MySQL - Explain深度剖析

文章目錄

生猛幹貨
官方文檔
Explain介紹
測試資料
explain 使用
explain重要列說明
- id
- select_type
- - simple
  - primary
  - subquery
  - derived
  - union
- table
- type
- - NULL
  - const, system
  - eq_ref
  - ref
  - range
  - index
  - ALL
- possible_keys
- key
- key_len
- - key_len計算規則
- rows
- Extra
- - Using index
  - Using where
  - Using index condition
  - Using temporary
  - Using filesort
  - Select tables optimized away
搞定MySQL實戰

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https://dev.mysql.com/doc/refman/5.7/en/explain-output.html

使用EXPLAIN關鍵字可以模拟優化器執行SQL語句，分查詢語句或是結構的性能瓶頸

在 select 語句之前增加 explain 關鍵字，MySQL 會在查詢上設定一個标記，執行查詢會傳回執行計劃的資訊，而不是執行這條SQL。

如果 from 中包含子查詢，仍會執行該子查詢，将結果放入臨時表中。

DB Version

mysql> select version();
+------------+
| version()  |
+------------+
| 5.7.29-log |
+------------+
1 row in set (0.00 sec)

mysql>

DROP TABLE IF EXISTS `actor`;

CREATE TABLE `actor` (
 `id` int(11) NOT NULL,
 `name` varchar(45) DEFAULT NULL,
 `update_time` datetime DEFAULT NULL,
 PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `actor` (`id`, `name`, `update_time`) VALUES (1,'a','2017-12-22 15:27:18'), (2,'b','2017-12-22 15:27:18'), (3,'c','2017-12-22 15:27:18');

###############################

DROP TABLE IF EXISTS `film`;

CREATE TABLE `film` (
 `id` int(11) NOT NULL AUTO_INCREMENT,
 `name` varchar(10) DEFAULT NULL,
 PRIMARY KEY (`id`),
 KEY `idx_name` (`name`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `film` (`id`, `name`) VALUES (3,'film0'),(1,'film1'),(2,'film2');

###############################

DROP TABLE IF EXISTS `film_actor`;

CREATE TABLE `film_actor` (
 `id` int(11) NOT NULL,
 `film_id` int(11) NOT NULL,
 `actor_id` int(11) NOT NULL,
 `remark` varchar(255) DEFAULT NULL,
 PRIMARY KEY (`id`),
 KEY `idx_film_actor_id` (`film_id`,`actor_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

INSERT INTO `film_actor` (`id`, `film_id`, `actor_id`) VALUES (1,1,1),(2,1,2),(3,2,1);

explain 兩個擴充的使用

explain extended: 提供：額外一些查詢優化的資訊（‘EXTENDED’ is deprecated and will be removed in a future release.）

mysql> explain extended select * from film where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 2 warnings (0.00 sec)


#   2 warnings

# 可以通過 show warnings 指令檢視


mysql> show warnings;
+---------+------+----------------------------------------------------------------------------------+
| Level   | Code | Message                                                                          |
+---------+------+----------------------------------------------------------------------------------+
| Warning | 1681 | 'EXTENDED' is deprecated and will be removed in a future release.                |
| Note    | 1003 | /* select#1 */ select '1' AS `id`,'film1' AS `name` from `dbtest`.`film` where 1 |
+---------+------+----------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

mysql>

filtered 列：百分比，計算公式 rows * filtered/100 可以估算出将要和 explain 中前一個表進行連接配接的行數（前一個表指 explain 中的id值比目前表id值小的表），供參考

第二個 ‘PARTITIONS’ is deprecated and will be removed in a future

mysql> explain partitions select * from film where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 2 warnings (0.00 sec)

mysql> show warnings;
+---------+------+----------------------------------------------------------------------------------+
| Level   | Code | Message                                                                          |
+---------+------+----------------------------------------------------------------------------------+
| Warning | 1681 | 'PARTITIONS' is deprecated and will be removed in a future release.              |
| Note    | 1003 | /* select#1 */ select '1' AS `id`,'film1' AS `name` from `dbtest`.`film` where 1 |
+---------+------+----------------------------------------------------------------------------------+
2 rows in set (0.00 sec)

mysql>

是以隻使用explain就足夠了。

mysql> explain select * from film_actor a where a.actor_id  = (select id from actor where name = 'a');
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | a     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
|  2 | SUBQUERY    | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

id列的編号是 select 的序列号，有幾個 select 就有幾個id，并且id的順序是按 select 出現的順序增長的。

執行順序：

id越大執行優先級越高，id相同則從上往下執行，id為NULL最後執行

表示的對應行是簡單還是複雜的查詢

簡單查詢，查詢不包含子查詢和union

mysql> explain select * from film where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film  | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql>

