MySQL explain詳解

Explain簡介

本文主要講述如何通過 explain 命令獲取 select 語句的執行計劃，通過 explain 我們可以知道以下信息：表的讀取順序，數據讀取操作的類型，哪些索引可以使用，哪些索引實際使用了，表之間的引用，每張表有多少行被優化器查詢等信息。

下面是使用 explain 的例子：

在 select 語句之前增加 explain 關鍵字，MySQL 會在查詢上設置一個標記，執行查詢時，會返回執行計劃的信息，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，將結果放入臨時表中）。

mysql> explain select * from actor;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL  |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+

在查詢中的每個表會輸出一行，如果有兩個表通過 join 連接查詢，那麼會輸出兩行。表的意義相當廣泛：可以是子查詢、一個 union 結果等。

explain 有兩個變種：

1）explain extended：會在 explain 的基礎上額外提供一些查詢優化的信息。緊隨其後通過 show warnings 命令可以得到優化後的查詢語句，從而看出優化器優化了什麼。額外還有 filtered 列，是一個半分比的值，rows * filtered/100 可以估算出將要和 explain 中前一個表進行連接的行數（前一個表指 explain 中的id值比當前表id值小的表）。

mysql> explain extended select * from film where id = 1;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | film  | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+----------+-------+

mysql> show warnings;
+-------+------+--------------------------------------------------------------------------------+
| Level | Code | Message                                                                        |
+-------+------+--------------------------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select '1' AS `id`,'film1' AS `name` from `test`.`film` where 1 |
+-------+------+--------------------------------------------------------------------------------+

2）explain partitions：相比 explain 多了個 partitions 字段，如果查詢是基於分區表的話，會顯示查詢將訪問的分區。

explain 中的列

接下來我們將展示 explain 中每個列的信息。

1. id列

id列的編號是 select 的序列號，有幾個 select 就有幾個id，並且id的順序是按 select 出現的順序增長的。MySQL將 select 查詢分爲簡單查詢和複雜查詢。複雜查詢分爲三類：簡單子查詢、派生表（from語句中的子查詢）、union 查詢。

1）簡單子查詢

mysql> explain select (select 1 from actor limit 1) from film;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
|  1 | PRIMARY     | film  | index | NULL          | idx_name | 32      | NULL |    1 | Using index |
|  2 | SUBQUERY    | actor | index | NULL          | PRIMARY  | 4       | NULL |    2 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

2）from子句中的子查詢

mysql> explain select id from (select id from film) as der;
+----+-------------+------------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table      | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+------------+-------+---------------+----------+---------+------+------+-------------+
|  1 | PRIMARY     | <derived2> | ALL   | NULL          | NULL     | NULL    | NULL |    2 | NULL        |
|  2 | DERIVED     | film       | index | NULL          | idx_name | 32      | NULL |    1 | Using index |
+----+-------------+------------+-------+---------------+----------+---------+------+------+-------------+

這個查詢執行時有個臨時表別名爲der，外部 select 查詢引用了這個臨時表

3）union查詢

mysql> explain select 1 union all select 1;
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra           |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
|  1 | PRIMARY      | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used  |
|  2 | UNION        | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used  |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+

union結果總是放在一個匿名臨時表中，臨時表不在SQL總出現，因此它的id是NULL。

2. select_type列

select_type 表示對應行是是簡單還是複雜的查詢，如果是複雜的查詢，又是上述三種複雜查詢中的哪一種。

1）simple：簡單查詢。查詢不包含子查詢和union

mysql> explain select * from film where id = 2;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
|  1 | SIMPLE      | film  | const | PRIMARY       | PRIMARY | 4       | const |    1 | NULL  |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+

2）primary：複雜查詢中最外層的 select

3）subquery：包含在 select 中的子查詢（不在 from 子句中）

4）derived：包含在 from 子句中的子查詢。MySQL會將結果存放在一個臨時表中，也稱爲派生表（derived的英文含義）

用這個例子來了解 primary、subquery 和 derived 類型

mysql> explain select (select 1 from actor where id = 1) from (select * from film where id = 1) der;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra       |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+
|  1 | PRIMARY     | <derived3> | system | NULL          | NULL    | NULL    | NULL  |    1 | NULL        |
|  3 | DERIVED     | film       | const  | PRIMARY       | PRIMARY | 4       | const |    1 | NULL        |
|  2 | SUBQUERY    | actor      | const  | PRIMARY       | PRIMARY | 4       | const |    1 | Using index |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------------+

