 warning (0.00 sec)
2.10,uncacheable union
表示union的查询结果不能被缓存
3,table
查询涉及到的表。
- 通常就是用户操作的用户表
- <unionM,N>:由 ID 等于 M,N 的语句 union 得到的结果表
- < derivedN>:派生表,由 ID 等于 N 的语句查询得到的结果表
- < subqueryN>:由子查询物化产生的表,由 ID 等于 N 的语句查询得到的结果表
4,type
type显示的是访问类型,访问类型表示我是以何种方式去访问我们的数据,最容易想的是全表扫描,直接暴力的遍历一张表去寻找需要的数据,效率非常低下,访问的类型有很多,效率从最好到最坏依次是:
system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
一般情况下,得保证查询至少达到range级别,最好能达到ref
- all:全表扫描,一般情况下出现这样的sql语句而且数据量比较大的话那么就需要进行优化。
- index:全索引扫描这个比all的效率要好,主要有两种情况,一种是当前的查询时覆盖索引,即我们需要的数据在索引中就可以索取,或者是使用了索引进行排序,这样就避免数据的重排序
- range:表示利用索引查询的时候限制了范围,在指定范围内进行查询,这样避免了index的全索引扫描,适用的操作符: =, <>, >, >=, <, <=, IS NULL, BETWEEN, LIKE, or IN()
- index_subquery:利用索引来关联子查询,不再扫描全表
- unique_subquery:该连接类型类似与index_subquery,使用的是唯一索引
- index_merge:在查询过程中需要多个索引组合使用
- ref_or_null:对于某个字段即需要关联条件,也需要null值的情况下,查询优化器会选择这种访问方式
- ref:使用了非唯一性索引进行数据的查找
- const:这个表至多有一个匹配行
- system:表只有一行记录(等于系统表),这是const类型的特例,平时不会出现
4.1,all
全表扫描,一般情况下出现这样的sql语句而且数据量比较大的话那么就需要进行优化。
mysql> explain select * from emp;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
4.2,index
全索引扫描这个比all的效率要好,主要有两种情况,一种是当前的查询时覆盖索引,即我们需要的数据在索引中就可以索取,或者是使用了索引进行排序,这样就避免数据的重排序
mysql> explain select empno from emp;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | emp | NULL | index | NULL | PRIMARY | 4 | NULL | 12 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
4.3,range
表示利用索引查询的时候限制了范围,在指定范围内进行查询,这样避免了index的全索引扫描,适用的操作符: =, <>, >, >=, <, <=, IS NULL, BETWEEN, LIKE, or IN()
mysql> explain select * from emp where empno between 7000 and 7500;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | emp | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 2 | 100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
4.4,index_subquery
利用索引来关联子查询,不再扫描全表
explain select * from emp where emp.job in (select job from t_job);
4.5,unique_subquery
该连接类型类似与index_subquery,使用的是唯一索引
explain select * from emp e where e.deptno in (select distinct deptno from dept);
4.6,index_merge
在查询过程中需要多个索引组合使用
4.7,ref_or_null
对于某个字段即需要关联条件,也需要null值的情况下,查询优化器会选择这种访问方式
explain select * from emp e where e.mgr is null or e.mgr=7369;
4.8,ref
使用了非唯一性索引进行数据的查找
mysql> explain select * from emp e,dept d where e.deptno =d.deptno;
+----+-------------+-------+------------+------+---------------+-------+---------+---------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------+------+----------+-------+
| 1 | SIMPLE | d | NULL | ALL | PRIMARY | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 1 | SIMPLE | e | NULL | ref | idx_3 | idx_3 | 5 | bobo.d.DEPTNO | 4 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+-------+---------+---------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
4.9,eq_ref
使用唯一性索引进行数据查找
explain select * from emp,emp2 where emp.empno = emp2.empno;
4.10,const
这个表至多有一个匹配行
mysql> explain select * from emp where empno = 7369;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | emp | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
4.11,system
表只有一行记录(等于系统表),这是const类型的特例,平时不会出现
5,possible_keys
显示可能应用在这张表中的索引,一个或多个,查询涉及到的字段上若存在索引,则该索引将被列出,但不一定被查询实际使用