複雜查詢中最外層的 select

mysql> explain select * from film_actor a where a.actor_id  = (select id from actor where name = 'a');
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | a     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
|  2 | SUBQUERY    | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

包含在 select 中的子查詢（不在 from 子句中）

mysql> explain select * from film_actor a where a.actor_id  = (select id from actor where name = 'a');
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | PRIMARY     | a     | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
|  2 | SUBQUERY    | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

包含在 from 子句中的子查詢。

MySQL會将結果存放在一個臨時表中，也稱為派生表

derived: 衍生的

mysql> set session optimizer_switch='derived_merge=off'; #關閉mysql5.7新特性對衍生表的合并優化
Query OK, 0 rows affected (0.00 sec)

mysql> explain select (select 1 from  actor where id = 1 ) from (select * from film where id =1 ) t ;
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table      | partitions | type   | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | <derived3> | NULL       | system | NULL          | NULL    | NULL    | NULL  |    1 |   100.00 | NULL        |
|  3 | DERIVED     | film       | NULL       | const  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | actor      | NULL       | const  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)

mysql>

在 union 中的第二個和随後的 select

mysql> EXPLAIN select 1 union select 1 ;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 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 tables used  |
|  2 | UNION        | NULL       | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used  |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)

mysql>

表示 explain 的一行正在通路哪個表

mysql> explain select (select 1 from  actor where id = 1 ) from (select * from film where id =1 ) t ;
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table      | partitions | type   | possible_keys | key     | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
|  1 | PRIMARY     | <derived3> | NULL       | system | NULL          | NULL    | NULL    | NULL  |    1 |   100.00 | NULL        |
|  3 | DERIVED     | film       | NULL       | const  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL        |
|  2 | SUBQUERY    | actor      | NULL       | const  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | Using index |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)

當 from 子句中有子查詢時，table列是

<derivenN>

格式，表示目前查詢依賴 id=N 的查詢，于是先執行 id=N 的查詢。

mysql> EXPLAIN select 1 union select 1 ;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 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 tables used  |
|  2 | UNION        | NULL       | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL |     NULL | No tables used  |
| NULL | UNION RESULT | <union1,2> | NULL       | ALL  | NULL          | NULL | NULL    | NULL | NULL |     NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)

mysql>

當有 union 時，UNION RESULT 的 table 列的值為<union1,2>，1和2表示參與 union 的 select 行id。

表示關聯類型或通路類型，即MySQL決定如何查找表中的行，查找資料行記錄的大概範圍。

依次從最優到最差分别為：system > const > eq_ref > ref > range > index > ALL

一般來說，得保證查詢達到range級别，最好達到ref

mysql能夠在優化階段分解查詢語句，在執行階段用不着再通路表或索引.

例如：在索引列中選取最小值，可以單獨查找索引來完成，不需要在執行時通路表

mysql> explain select min(id) from actor;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| 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)

mysql能對查詢的某部分進行優化并将其轉化成一個常量（可以看show warnings 的結果）。用于primary key 或 unique key 的所有列與常數比較時，是以表最多有一個比對行，讀取1次，速度比較快。

system是const的特例，表裡隻有一條元組比對時為system.

mysql> EXPLAIN select * from (select * from film where id=1) t ;
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table      | partitions | type   | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
|  1 | PRIMARY     | <derived2> | NULL       | system | NULL          | NULL    | NULL    | NULL  |    1 |   100.00 | NULL  |
|  2 | DERIVED     | film       | NULL       | const  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+---------------------------------------------------------------+
| Level | Code | Message                                                       |
+-------+------+---------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select '1' AS `id`,'film1' AS `name` from dual |
+-------+------+---------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>

primary key 或 unique key 索引的所有部分被連接配接使用，最多隻會傳回一條符合條件的記錄。這可能是在const 之外最好的聯接類型了，簡單的 select 查詢不會出現這種 type。

mysql> EXPLAIN select * from film_actor a  left join film b on a.film_id = b.id ;
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref              | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
|  1 | SIMPLE      | a     | NULL       | ALL    | NULL          | NULL    | NULL    | NULL             |    3 |   100.00 | NULL  |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 4       | dbtest.a.film_id |    1 |   100.00 | NULL  |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)

mysql> show warnings;
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message                                                                                                                                                                                                                                                                                                                                   |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select `dbtest`.`a`.`id` AS `id`,`dbtest`.`a`.`film_id` AS `film_id`,`dbtest`.`a`.`actor_id` AS `actor_id`,`dbtest`.`a`.`remark` AS `remark`,`dbtest`.`b`.`id` AS `id`,`dbtest`.`b`.`name` AS `name` from `dbtest`.`film_actor` `a` left join `dbtest`.`film` `b` on((`dbtest`.`b`.`id` = `dbtest`.`a`.`film_id`)) where 1 |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>