5）union：在 union 中的第二個和隨後的 select

6）union result：從 union 臨時表檢索結果的 select

用這個例子來了解 union 和 union result 類型：

mysql> explain select 1 union all select 1;
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra           |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
|  1 | PRIMARY      | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used  |
|  2 | UNION        | NULL       | NULL | NULL          | NULL | NULL    | NULL | NULL | No tables used  |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+

3. table列

這一列表示 explain 的一行正在訪問哪個表。

當 from 子句中有子查詢時，table列是 <derivenN> 格式，表示當前查詢依賴 id=N 的查詢，於是先執行 id=N 的查詢。當有 union 時，UNION RESULT 的 table 列的值爲 <union1,2>，1和2表示參與 union 的 select 行id。

4. type列

這一列表示關聯類型或訪問類型，即MySQL決定如何查找表中的行。

依次從最優到最差分別爲：system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL

NULL：mysql能夠在優化階段分解查詢語句，在執行階段用不着再訪問表或索引。例如：在索引列中選取最小值，可以單獨查找索引來完成，不需要在執行時訪問表

mysql> explain select min(id) from film;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                        |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+

const, system：mysql能對查詢的某部分進行優化並將其轉化成一個常量（可以看show warnings 的結果）。用於 primary key 或 unique key 的所有列與常數比較時，所以表最多有一個匹配行，讀取1次，速度比較快。

mysql> explain extended select * from (select * from film where id = 1) tmp;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL  |    1 |   100.00 | NULL  |
|  2 | DERIVED     | film       | const  | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+

mysql> show warnings;
+-------+------+---------------------------------------------------------------+
| Level | Code | Message                                                       |
+-------+------+---------------------------------------------------------------+
| Note  | 1003 | /* select#1 */ select '1' AS `id`,'film1' AS `name` from dual |
+-------+------+---------------------------------------------------------------+

eq_ref：primary key 或 unique key 索引的所有部分被連接使用，最多隻會返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 left join film on film_actor.film_id = film.id;
+----+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+
| id | select_type | table      | type   | possible_keys | key               | key_len | ref                     | rows | Extra       |
+----+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+
|  1 | SIMPLE      | film_actor | index  | NULL          | idx_film_actor_id | 8       | NULL                    |    3 | Using index |
|  1 | SIMPLE      | film       | eq_ref | PRIMARY       | PRIMARY           | 4       | test.film_actor.film_id |    1 | NULL        |
+----+-------------+------------+--------+---------------+-------------------+---------+-------------------------+------+-------------+

ref：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前綴，索引要和某個值相比較，可能會找到多個符合條件的行。

1. 簡單 select 查詢，name是普通索引（非唯一索引）
mysql> explain select * from film where name = "film1";
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table | type | possible_keys | key      | key_len | ref   | rows | Extra                    |
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | film  | ref  | idx_name      | idx_name | 33      | const |    1 | Using where; Using index |
+----+-------------+-------+------+---------------+----------+---------+-------+------+--------------------------+

2.關聯表查詢，idx_film_actor_id是film_id和actor_id的聯合索引，這裏使用到了film_actor的左邊前綴film_id部分。
mysql> explain select * from film left join film_actor on film.id = film_actor.film_id;
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+
| id | select_type | table      | type  | possible_keys     | key               | key_len | ref          | rows | Extra       |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+
|  1 | SIMPLE      | film       | index | NULL              | idx_name          | 33      | NULL         |    3 | Using index |
|  1 | SIMPLE      | film_actor | ref   | idx_film_actor_id | idx_film_actor_id | 4       | test.film.id |    1 | Using index |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+-------------+

ref_or_null：類似ref，但是可以搜索值爲NULL的行。

mysql> explain select * from film where name = "film1" or name is null;
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+
| id | select_type | table | type        | possible_keys | key      | key_len | ref   | rows | Extra                    |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+
|  1 | SIMPLE      | film  | ref_or_null | idx_name      | idx_name | 33      | const |    2 | Using where; Using index |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+--------------------------+

index_merge：表示使用了索引合併的優化方法。例如下表：id是主鍵，tenant_id是普通索引。or 的時候沒有用 primary key，而是使用了 primary key(id) 和 tenant_id 索引

mysql> explain select * from role where id = 11011 or tenant_id = 8888;
+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+
| id | select_type | table | type        | possible_keys         | key                   | key_len | ref  | rows | Extra                                           |
+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+
|  1 | SIMPLE      | role  | index_merge | PRIMARY,idx_tenant_id | PRIMARY,idx_tenant_id | 4,4     | NULL |  134 | Using union(PRIMARY,idx_tenant_id); Using where |
+----+-------------+-------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+

range：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 | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | actor | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+

index：和ALL一樣，不同就是mysql只需掃描索引樹，這通常比ALL快一些。

mysql> explain select count(*) from film;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
|  1 | SIMPLE      | film  | index | NULL          | idx_name | 33      | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