mysql> explain select * from emp,dept where emp.deptno = dept.deptno and emp.deptno = 10;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | dept | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | emp | NULL | ref | idx_3 | idx_3 | 5 | const | 3 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
6,key
实际使用的索引,如果为null,则没有使用索引,查询中若使用了覆盖索引,则该索引和查询的select字段重叠。
mysql> explain select * from emp,dept where emp.deptno = dept.deptno and emp.deptno = 10;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | dept | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | emp | NULL | ref | idx_3 | idx_3 | 5 | const | 3 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
7,key_len
表示索引中使用的字节数,可以通过key_len计算查询中使用的索引长度,在不损失精度的情况下长度越短越好。
mysql> explain select * from emp,dept where emp.deptno = dept.deptno and emp.deptno = 10;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | dept | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | emp | NULL | ref | idx_3 | idx_3 | 5 | const | 3 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
8,ref
显示索引的哪一列被使用了,如果可能的话,是一个常数
mysql> explain select * from emp,dept where emp.deptno = dept.deptno and emp.deptno = 10;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | dept | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | emp | NULL | ref | idx_3 | idx_3 | 5 | const | 3 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
9,rows
根据表的统计信息及索引使用情况,大致估算出找出所需记录需要读取的行数,此参数很重要,直接反应的sql找了多少数据,在完成目的的情况下越少越好
mysql> select * from emp;
+-------+--------+-----------+------+------------+---------+---------+--------+
| EMPNO | ENAME | JOB | MGR | HIREDATE | SAL | COMM | DEPTNO |
+-------+--------+-----------+------+------------+---------+---------+--------+
| 7369 | SMITH | CLERK | 7902 | 1980-12-17 | 800.00 | NULL | 20 |
| 7499 | ALLEN | SALESMAN | 7698 | 1981-02-20 | 1600.00 | 300.00 | 30 |
| 7521 | WARD | SALESMAN | 7698 | 1981-02-22 | 1250.00 | 500.00 | 30 |
| 7566 | JONES | MANAGER | 7839 | 1981-02-02 | 2975.00 | NULL | 20 |
| 7654 | MARTIN | SALESMAN | 7698 | 1981-09-28 | 1250.00 | 1400.00 | 30 |
| 7698 | BLAKE | MANAGER | 7839 | 1981-01-05 | 2850.00 | NULL | 30 |
| 7782 | CLARK | MANAGER | 7839 | 1981-09-06 | 2450.00 | NULL | 10 |
| 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000.00 | NULL | 10 |
| 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500.00 | 0.00 | 30 |
| 7900 | JAMES | CLERK | 7698 | 1981-12-03 | 950.00 | NULL | 30 |
| 7902 | FORD | ANALYST | 7566 | 1981-12-03 | 3000.00 | NULL | 20 |
| 7934 | MILLER | CLERK | 7782 | 1982-01-23 | 1300.00 | NULL | 10 |
+-------+--------+-----------+------+------------+---------+---------+--------+
12 rows in set (0.00 sec)
mysql> explain select * from emp;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | emp | NULL | ALL | NULL | NULL | NULL | NULL | 12 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
10,extra
包含额外的信息。
Using filesort:说明mysql无法利用索引进行排序,只能利用排序算法进行排序,会消耗额外的位置
Using index:表示 SQL 操作中使用了覆盖索引(Covering Index),避免了访问表的数据行,效率高。
