使用explain的12个字段说明
0. 前情提要: 用到的几个表说明:
0.1. goods表和goods2两个表结构和数据相同(复制的表)-test库
mysql> show create table goods;
CREATE TABLE `goods` (
`id` bigint(20) unsigned NOT NULL AUTO_INCREMENT,
`name` varchar(10) DEFAULT NULL,
`price` double DEFAULT NULL,
UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=150001 DEFAULT CHARSET=utf8
mysql> show create table goods2;
CREATE TABLE `goods2` (
`id` bigint(20) unsigned NOT NULL DEFAULT '0',
`name` varchar(10) CHARACTER SET utf8 DEFAULT NULL,
`price` double DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
mysql> select count(*) from goods;
+----------+
| count(*) |
+----------+
| 150000 |
+----------+
1 row in set (0.02 sec)
mysql> select count(*) from goods2;
+----------+
| count(*) |
+----------+
| 150000 |
+----------+
1 row in set (0.05 sec)
0.2. t2表是goods表中的前10条-test库
mysql> select * from t2;
+----+---------+--------+
| id | name | price |
+----+---------+--------+
| 1 | 商品1 | 200.17 |
| 2 | 商品2 | 200.87 |
| 3 | 商品3 | 200.81 |
| 4 | 商品4 | 200.43 |
| 5 | 商品5 | 200.73 |
| 6 | 商品6 | 200.36 |
| 7 | 商品7 | 200.61 |
| 8 | 商品8 | 200.98 |
| 9 | 商品9 | 200.06 |
| 10 | 商品0 | 200.38 |
+----+---------+--------+
10 rows in set (0.00 sec)
mysql> show create table t2;
CREATE TABLE `t2` (
`id` bigint(20) unsigned NOT NULL DEFAULT '0',
`name` varchar(10) CHARACTER SET utf8 DEFAULT NULL,
`price` double DEFAULT NULL,
UNIQUE KEY `id` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb40.3. sakila库的film表:
mysql> show create table film;
CREATE TABLE `film` (
`film_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`title` varchar(128) NOT NULL,
`description` text,
`release_year` year(4) DEFAULT NULL,
`language_id` tinyint(3) unsigned NOT NULL,
`original_language_id` tinyint(3) unsigned DEFAULT NULL,
`rental_duration` tinyint(3) unsigned NOT NULL DEFAULT '3',
`rental_rate` decimal(4,2) NOT NULL DEFAULT '4.99',
`length` smallint(5) unsigned DEFAULT NULL,
`replacement_cost` decimal(5,2) NOT NULL DEFAULT '19.99',
`rating` enum('G','PG','PG-13','R','NC-17') DEFAULT 'G',
`special_features` set('Trailers','Commentaries','Deleted Scenes','Behind the Scenes') DEFAULT NULL,
`last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`film_id`),
KEY `idx_title` (`title`),
KEY `idx_fk_language_id` (`language_id`),
KEY `idx_fk_original_language_id` (`original_language_id`),
CONSTRAINT `fk_film_language` FOREIGN KEY (`language_id`) REFERENCES `language` (`language_id`) ON UPDATE CASCADE,
CONSTRAINT `fk_film_language_original` FOREIGN KEY (`original_language_id`) REFERENCES `language` (`language_id`) ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=1001 DEFAULT CHARSET=utf8mb4;
1 row in set (0.00 sec)
mysql> select count(*) from film;
+----------+
| count(*) |
+----------+
| 1000 |
+----------+
1 row in set (0.00 sec)0.4. sakila库的film_category表
mysql> show create table film_actor;
CREATE TABLE `film_actor` (
`actor_id` smallint(5) unsigned NOT NULL,
`film_id` smallint(5) unsigned NOT NULL,
`last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`actor_id`,`film_id`),
KEY `idx_fk_film_id` (`film_id`),
CONSTRAINT `fk_film_actor_actor` FOREIGN KEY (`actor_id`) REFERENCES `actor` (`actor_id`) ON UPDATE CASCADE,
CONSTRAINT `fk_film_actor_film` FOREIGN KEY (`film_id`) REFERENCES `film` (`film_id`) ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
mysql> select count(*)from film_actor;
+----------+
| count(*) |
+----------+
| 5462 |
+----------+
1 row in set (0.00 sec)0.5. sakila库的film_category表
