mysql explain 詳解
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當咱們寫下一條複雜的sql時，不是盲目去執行，選擇提早explain分析sql，是一個不錯的選擇，避免沒用到索引或者用到錯誤的索引致使mysql大量的掃表，形成線上很差的後果。
mysql提供的explain命令能夠獲取select語句的執行計劃，經過explain咱們能夠知道：表的讀取順序，數據讀取操做的類型，哪些索引可使用，哪些索引實際使用了等等。前端
mysql> explain select * from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
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如下所有基於mysql5.7.32mysql
mysql> select version();
+-----------+
| version() |
+-----------+
| 5.7.32 |
+-----------+
1 row in set (0.00 sec)
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id
mysql> explain select * from user a left join user_info b on a.id=b.user_id where a.id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | a | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | b | NULL | ref | user_id | user_id | 8 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
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id相同的時候，從上往下順序執行sql
mysql> explain select * from user where id = (select user_id from user_info where age=10);
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | user | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
| 2 | SUBQUERY | user_info | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
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id不一樣的時候，越大的id越先執行，對於這種子查詢，優先執行子sql拿到結果，纔去執行主sql後端
mysql> explain select * from user union select * from user;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | user | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
| 2 | UNION | user | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
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對於union查詢，會建立一個臨時表，對應的id是null服務器
select_type
SIMPLE
simple是簡單查詢，mysql認爲比較簡單的查詢，不包含子查詢和union查詢都是simple，哪怕簡單的join都是simplemarkdown
mysql> explain select * from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
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mysql> explain select * from user a left join user_info b on a.id=b.user_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
| 1 | SIMPLE | a | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
| 1 | SIMPLE | b | NULL | ref | user_id | user_id | 8 | test.a.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
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PRIMARY
查詢中若包含任何複雜的子部分，最外層查詢則被標記爲primarypost
mysql> explain select * from user where id = (select id from user_info where age=10);
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | user | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
| 2 | SUBQUERY | user_info | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
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mysql> explain select * from user union all select * from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | user | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
| 2 | UNION | user | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
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SUBQUERY
在SELECT或WHERE列表中包含了子查詢的語句優化
mysql> explain select (select id from user) from user;
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | user | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
| 2 | SUBQUERY | user | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
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mysql> explain select * from user where id= (select user_id from user_info where age=10);
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | user | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
| 2 | SUBQUERY | user_info | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
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DERIVED
派生表的select(from子句的子查詢)搜索引擎
mysql> explain select * from (select * from user union select * from user) c;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 2 | DERIVED | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| 3 | UNION | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.00 sec)
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UNION
union中後面的select語句編碼
mysql> explain select * from user union select * from user;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| 2 | UNION | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.02 sec)
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DEPENDENT UNION
出如今union或union all語句中，可是這個查詢要受到外部查詢的影響
mysql> explain select * from user where id in(select id from user union select id from user);
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
| 1 | PRIMARY | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using where; Using index |
