技術標籤：Mysql

explain 返回值

• 參考：https://blog.csdn.net/why15732625998/article/details/80388236

type

type 所顯示的是查詢使用了哪種型別，type 包含的型別包括如下圖所示的幾種：

效能：system > const > eq_ref > ref > range > index > all

• system 表只有一行記錄（等於系統表），這是const型別的特列，平時不會出現，這個也可以忽略不計
• const 表示通過索引一次就找到了，const用於比較primary key 或者unique索引。因為只匹配一行資料，所以很快。如將主鍵置於where列表中，MySQL就能將該查詢轉換為一個常量。
• eq_ref 唯一性索引掃描，對於每個索引鍵，表中只有一條記錄與之匹配。常見於主鍵或唯一索引掃描。
• 非唯一性索引掃描，返回匹配某個單獨值的所有行，本質上也是一種索引訪問，它返回所有匹配某個單獨值的行，然而，它可能會找到多個符合條件的行，所以他應該屬於查詢和掃描的混合體。
• range 只檢索給定範圍的行，使用一個索引來選擇行，key列顯示使用了哪個索引，一般就是在你的 where 語句中出現 between、< 、>、in等的查詢，這種範圍掃描索引比全表掃描要好，因為它只需要開始於索引的某一點，而結束於另一點，不用掃描全部索引。
• index，Full Index Scan，Index 與 All 區別為 index 型別只遍歷索引樹。這通常比 ALL 快，因為索引檔案通常比資料檔案小。（也就是說雖然 all 和Index 都是讀全表，但 index 是從索引中讀取的，而 all 是從硬碟讀取的）。
• all，Full Table Scan 將遍歷全表以找到匹配的行。

possible_keys 和 key

• possible_keys，顯示可能應用在這張表中的索引，一個或多個。查詢涉及到的欄位上若存在索引，則該索引將被列出，但不一定被查詢實際使用。
  key，實際使用的索引，如果為NULL，則沒有使用索引。（可能原因包括沒有建立索引或索引失效）

唯一主鍵按照主鍵查詢

• type: const
• possible_keys：PRIMARY

create table has_pk(id int, value int, primary key(id));
insert into has_pk values
 
(1,1),(2,2);
explain select * from has_pk where id = 1;
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table  | partitions | type  | possible_keys | key     | key_len | ref   | rows | filtered | Extra |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
|  1 | SIMPLE      | has_pk | NULL       | const | PRIMARY       | PRIMARY | 4       | const |    1 |   100.00 | NULL  |
+----+-------------+--------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

聯合唯一鍵

create table multi_uk(id int, value int, age int, name int, unique key uk(id, value, age));
insert into multi_uk values(0,0,0,0),(0,0,1,0),(0,1,0,0),(0,1,1,0),(1,0,0,0),(1,0,1,0),(1,1,0,0),(1,1,1,0);

通過聯合唯一鍵查詢

• type = const
• possible_keys = uk
• 可見用到了 uk，效能高

explain select * from multi_uk where id=1 and value=1 and age = 0;
+----+-------------+----------+------------+-------+---------------+------+---------+-------------------+------+----------+-------+
| id | select_type | table    | partitions | type  | possible_keys | key  | key_len | ref               | rows | filtered | Extra |
+----+-------------+----------+------------+-------+---------------+------+---------+-------------------+------+----------+-------+
|  1 | SIMPLE      | multi_uk | NULL       | const | uk            | uk   | 15      | const,const,const |    1 |   100.00 | NULL  |
+----+-------------+----------+------------+-------+---------------+------+---------+-------------------+------+----------+-------+

通過聯合唯一鍵前兩個鍵查詢

• type = ref
• possible_keys = uk

explain select * from multi_uk where id=1 and value=1;
+----+-------------+----------+------------+------+---------------+------+---------+-------------+------+----------+-------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref         | rows | filtered | Extra |
+----+-------------+----------+------------+------+---------------+------+---------+-------------+------+----------+-------+
|  1 | SIMPLE      | multi_uk | NULL       | ref  | uk            | uk   | 10      | const,const |    2 |   100.00 | NULL  |
+----+-------------+----------+------------+------+---------------+------+---------+-------------+------+----------+-------+

通過聯合唯一鍵前兩個鍵查詢範圍

• type = range
• possible_keys = uk

explain select * from multi_uk where id>=0 and value>=0 limit 0,4;
+----+-------------+----------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+
| id | select_type | table    | partitions | type  | possible_keys | key  | key_len | ref  | rows | filtered | Extra                 |
+----+-------------+----------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+
|  1 | SIMPLE      | multi_uk | NULL       | range | uk            | uk   | 5       | NULL |    4 |    33.33 | Using index condition |
+----+-------------+----------+------------+-------+---------------+------+---------+------+------+----------+-----------------------+

單獨通過聯合唯一鍵的第一個鍵查詢

• type = ALL
• possible_keys = uk

explain select * from multi_uk where id = 0;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | multi_uk | NULL       | ALL  | uk            | NULL | NULL    | NULL |    8 |    50.00 | Using where |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+

單獨通過聯合唯一鍵的第二個鍵查詢

• type = ALL
• possible_keys = NULL

explain select * from multi_uk where value = 0;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | multi_uk | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |    12.50 | Using where |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+

單獨通過聯合唯一鍵的第三個鍵查詢

• type = ALL
• possible_keys = NULL

explain select * from multi_uk where age = 0;
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table    | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | multi_uk | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    8 |    12.50 | Using where |
+----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------------+

無主鍵表

• 查詢時不能用到任務 key，效率最差。
• type = ALL
• possible_keys = NULL

create table no_pk_no_uk(id int, value int);
explain select * from no_pk_no_uk where id = 1;
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table       | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra       |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | no_pk_no_uk | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    1 |   100.00 | Using where |
+----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+