MYSQL-EXPLAIN 命令詳解（轉載）

阿新 • • 發佈：2018-09-02

test cacheable sel exp 打開慢一起 select語句方式命令詳解

在工作中，我們用於捕捉性能問題最常用的就是打開慢查詢，定位執行效率差的SQL，那麽當我們定位到一個SQL以後還不算完事，我們還需要知道該SQL的執行計劃，比如是全表掃描，還是索引掃描，這些都需要通過EXPLAIN去完成。EXPLAIN命令是查看優化器如何決定執行查詢的主要方法。可以幫助我們深入了解MySQL的基於開銷的優化器，還可以獲得很多可能被優化器考慮到的訪問策略的細節，以及當運行SQL語句時哪種策略預計會被優化器采用。需要註意的是，生成的QEP並不確定，它可能會根據很多因素發生改變。MySQL不會將一個QEP和某個給定查詢綁定，QEP將由SQL語句每次執行時的實際情況確定，即便使用存儲過程也是如此。盡管在存儲過程中SQL語句都是預先解析過的，但QEP仍然會在每次調用存儲過程的時候才被確定。

通過執行計劃可以知道什麽？

(root@yayun-mysql-server) [test]>explain select d1.age, t2.id from (select age,name from t1 where id in (1,2))d1, t2 where d1.age=t2.age group by d1.age, t2.id order by t2.id;
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
| id | select_type | table      | type  | possible_keys | key     | key_len | ref    | rows | Extra                           |
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
|  1 | PRIMARY     | <derived2> | ALL   | NULL          | NULL    | NULL    | NULL   |    2 | Using temporary; Using filesort |
|  1 | PRIMARY     | t2         | ref   | age           | age     | 5       | d1.age |    1 | Using where; Using index        |
|  2 | DERIVED     | t1         | range | PRIMARY       | PRIMARY | 4       | NULL   |    2 | Using where                     |
+----+-------------+------------+-------+---------------+---------+---------+--------+------+---------------------------------+
3 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

MySQL執行計劃調用方式
1.EXPLAIN SELECT ……
變體：
2.EXPLAIN EXTENDED SELECT ……
將執行計劃"反編譯"成SELECT語句，運行SHOW WARNINGS
可得到被MySQL優化器優化後的查詢語句
3.EXPLAIN PARTITIONS SELECT ……
用於分區表的EXPLAIN生成QEP的信息

執行計劃包含的信息

+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+

1. id：

包含一組數字，表示查詢中執行select子句或操作表的順序

Example（id相同，執行順序由上至下）

(root@yayun-mysql-server) [test]>explain select t2.* from t1, t2, t3 where t1.id=t2.id and t1.id=t3.id and t1.name=‘‘;
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref        | rows | Extra                    |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
|  1 | SIMPLE      | t1    | ref    | PRIMARY,name  | name    | 63      | const      |    1 | Using where; Using index |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.id |    1 |                          |
|  1 | SIMPLE      | t3    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.id |    1 | Using index              |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+--------------------------+
3 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

Example （如果是子查詢，id的序號會遞增，id值越大優先級越高，越先被執行）

(root@yayun-mysql-server) [test]>explain select t2.* from t2 where id = (select id from t1 where id = (select t3.id from t3 where t3.name=‘‘));
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                               |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
|  1 | PRIMARY     | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE noticed after reading const tables |
|  2 | SUBQUERY    | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | no matching row in const table                      |
|  3 | SUBQUERY    | t3    | ref  | name          | name | 63      |      |    1 | Using where; Using index                            |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
3 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

Example（id如果相同，可以認為是一組，從上往下順序執行；在所有組中，id值越大，優先級越高，越先執行）

(root@yayun-mysql-server) [test]>explain select t2.* from (select t3.id from t3 where t3.name=‘‘)s1, t2 where s1.id=t2.id;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref   | rows | Extra                    |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL  |    1 |                          |
|  1 | PRIMARY     | t2         | const  | PRIMARY       | PRIMARY | 4       | const |    1 |                          |
|  2 | DERIVED     | t3         | ref    | name          | name    | 63      |       |    1 | Using where; Using index |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+--------------------------+
3 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