| 2 | DEPENDENT SUBQUERY | user | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 100.00 | Using index |
| 3 | DEPENDENT UNION | user | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 100.00 | Using index |
| NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
4 rows in set, 1 warning (0.11 sec)
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DEPENDENT SUBQUERY
同DEPENDENT UNION 差很少，包含子查詢，且受到外部查詢的影響
mysql> explain select * from user where id in(select id from user union select id from user);
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
| 1 | PRIMARY | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using where; Using index |
| 2 | DEPENDENT SUBQUERY | user | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 100.00 | Using index |
| 3 | DEPENDENT UNION | user | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 100.00 | Using index |
| NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+
4 rows in set, 1 warning (0.11 sec)
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UNION RESULT
出如今union或union all中，表示的是一個結果集
mysql> explain select id from user union select id from user;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| 2 | UNION | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
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table
查詢的表名，有時候顯示的並非真正的表名
mysql> explain select * from (select * from user union select * from user) c;
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL |
| 2 | DERIVED | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| 3 | UNION | user | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+------+---------+------+------+----------+-----------------+
4 rows in set, 1 warning (0.00 sec)
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好比id=1的table是derived2，說明它查的是一個派生表，其中derived[num],這個num就是第幾步執行的結果，這裏是2，說明就是id=2的那一步執行的結果。
partitions
查詢匹配記錄的分區。對於非分區表，值爲NULL。
先建立一張分區表
CREATE TABLE users (
id INT NOT NULL ,
name varchar(100) NOT NULL
)
PARTITION BY RANGE (id) (
PARTITION p0 VALUES LESS THAN (10),
PARTITION p1 VALUES LESS THAN (20),
PARTITION p2 VALUES LESS THAN (30),
PARTITION p3 VALUES LESS THAN MAXVALUE
);
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經過id分了4個區，插入兩條數據
mysql> select * from users;
+----+-------+
| id | name |
+----+-------+
| 1 | Tom |
| 11 | jerry |
+----+-------+
2 rows in set (0.00 sec)
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mysql> explain select * from users where id=1;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | users | p0 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
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id=1 分佈在p0分區
mysql> explain select * from users where id=11;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | users | p1 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
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id=11 分佈在p1分區
type
type是很是重要的指標，它表示咱們使用什麼類型去查數據。下面由好到壞的介紹各個type類型
system
這種類型通常不會出現，官方解釋：
The table has only one row (= system table). This is a special case of the const join type.
表只有一行記錄，通常是系統表，是一種特殊的const類型
const
表最多有一個匹配行，由於只有一行，因此優化器的其他部分能夠將此行中列的值視爲常量。const表很是快，由於它們只讀取一次。
mysql> explain select * from user where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | user | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)
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const必定是用到primary key或者unique的索引時候纔會出現。
mysql> explain select * from user where name="Tom";
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | ref | name | name | 1023 | const | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
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雖然經過name也查出一條數據，可是因爲name是普通索引，因此不是const
eq_ref
除了system和const 它是最好的。通常出如今鏈接查詢中，且鏈接的條件是主鍵索引或者惟一索引時，就會用到它。
mysql> explain select * from user a left join user_info b on a.id=b.user_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| 1 | SIMPLE | a | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| 1 | SIMPLE | b | NULL | eq_ref | user_id | user_id | 4 | test.a.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
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a.id是primary key
b.user_id是 unique key
先查a表，a表關聯b表經過id和user_id來，由於id是主鍵索引，而user_id又是惟一索引，那麼a表中的每條記錄僅會關聯b表的一條記錄。因此b表的類型就是eq_ref。
ref
和eq_ref相比，不一樣的就是關聯表查詢的字段不是惟一或者主鍵索引，就是匹配到了，還要繼續匹配，可能有多條記錄。
mysql> explain select b.* from user a left join user_info b on a.id=b.user_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
| 1 | SIMPLE | a | NULL | index | NULL | name | 402 | NULL | 1 | 100.00 | Using index |