相比 eq_ref，不使用唯一索引，而是使用普通索引或者唯一性索引的部分字首，索引要和某個值相比較，可能會找到多個符合條件的行.

【簡單 select 查詢，name是普通索引（非唯一索引）】

mysql> show INDEX  from  film ;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| film  |          0 | PRIMARY  |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| film  |          1 | idx_name |            1 | name        | A         |           3 |     NULL | NULL   | YES  | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)

mysql> 
mysql> EXPLAIN select * from film a where a.name = 'film0';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key      | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | a     | NULL       | ref  | idx_name      | idx_name | 33      | const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

【關聯表查詢，idx_film_actor_id是film_id和actor_id的聯合索引，這裡使用到了film_actor的左邊字首film_id部分。】

mysql> show index from film_actor;
+------------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table      | Non_unique | Key_name          | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| film_actor |          0 | PRIMARY           |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
| film_actor |          1 | idx_film_actor_id |            1 | film_id     | A         |           2 |     NULL | NULL   |      | BTREE      |         |               |
| film_actor |          1 | idx_film_actor_id |            2 | actor_id    | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+------------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)

mysql> 
mysql> EXPLAIN select film_id from film  left join  film_actor  on film.id =film_actor.film_id ;
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys     | key               | key_len | ref            | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
|  1 | SIMPLE      | film       | NULL       | index | NULL              | idx_name          | 33      | NULL           |    3 |   100.00 | Using index |
|  1 | SIMPLE      | film_actor | NULL       | ref   | idx_film_actor_id | idx_film_actor_id | 4       | dbtest.film.id |    1 |   100.00 | Using index |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

mysql>

範圍掃描通常出現在 in(), between ,> ,<, >= 等操作中。使用一個索引來檢索給定範圍的行。

mysql> explain select * from actor  where id > 1 ;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key     | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | actor | NULL       | range | PRIMARY       | PRIMARY | 4       | NULL |    2 |   100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

掃描全索引就能拿到結果，一般是掃描某個二級索引，這種掃描不會從索引樹根節點開始快速查找，而是直接對二級索引的葉子節點周遊和掃描，速度還是比較慢的，這種查詢一般為使用覆寫索引，二級索引一般比較小，是以這種通常比ALL快一些

mysql> explain select * from film;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key      | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | index | NULL          | idx_name | 33      | NULL |    3 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

全表掃描，掃描你的聚簇索引的所有葉子節點。通常情況下這需要增加索引來進行優化了

mysql> explain select * from actor ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

顯示查詢可能使用哪些索引來查找

explain 時可能出現 possible_keys 有列，而 key 顯示 NULL 的情況，這種情況是因為表中資料不多，mysql認為索引對此查詢幫助不大，選擇了全表查詢。

如果該列是NULL，則沒有相關的索引。在這種情況下，可以通過檢查 where 子句看是否可以創造一個适當的索引來提高查詢性能，然後用 explain 檢視效果。

mysql實際采用哪個索引來優化對該表的通路。

如果沒有使用索引，則該列是 NULL。如果想強制mysql使用或忽視possible_keys列中的索引，在查詢中使用 force index、ignore index。

顯示了mysql在索引裡使用的位元組數，通過這個值可以算出具體使用了索引中的哪些列。

舉個例子：

film_actor的聯合索引 idx_film_actor_id 由 film_id 和 actor_id 兩個int列組成，并且每個int是4位元組。

mysql> explain select * from film_actor where film_id=1;
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
| id | select_type | table      | partitions | type | possible_keys     | key               | key_len | ref   | rows | filtered | Extra |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film_actor | NULL       | ref  | idx_film_actor_id | idx_film_actor_id | 4       | const |    2 |   100.00 | NULL  |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql>