mysql> show create table film_category;
CREATE TABLE `film_category` (
`film_id` smallint(5) unsigned NOT NULL,
`category_id` tinyint(3) unsigned NOT NULL,
`last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`film_id`,`category_id`),
KEY `fk_film_category_category` (`category_id`),
CONSTRAINT `fk_film_category_category` FOREIGN KEY (`category_id`) REFERENCES `category` (`category_id`) ON UPDATE CASCADE,
CONSTRAINT `fk_film_category_film` FOREIGN KEY (`film_id`) REFERENCES `film` (`film_id`) ON UPDATE CASCADE
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4
mysql> select count(*) from film_category;
+----------+
| count(*) |
+----------+
| 1000 |
+----------+
1 row in set (0.00 sec)0.6. sakila库的actor表
mysql> show create table actor;
CREATE TABLE `actor` (
`actor_id` smallint(5) unsigned NOT NULL AUTO_INCREMENT,
`first_name` varchar(45) NOT NULL,
`last_name` varchar(45) NOT NULL,
`last_update` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
PRIMARY KEY (`actor_id`),
KEY `idx_actor_last_name` (`last_name`)
) ENGINE=InnoDB AUTO_INCREMENT=201 DEFAULT CHARSET=utf8mb4
mysql> select count(*) from actor;
+----------+
| count(*) |
+----------+
| 200 |
+----------+
1 row in set (0.00 sec)explain一共有12个字段, 下面具体介绍:
1. id=步骤编号: 表示当前执行计划的第几步
执行计划可能分很多步, 每一行表示一个步骤, id=1表示是第1步;如果编号id相同, 执行顺序就是从上到下;
id越大越先执行
id:1
1.1 id越大的越先执行
mysql> explain select (select 1 from actor limit 1) from film;
+----+-------------+-------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
| 1 | PRIMARY | film | NULL | index | NULL | idx_fk_language_id | 1 | NULL | 1000 | 100.00 | Using index |
| 2 | SUBQUERY | actor | NULL | index | NULL | idx_actor_last_name | 182 | NULL | 200 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)下面的 2.row id=2 最大, 第二步就先执行;
第二步:
- 是子查询;
- 连接类型是 index;
- 用到的索引名是 idx_actor_last_name;
- 扫描的索引长度182;
- 可能会检测的行数200;
- 扩展信息: 使用了索引;
1.2 id相同的前面的先执行
2. select_type=查询类型: 表示是简单查询还是复杂查询
select_type=primary 复杂查询=包含union查询或者包含子查询select_type=simple 简单查询=不包含union, 也不包含子查询;
select_type=union
select_type=union result
select_type=dependent union
select_type=subquery
select_type=derived
select_type=materializaton
2.1. select_type=simple: 简单查询
表示不需要 union 操作, 或者不包含子查询有连接查询时, 外层的查询为simple, 且只有一个;
2.2. select_type=primary: 复杂查询(有union操作或有子查询)
复杂查询: 有union操作; 或者有子查询;
2.2.1 union查询实例:
mysql> explain select film_id from film union all select film_id from film_actor;
+----+-------------+------------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
| 1 | PRIMARY | film | NULL | index | NULL | idx_fk_language_id | 1 | NULL | 1000 | 100.00 | Using index |
| 2 | UNION | film_actor | NULL | index | NULL | idx_fk_film_id | 2 | NULL | 5462 | 100.00 | Using index |
+----+-------------+------------+------------+-------+---------------+--------------------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)有union查询的步骤, 位于最外层的查询的select_type即为 primary, 且只有一个;
2.3 select_type=union 连接查询
union查询=连接查询, 连接的是2个select查询:
第一个查询是: derived派生的表; 除了一个之外, 第二个以后的表select_type都是union;
2.4 select_type=union result 此类型id为空(mysql5.7.30以后都没有了)
2.5 select_type=dependent union
此查询跟union 一样, 出现在union或union all 语句中, 但是这个查询要受外部查询的影响;
mysql> explain select * from film_category where film_id in (select film_id from film union all select film_id from film_actor);
+----+--------------------+---------------+------------+--------+----------------+----------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+---------------+------------+--------+----------------+----------------+---------+------+------+----------+-------------+