| 1 | SIMPLE | b | NULL | ref | user_id | user_id | 8 | test.a.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.01 sec)
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a.id是primary key
b.user_id是普通的索引key
a可能關聯b的多條記錄，可是起碼有索引，因此的b的type就是ref。
fulltext
全文索引的話，innodb不支持，隨着各類搜索引擎的出現，通常出現須要全文索引的地方，都會用相似es擅長分詞的存儲。
ref_or_null
這種類型相似於ref，可是MySQL會額外搜索包含空值的行。
mysql> explain select * from user_info where user_id is null or user_id=1;
+----+-------------+-----------+------------+-------------+---------------+---------+---------+-------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------------+---------------+---------+---------+-------+------+----------+-----------------------+
| 1 | SIMPLE | user_info | NULL | ref_or_null | user_id | user_id | 9 | const | 108 | 100.00 | Using index condition |
+----+-------------+-----------+------------+-------------+---------------+---------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
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user_id是普通的索引key，且容許爲null。
index_merge
索引合併
mysql> explain select * from user_info where id=1 or user_id=999;
+----+-------------+-----------+------------+-------------+-----------------+-----------------+---------+------+------+----------+-------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------------+-----------------+-----------------+---------+------+------+----------+-------------------------------------------+
| 1 | SIMPLE | user_info | NULL | index_merge | PRIMARY,user_id | PRIMARY,user_id | 8,9 | NULL | 2 | 100.00 | Using union(PRIMARY,user_id); Using where |
+----+-------------+-----------+------------+-------------+-----------------+-----------------+---------+------+------+----------+-------------------------------------------+
1 row in set, 1 warning (0.00 sec)
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id是主鍵索引，user_id是普通索引，結果用了兩個索引的合併
unique_subquery
用於where中的in形式子查詢，子查詢返回不重複值惟一值，能夠徹底替換子查詢，效率更高。 該類型替換了下面形式的IN子查詢的ref。
explain select * from user_info where user_id in (select id from user where id>10);
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子查詢的id爲primary key或者unique key。
index_subquery
相似於unique_subquery子查詢，可是子查詢返回的是非惟一索引。
explain select * from user_info where user_id in (select id from user where id>10);
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子查詢的id不是primary key和unique key。
range
索引範圍掃描，通常條件使用了 >，<，between，in等運算符的查詢。
mysql> explain select * from user where id>=1 and id <=100;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user | NULL | range | PRIMARY | PRIMARY | 8 | NULL | 100 | 100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
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index
和全表掃差很少，可是隻有索引樹被掃描，這一般比全表掃快，由於索引文件一般比數據文件小。
mysql> explain select user_id from user_info;
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | user_info | NULL | index | NULL | user_id | 9 | NULL | 100100 | 100.00 | Using index |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
複製代碼
ALL
全表掃描
mysql> explain select * from user_info where age=1;
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | user_info | NULL | ALL | NULL | NULL | NULL | NULL | 100100 | 10.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
複製代碼
age沒有索引，引擎層作了全表掃。
possible_keys
查詢可能使用到的索引。
mysql> explain select * from user_info where id>1 and user_id >2;
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | user_info | NULL | range | PRIMARY,user_id | PRIMARY | 8 | NULL | 50050 | 50.00 | Using where |
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
複製代碼
id和user_id都有索引
key
執行器最終選擇的索引
mysql> explain select * from user_info where id>1 and user_id >2;
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
| 1 | SIMPLE | user_info | NULL | range | PRIMARY,user_id | PRIMARY | 8 | NULL | 50050 | 50.00 | Using where |
+----+-------------+-----------+------------+-------+-----------------+---------+---------+------+-------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
複製代碼
這裏選擇了主鍵索引。
key_len
使用的索引的長度，這裏面的狀況仍是挺複雜的，特別對於一些複合索引。
假設存在這樣的表
CREATE TABLE `testlen` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`name1` varchar(10) not null default "",
`name2` varchar(10),
`num1` int(10) not null default 0,
`num2` int(10),
PRIMARY KEY (`id`),
key(`name1`),
key(`name2`),
key(`num1`),
key(`num2`)
) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;
複製代碼
- 當字段爲定長時，如char，int這些，須要有一個字節來標記是否爲空，not null的話就不須要。
mysql> explain select * from testlen where num1=1;
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