Using index condition:表示 SQL 操作命中了索引,但不是所有的列数据都在索引树上,还需要访问实际的行记录。
Using index for group by:优化器只需要使用索引就能处理 group by 或 distinct 语句。
Using join buffer (Block Nested Loop):表示 SQL 操作使用了关联查询或者子查询,且需要进行嵌套循环计算。
Using MRR:优化器使用 MRR 优化
Using temporary:使用临时表保存中间结果,也就是说 MySQL 在对查询结果排序时使用了临时表,常见于order by 或 group by。
Using where:表示 SQL 操作使用了 where 过滤条件。
Select tables optimized away:基于索引优化 MIN/MAX 操作或者 MyISAM 存储引擎优化 COUNT(*) 操作,不必等到执行阶段再进行计算,查询执行计划生成的阶段即可完成优化。
准备数据
mysql> CREATE TABLE `user` (
-> `id` int(11) NOT NULL,
-> `name` varchar(20) DEFAULT NULL,
-> `sex` varchar(5) DEFAULT NULL,
-> PRIMARY KEY (`id`),
-> KEY `name` (`name`)
-> );
Query OK, 0 rows affected, 1 warning (0.44 sec)
mysql> insert into user values(1, 'bobo','no');
Query OK, 1 row affected (0.07 sec)
mysql> insert into user values(2, 'zhangsan','no');
Query OK, 1 row affected (0.05 sec)
mysql> insert into user values(3, 'lisi', 'yes');
Query OK, 1 row affected (0.05 sec)
mysql> insert into user values(4, 'lisi', 'no');
Query OK, 1 row affected (0.03 sec)
mysql> select * from user;
+----+----------+------+
| id | name | sex |
+----+----------+------+
| 1 | bobo | no |
| 2 | zhangsan | no |
| 3 | lisi | yes |
| 4 | lisi | no |
+----+----------+------+
4 rows in set (0.00 sec)
用户表:id 主键索引,name 普通索引(非唯一),sex 无索引。
四行记录:其中 name 普通索引存在重复记录 lisi。
10.1,Using filesort
mysql无法利用索引进行排序,只能利用排序算法进行排序,会消耗额外的位置。
mysql> explain select * from user order by sex;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | user | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
这类 SQL 语句性能极差,需要进行优化。
典型的,在一个没有建立索引的列上进行了 order by,就会触发 filesort,常见的优化方案是,在 order by 的列上添加索引,避免每次查询都全量排序。
10.2,Using temporary
Extra 为 Using temporary 说明,需要建立临时表(temporary table)来暂存中间结果。
explain select * from user group by name order by sex;
这类 SQL 语句性能较低,往往也需要进行优化。
典型的 group by 和 order by 同时存在,且作用于不同的字段时,就会建立临时表,以便计算出最终的结果集。
临时表存在两种引擎,一种是 Memory 引擎,一种是 MyISAM 引擎,如果返回的数据在 16M 以内(默认),且没有大字段的情况下,使用 Memory 引擎,否则使用 MyISAM 引擎。
10.3,Using index
这个表示当前的查询时覆盖索引的,直接从索引中读取数据,而不用访问数据表。如果同时出现using where 表名索引被用来执行索引键值的查找,如果没有,表面索引被用来读取数据,而不是真的查找
mysql> explain select id from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | index | NULL | name | 83 | NULL | 4 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
这类 SQL 语句往往性能较好。
10.4,Using index condition
确实命中了索引,但不是所有的列数据都在索引树上,还需要访问实际的行记录。
explain select * from user where name='bobo';
这类 SQL 语句性能也较高,但不如 Using index。
10.5,Using where
查询的结果集使用了 where 过滤条件
mysql> explain select * from user where sex='no';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 25.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
10.6,Select tables optimized away
mysql> explain select max(id) from user;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)
比如上面的语句查询 id 的最大值,因为 id 是主键索引,根据 B+Tree 的结构,天然就是有序存放的,所以不需要等到执行阶段再进行计算,查询执行计划生成的阶段即可完成优化。
10.7,Using join buffer (Block Nested Loop)
表示 SQL 操作使用了关联查询或者子查询,且需要进行嵌套循环计算。
explain select * from user where id in (select id from user where sex='no');
这类 SQL 语句性能往往也较低,需要进行优化。
典型的两个关联表 join,关联字段均未建立索引,就会出现这种情况。常见的优化方案是,在关联字段上添加索引,避免每次嵌套循环计算。