| 1 | PRIMARY | film_category | NULL | ALL | NULL | NULL | NULL | NULL | 1000 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | film | NULL | eq_ref | PRIMARY | PRIMARY | 2 | func | 1 | 100.00 | Using index |
| 3 | DEPENDENT UNION | film_actor | NULL | ref | idx_fk_film_id | idx_fk_film_id | 2 | func | 5 | 100.00 | Using index |
+----+--------------------+---------------+------------+--------+----------------+----------------+---------+------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
2.6 select_type=subquery 子查询
除了from子句中包含的子查询外, 其他地方出现的子查询都可能是 subquery;
mysql> explain select (select 1 from actor limit 1) from film;
+----+-------------+-------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
| 1 | PRIMARY | film | NULL | index | NULL | idx_fk_language_id | 1 | NULL | 1000 | 100.00 | Using index |
| 2 | SUBQUERY | actor | NULL | index | NULL | idx_actor_last_name | 182 | NULL | 200 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+---------------------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)2.7 select_type=derived from子句中出现的子查询也叫派生表
但是在5.7以后的版本中没有 derived这个类型了, 做了优化;
5.6还是有的, 看: 5.7.30:
mysql> explain select (select 1 from actor where film_id=1) from (select * from film where film_id=1) der;
+----+--------------------+-------+------------+-------+---------------+---------------------+---------+-------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------+------------+-------+---------------+---------------------+---------+-------+------+----------+--------------------------+
| 1 | PRIMARY | film | NULL | const | PRIMARY | PRIMARY | 2 | const | 1 | 100.00 | Using index |
| 2 | DEPENDENT SUBQUERY | actor | NULL | index | NULL | idx_actor_last_name | 182 | NULL | 200 | 100.00 | Using where; Using index |
+----+--------------------+-------+------------+-------+---------------+---------------------+---------+-------+------+----------+--------------------------+
2 rows in set, 2 warnings (0.00 sec)2.8 select_type=materializaton 物化(具体化): 将子查询结果作为一个临时表来加快执行速度
正常来讲是常驻内存, 下次查询会再次引用临时表;
如果是一个大表, 大表作为一个子查询, 就会在第一次查询时生成一个临时表, 后面每次引用;
mysql> explain select * from (select * from goods) s where id in (select id from goods2);
+----+--------------+-------------+------------+--------+---------------+------------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+-------------+------------+--------+---------------+------------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | goods | NULL | ALL | id | NULL | NULL | NULL | 149548 | 100.00 | NULL |
| 1 | SIMPLE | | NULL | eq_ref | | | 8 | func | 1 | 100.00 | Using where |
| 3 | MATERIALIZED | goods2 | NULL | ALL | NULL | NULL | NULL | NULL | 149692 | 100.00 | NULL |
+----+--------------+-------------+------------+--------+---------------+------------+---------+------+--------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
subquery3的子查询依赖3的物化的, 物化先执行, 然后是goods, 最后在是子查询;
物化前提是: 表记录比较多
3. table=表名: 表示当前这一步涉及的表都有哪些
- table表示查询使用的表名, 如果查询使用了别名, 显示的就是别名;
- 如果并不涉及数据表的操作, table=null;
- 如果显示为
就表示这是个临时表, N就是步骤的id; - 如果显示为
也是个临时表, 表示, 是M, N两个步骤id的结果集;
table:film
4. partitions=分区状况
partitions:NULL
5. type=连接类型: 用的全表扫描/有没有走索引
type的效率由高到低排序:
type=system 最高效;
type=const 用到索引: 且是唯一索引或主键且+ where返回了只有1行;
type=eq_ref 用到索引: 连接查询中前表每个结果, 都只匹配后表一行结果. 且比较是唯一索引, 查询效率较高.
type=ref 用到索引: 多表的 join 查询, 非唯一或非主键索引, 或者是使用了最左前缀 规则索引的查询.