| 1 | SIMPLE | testlen | NULL | ref | num1 | num1 | 4 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.03 sec)
複製代碼
mysql> explain select * from testlen where num2=1;
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
| 1 | SIMPLE | testlen | NULL | ref | num2 | num2 | 5 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
複製代碼
- 當字段爲變長的時候，如varchar這些，除了是否須要用一個字節來標記非not null的，還須要額外的兩個字節標記長度
3.對於char、varchar這些，utf8編碼的一個字符佔用3個字節，utf8mb4一個字符佔用4個字節
mysql> explain select * from testlen where name1='1';
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| 1 | SIMPLE | testlen | NULL | ref | name1 | name1 | 42 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)
複製代碼
對於utf8mb4編碼的來講，not null的varchar(10) 最終長度=10*4+2 = 42
mysql> explain select * from testlen where name2='1';
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
| 1 | SIMPLE | testlen | NULL | ref | name2 | name2 | 43 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+-------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
複製代碼
對於utf8mb4編碼的來講，容許爲null的varchar(10) 最終長度=10*4+2+1 = 43。
ref
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。
mysql> explain select * from user where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | user | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.01 sec)
複製代碼
where 條件是id=1，那麼對應的ref列就是常量const。
mysql> explain select * from user a left join user_info b on a.id=b.user_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
| 1 | SIMPLE | a | NULL | index | NULL | name | 1023 | NULL | 1 | 100.00 | Using index |
| 1 | SIMPLE | b | NULL | eq_ref | user_id | user_id | 4 | test.a.id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+-----------+------+----------+-------------+
2 rows in set, 1 warning (0.02 sec)
複製代碼
user_info關聯user表，關聯的條件是user.id，故ref=test.a.id（test是庫名）
rows
掃描出的行數，這個是個估算的值，並非真正的結果集
filtered
filtered表示返回結果的行數佔需讀取行數的百分比，filtered列的值依賴於統計信息。
extra
一些額外的信息，主要說明如何找到數據的。
using index
使用覆蓋索引的時候就會出現，只查找索引列的值。
mysql> explain select user_id from user_info;
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | user_info | NULL | index | NULL | user_id | 4 | NULL | 2 | 100.00 | Using index |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
複製代碼
using where
MYSQL服務器層將在存儲引擎層返回行之後再應用WHERE過濾條件，通常發生在不能走索引掃描的狀況下或者走索引掃描，可是有些查詢列不在索引當中的狀況下。
mysql> explain select * from user_info where user_id>10;
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
| 1 | SIMPLE | user_info | NULL | ALL | user_id | NULL | NULL | NULL | 100100 | 50.00 | Using where |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+-------------+
1 row in set, 1 warning (0.01 sec)
複製代碼
Using temporary
對於一些order by、group by可能會用到臨時表。
mysql> explain select * from user a left join user_info b on a.id=b.user_id where a.id>=1 order by b.user_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+----------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+----------------------------------------------+
| 1 | SIMPLE | a | NULL | range | PRIMARY | PRIMARY | 8 | NULL | 5187 | 100.00 | Using where; Using temporary; Using filesort |
| 1 | SIMPLE | b | NULL | ref | user_id | user_id | 9 | test.a.id | 1 | 100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+------+----------+----------------------------------------------+
2 rows in set, 1 warning (0.01 sec)
複製代碼
Using filesort
通常order by相關的沒用到索引，就要文件排序。
mysql> explain select * from user_info order by age desc;
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+----------------+
| 1 | SIMPLE | user_info | NULL | ALL | NULL | NULL | NULL | NULL | 100100 | 100.00 | Using filesort |
+----+-------------+-----------+------------+------+---------------+------+---------+------+--------+----------+----------------+
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Using index condition
相比using where，Using index condition把where條件用到的索引放在引擎層過濾一下，沒用到索引的列，在server層再過濾一遍。
mysql> explain select * from user a left join user_info b on a.id=b.user_id where b.user_id>1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+-------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+-------+----------+-----------------------+
| 1 | SIMPLE | a | NULL | index | PRIMARY | name | 402 | NULL | 10375 | 100.00 | Using index |
| 1 | SIMPLE | b | NULL | ref | user_id | user_id | 9 | test.a.id | 1 | 100.00 | Using index condition |
+----+-------------+-------+------------+-------+---------------+---------+---------+-----------+-------+----------+-----------------------+
2 rows in set, 1 warning (0.00 sec)
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