通過結果中的key_len=4可推斷出查詢使用了第一個列：film_id列來執行索引查找。

【字元串】

char(n)：n位元組長度
varchar(n)：如果是utf-8，則長度 3n + 2 位元組，加的2位元組用來存儲字元串長度

【數值類型】

tinyint：1位元組
smallint：2位元組
int：4位元組
bigint：8位元組

【時間類型】

date：3位元組
timestamp：4位元組
datetime：8位元組

如果字段允許為 NULL，需要1位元組記錄是否為 NULL

索引最大長度是768位元組，當字元串過長時，mysql會做一個類似左字首索引的處理，将前半部分的字元提取出來做索引

顯示了在key列記錄的索引中，表查找值所用到的列或常量，常見的有：const（常量），字段名（例：film.id）

mysql> explain select * from film_actor where film_id=1;
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
| id | select_type | table      | partitions | type | possible_keys     | key               | key_len | ref   | rows | filtered | Extra |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film_actor | NULL       | ref  | idx_film_actor_id | idx_film_actor_id | 4       | const |    2 |   100.00 | NULL  |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)

mysql> EXPLAIN select film_id from film  left join  film_actor  on film.id =film_actor.film_id ;
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
| id | select_type | table      | partitions | type  | possible_keys     | key               | key_len | ref            | rows | filtered | Extra       |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
|  1 | SIMPLE      | film       | NULL       | index | NULL              | idx_name          | 33      | NULL           |    3 |   100.00 | Using index |
|  1 | SIMPLE      | film_actor | NULL       | ref   | idx_film_actor_id | idx_film_actor_id | 4       | dbtest.film.id |    1 |   100.00 | Using index |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)

mysql估計要讀取并檢測的行數，注意這個不是結果集裡的行數。

展示的是額外資訊。

列舉幾個常見的值

使用覆寫索引：無需回表

mysql執行計劃explain結果裡的key有使用索引，如果select後面查詢的字段都可以從這個索引的樹中擷取，這種情況一般可以說是用到了覆寫索引，extra裡一般都有using index；

覆寫索引一般針對的是輔助索引，整個查詢結果隻通過輔助索引就能拿到結果，不需要通過輔助索引樹找到主鍵，再通過主鍵去主鍵索引樹裡擷取其它字段值

mysql> explain select film_id from film_actor where film_id=1;
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------------+
| id | select_type | table      | partitions | type | possible_keys     | key               | key_len | ref   | rows | filtered | Extra       |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | film_actor | NULL       | ref  | idx_film_actor_id | idx_film_actor_id | 4       | const |    2 |   100.00 | Using index |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

使用 where 語句來處理結果，并且查詢的列未被索引覆寫

mysql> show index from actor;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| actor |          0 | PRIMARY  |            1 | id          | A         |           3 |     NULL | NULL   |      | BTREE      |         |               |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)

mysql> explain select * from actor where name = 'a';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |    33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

查詢的列不完全被索引覆寫，where條件中是一個前導列的範圍；

mysql> explain select * from film_actor where film_id > 1 ;
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
| id | select_type | table      | partitions | type  | possible_keys     | key               | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | film_actor | NULL       | range | idx_film_actor_id | idx_film_actor_id | 4       | NULL |    1 |   100.00 | Using index condition |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

mysql>

mysql需要建立一張臨時表來處理查詢。出現這種情況一般是要進行優化的，首先是想到用索引來優化。

【actor.name沒有索引，此時建立了張臨時表來distinct】

mysql> explain select distinct name from actor;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra           |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
|  1 | SIMPLE      | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)

【film.name建立了idx_name索引，此時查詢時extra是using index,沒有用臨時表】

mysql> explain select distinct name from film;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key      | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | index | idx_name      | idx_name | 33      | NULL |    3 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

将用外部排序而不是索引排序，資料較小時從記憶體排序，否則需要在磁盤完成排序。這種情況下一般也是要考慮使用索引來優化的

【 actor.name未建立索引，會浏覽actor整個表，儲存排序關鍵字name和對應的id，然後排序name并檢索行記錄】

mysql> explain select * from actor order by name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra          |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
|  1 | SIMPLE      | actor | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    3 |   100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)

mysql>

【film.name建立了idx_name索引,此時查詢時extra是using index】

mysql> explain select * from film order by name ;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type  | possible_keys | key      | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | film  | NULL       | index | NULL          | idx_name | 33      | NULL |    3 |   100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql>

使用某些聚合函數（比如 max、min）來通路存在索引的某個字段

MySQL - Explain深度剖析

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