type=fulltext 用到全文索引
type=ref_or_null 用到索引, 相当于 ref+ xxx is null的条件
type=unique_subquery 用于在in形式查询,子查询返回不重复的唯一值(略)
type=index_subquery 用于in形式子查询用到辅助索引或者in常数列表,子查询可能返回重复值 (略)
type=range 字段上有索引的范围扫描: 常见在索引字段使用 >, <, is null, between, in, like等运算符的查询中;
type=index_merge 使用了2个以上的索引, 常见: and, or的多个列都有索引, 根据索引查出来进行合并;
type=index 索引从头到尾扫一遍; 不用查表了; select name from t2; name上有索引, 但是不指定where条件;
type=all 性能最差: 全表扫描数据, 然后在server层返回数据过滤返回符合的数据; 5.1 type=system 最高效
type=system 表只有1行数据 或是空表; 且只用于 myisam和memory表; innodb的type列通常为all或index;
5.2 type=const 唯一索引或主键+返回一行记录时
使用唯一索引或主键, 返回结果一定是1行记录的等值where条件时, type=const通常
先来一个普通索引:
mysql> alter table t2 add index(id);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> select * from t2 where id=2;
+----+---------+--------+
| id | name | price |
+----+---------+--------+
| 2 | 商品2 | 200.87 |
+----+---------+--------+
1 row in set (0.00 sec)
mysql> explain select * from t2 where id=2;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
| 1 | SIMPLE | t2 | NULL | ref | id | id | 8 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
看到type=试试ref; 不是const;
改为唯一索引试试, 先删掉普通索引
mysql> show index from t2;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| t2 | 1 | id | 1 | id | A | 10 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
mysql> drop index id on t2;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show index from t2;
Empty set (0.00 sec)新建唯一索引, 再试试:
mysql> alter table t2 add unique(id);
Query OK, 0 rows affected (0.11 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select * from t2 where id>3;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t2 | NULL | range | id | id | 8 | NULL | 7 | 100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select * from t2 where id=2;
+----+-------------+-------+------------+-------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+-------+------+----------+-------+
| 1 | SIMPLE | t2 | NULL | const | id | id | 8 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)id>3是 type=range; id=2时, 唯一索引, type=const
5.3 type=eq_ref 多表join查询时, 对于前表的每一行数据, 后表中只返回一行匹配
说明: t2和goods表中数据是一样的, 只是goods中有近15万条数据, t2中只有10条且t2表id有unique索引;
mysql> explain select * from t2 left join goods on t2.id=goods.id;
+----+-------------+-------+------------+--------+---------------+------+---------+------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+------+---------+------------+------+----------+-------+
| 1 | SIMPLE | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
| 1 | SIMPLE | goods | NULL | eq_ref | id | id | 8 | test.t2.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+------+---------+------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
mysql> explain select * from t2 right join goods on t2.id=goods.id;
+----+-------------+-------+------------+--------+---------------+------+---------+---------------+--------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+------+---------+---------------+--------+----------+-------+
| 1 | SIMPLE | goods | NULL | ALL | NULL | NULL | NULL | NULL | 149548 | 100.00 | NULL |
| 1 | SIMPLE | t2 | NULL | eq_ref | id | id | 8 | test.goods.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+------+---------+---------------+--------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
mysql> 5.4 type=ref 多表的 join 查询, 非唯一或非主键索引, 或者是使用了最左前缀规则索引的查询.