ALL：即全表掃描，意味着mysql需要從頭到尾去查找所需要的行。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 | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL  |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+

5. possible_keys列

這一列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，這種情況是因爲表中數據不多，mysql認爲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適當的索引來提高查詢性能，然後用 explain 查看效果。

6. key列

這一列顯示mysql實際採用哪個索引來優化對該表的訪問。

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。

7. key_len列

這一列顯示了mysql在索引裏使用的字節數，通過這個值可以算出具體使用了索引中的哪些列。

舉例來說，film_actor的聯合索引 idx_film_actor_id 由 film_id 和 actor_id 兩個int列組成，並且每個int是4字節。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。

mysql> explain select * from film_actor where film_id = 2;
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+
| id | select_type | table      | type | possible_keys     | key               | key_len | ref   | rows | Extra       |
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+
|  1 | SIMPLE      | film_actor | ref  | idx_film_actor_id | idx_film_actor_id | 4       | const |    1 | Using index |
+----+-------------+------------+------+-------------------+-------------------+---------+-------+------+-------------+

key_len計算規則如下：

字符串
- char(n)：n字節長度
- varchar(n)：2字節存儲字符串長度，如果是utf-8，則長度 3n + 2
數值類型
- tinyint：1字節
- smallint：2字節
- int：4字節
- bigint：8字節　　
時間類型　
- date：3字節
- timestamp：4字節
- datetime：8字節
如果字段允許爲 NULL，需要1字節記錄是否爲 NULL

索引最大長度是768字節，當字符串過長時，mysql會做一個類似左前綴索引的處理，將前半部分的字符提取出來做索引。

8. ref列

這一列顯示了在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），func，NULL，字段名（例：film.id）

9. rows列

這一列是mysql估計要讀取並檢測的行數，注意這個不是結果集裏的行數。

10. Extra列

這一列展示的是額外信息。常見的重要值如下：

distinct: 一旦mysql找到了與行相聯合匹配的行，就不再搜索了

mysql> explain select distinct name from film left join film_actor on film.id = film_actor.film_id;
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+------------------------------+
| id | select_type | table      | type  | possible_keys     | key               | key_len | ref          | rows | Extra                        |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+------------------------------+
|  1 | SIMPLE      | film       | index | idx_name          | idx_name          | 33      | NULL         |    3 | Using index; Using temporary |
|  1 | SIMPLE      | film_actor | ref   | idx_film_actor_id | idx_film_actor_id | 4       | test.film.id |    1 | Using index; Distinct        |
+----+-------------+------------+-------+-------------------+-------------------+---------+--------------+------+------------------------------+

Using index：這發生在對錶的請求列都是同一索引的部分的時候，返回的列數據只使用了索引中的信息，而沒有再去訪問表中的行記錄。是性能高的表現。

mysql> explain select id from film order by id;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | film  | index | NULL          | PRIMARY | 4       | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+

Using where：mysql服務器將在存儲引擎檢索行後再進行過濾。就是先讀取整行數據，再按 where 條件進行檢查，符合就留下，不符合就丟棄。

mysql> explain select * from film where id > 1;
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key      | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+
|  1 | SIMPLE      | film  | index | PRIMARY       | idx_name | 33      | NULL |    3 | Using where; Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+--------------------------+

Using temporary：mysql需要創建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。

1. actor.name沒有索引，此時創建了張臨時表來distinct
mysql> explain select distinct name from actor;
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra           |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+
|  1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using temporary |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------+

2. 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 | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
|  1 | SIMPLE      | film  | index | idx_name      | idx_name | 33      | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

Using filesort：mysql 會對結果使用一個外部索引排序，而不是按索引次序從表裏讀取行。此時mysql會根據聯接類型瀏覽所有符合條件的記錄，並保存排序關鍵字和行指針，然後排序關鍵字並按順序檢索行信息。這種情況下一般也是要考慮使用索引來優化的。

1. actor.name未創建索引，會瀏覽actor整個表，保存排序關鍵字name和對應的id，然後排序name並檢索行記錄
mysql> explain select * from actor order by name;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | actor | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+

2. 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 | type  | possible_keys | key      | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+
|  1 | SIMPLE      | film  | index | NULL          | idx_name | 33      | NULL |    3 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

使用的表

以上所有sql使用的表和數據：

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,
  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);

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MySQL explain詳解

Explain簡介

explain 中的列

1. id列

2. select_type列

3. table列

4. type列

5. possible_keys列

6. key列

7. key_len列

8. ref列

9. rows列

10. Extra列

使用的表

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