mysql> explain select * from film where title = 'film2';
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | film | NULL | ref | idx_title | idx_title | 514 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+-----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql>title列不是唯一索引, 也不是主键列; 但是它也是索引列:
KEYidx_title(title),
5.5 type=fulltext 用到全文索引
全文索引的优先级很高, 如果全文索引和普通索引同时存在 ,mysql 不管性能代价, 会优先使用全文索引;
5.6 type=ref_or_nul 实际用的少: 类似ref, 但增加了null值的比较
mysql> alter table t2 add index(name);
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select * from t2 where name='商品1' or name is null;
+----+-------------+-------+------------+-------------+---------------+------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+---------------+------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | t2 | NULL | ref_or_null | name | name | 33 | const | 2 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------------+---------------+------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)如果没有 null判断, 就是ref了;
5.7 type=unique_subquery 有索引:在where中in,子查询返回不重复的唯一值
5.8 type=index_subquery 用于in形式子查询用到辅助索引或者in常数列表,子查询可能返回重复值
5.9 type=range 有索引的范围扫描: 常见在索引字段使用 >, <, is null, between, in, like等运算符的查询中;
5.10 type=index_merge 使用了2个以上的索引, 常见: and, or的多个列都有索引, 根据索引查出来进行合并;
5.11 type=index 索引从头到尾扫一遍; 不查表; select name from t2; name上有索引, 但是不指定where条件;
5.12 type=all 性能最差: 全表扫描数据, 然后在server层返回数据过滤返回符合的数据;
小结: 好的查询一般至少达到 range级别, 最好达到ref;
缩小下范围: 常见的有:
system/const/eq_ref/ref/range/index/all
6. possible_keys=可能用到的索引:当前这一步有可能用到的索引有哪些,都列出来
possible_keys:NULL
7. key=索引: 确定用了的索引
key:NULL
8. key_len=索引长度: 越小越好(越短越好)
key_len:NULL
9. ref=索引具体在哪一列上
ref:NULL
9.1 如果使用的常数等值查询, 会显示const;
9.2 如果是连接查询, 被驱动表的执行计划此处会显示 驱动表的关联字段;
mysql> explain select * from t2 left join goods on t2.id=goods.id;
+----+-------------+-------+------------+--------+---------------+------+---------+------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+------+---------+------------+------+----------+-------+
| 1 | SIMPLE | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
| 1 | SIMPLE | goods | NULL | eq_ref | id | id | 8 | test.t2.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+------+---------+------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)9.3 如果是条件使用了表达式或函数, 或条件列发生了内部隐式转换, 此处显示为func;
mysql> explain select * from film_category where film_id in (select film_id from film union all select film_id from film_actor);
+----+--------------------+---------------+------------+--------+----------------+----------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+---------------+------------+--------+----------------+----------------+---------+------+------+----------+-------------+
| 1 | PRIMARY | film_category | NULL | ALL | NULL | NULL | NULL | NULL | 1000 | 100.00 | Using where |
| 2 | DEPENDENT SUBQUERY | film | NULL | eq_ref | PRIMARY | PRIMARY | 2 | func | 1 | 100.00 | Using index |
| 3 | DEPENDENT UNION | film_actor | NULL | ref | idx_fk_film_id | idx_fk_film_id | 2 | func | 5 | 100.00 | Using index |
+----+--------------------+---------------+------------+--------+----------------+----------------+---------+------+------+----------+-------------+10. rows=当前这一步可能会检测的行数
rows:1000
估算行, 非精确值;
11. filtered=过滤后返回数据的百分比: 经过server过滤后实际返回客户端的百分比
filtered:100.00
12. Extra=扩展信息: 有没有排序/有没有用临时表, 很多种类型
Extra:NULL
12.1 no table used
mysql> explain select 1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 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 | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)12.2 using index 使用到了索引
mysql> explain select name from t2 ;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t2 | NULL | index | NULL | name | 33 | NULL | 10 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
12.3 NULL: 查询到的列有未被索引覆盖到的(就是查了几个列, 其中有的没索引)
mysql> explain select name, price from t2 ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)不是纯粹用索引, 但是也用到了索引;
或者, 就是未被索引覆盖到; 查了没用索引的列, 也都是 NULL;
12.4 using where 查询的where条件是没有索引的
mysql> explain select price from t2 where price=2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | t2 | NULL | ALL | NULL | NULL | NULL | NULL | 10 | 10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> explain select price from t2 where name='s' and price=2;
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | t2 | NULL | ref | name | name | 33 | const | 1 | 10.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
where 语句中有的列没有索引;
12.5 using where using index: 查询的符合索引,但是不是第一个列, 用不到索引
mysql> alter table t2 add index(name, price);
Query OK, 0 rows affected (0.03 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> explain select * from t2 where price=2;
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+
| 1 | SIMPLE | t2 | NULL | index | NULL | name_2 | 42 | NULL | 10 | 10.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+--------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)相当于又找到索引, 又用了where, 为什么呢? 有索引, 但是用不到, 最后还是用了where扫表;