2. select_type

示查詢中每個select子句的類型（簡單OR復雜）

a. SIMPLE：查詢中不包含子查詢或者UNION
b. 查詢中若包含任何復雜的子部分，最外層查詢則被標記為：PRIMARY
c. 在SELECT或WHERE列表中包含了子查詢，該子查詢被標記為：SUBQUERY
d. 在FROM列表中包含的子查詢被標記為：DERIVED（衍生）用來表示包含在from子句中的子查詢的select，mysql會遞歸執行並將結果放到一個臨時表中。服務器內部稱為"派生表"，因為該臨時表是從子查詢中派生出來的
e. 若第二個SELECT出現在UNION之後，則被標記為UNION；若UNION包含在FROM子句的子查詢中，外層SELECT將被標記為：DERIVED
f. 從UNION表獲取結果的SELECT被標記為：UNION RESULT

SUBQUERY和UNION還可以被標記為DEPENDENT和UNCACHEABLE。
DEPENDENT意味著select依賴於外層查詢中發現的數據。
UNCACHEABLE意味著select中的某些特性阻止結果被緩存於一個item_cache中。

Example

(root@yayun-mysql-server) [test]>explain select d1.name, ( select id from t3) d2 from (select id,name from t1 where name=‘‘)d1 union (select name,id from t2);
+----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+
| id | select_type  | table      | type   | possible_keys | key  | key_len | ref  | rows | Extra                    |
+----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+
|  1 | PRIMARY      | <derived3> | system | NULL          | NULL | NULL    | NULL |    0 | const row not found      |
|  3 | DERIVED      | t1         | ref    | name          | name | 63      |      |    1 | Using where; Using index |
|  2 | SUBQUERY     | t3         | index  | NULL          | age  | 5       | NULL |    6 | Using index              |
|  4 | UNION        | t2         | index  | NULL          | name | 63      | NULL |    4 | Using index              |
| NULL | UNION RESULT | <union1,4> | ALL    | NULL          | NULL | NULL    | NULL | NULL |                          |
+----+--------------+------------+--------+---------------+------+---------+------+------+--------------------------+
5 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

第一行：id列為1，表示第一個select，select_type列的primary表示該查詢為外層查詢，table列被標記為<derived3>，表示查詢結果來自一個衍生表，其中3代表該查詢衍生自第三個select查詢，即id為3的select。
第二行：id為3，表示該查詢的執行次序為2（ 4 => 3），是整個查詢中第三個select的一部分。因查詢包含在from中，所以為derived。
第三行：select列表中的子查詢，select_type為subquery，為整個查詢中的第二個select。
第四行：select_type為union，說明第四個select是union裏的第二個select，最先執行。
第五行：代表從union的臨時表中讀取行的階段，table列的<union1,4>表示用第一個和第四個select的結果進行union操作。

3. type

表示MySQL在表中找到所需行的方式，又稱“訪問類型”，常見類型如下:

ALL, index, range, ref, eq_ref, const, system, NULL

從左到右，性能從最差到最好

Example

a. ALL：Full Table Scan， MySQL將遍歷全表以找到匹配的行

(root@yayun-mysql-server) [test]>explain select * from t1 where email=‘‘;
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

b. index：Full Index Scan，index與ALL區別為index類型只遍歷索引樹

(root@yayun-mysql-server) [test]>explain select id from t1;
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

c. range:索引範圍掃描，對索引的掃描開始於某一點，返回匹配值域的行。顯而易見的索引範圍掃描是帶有between或者where子句裏帶有<, >查詢。當mysql使用索引去查找一系列值時，例如IN()和OR列表，也會顯示range（範圍掃描）,當然性能上面是有差異的。

(root@yayun-mysql-server) [test]>explain select * from t1 where id in (1,4);
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>explain select * from t1 where id between 1 and 4;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>explain select * from t1 where id=1 or id=4;       
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.01 sec)

(root@yayun-mysql-server) [test]>explain select * from t1 where id > 1;      
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    3 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

d. ref：使用非唯一索引掃描或者唯一索引的前綴掃描，返回匹配某個單獨值的記錄行

(root@yayun-mysql-server) [test]>explain select * from t1 where name=‘yayun‘;
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref   | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
|  1 | SIMPLE      | t1    | ref  | name          | name | 63      | const |    1 | Using where |
+----+-------------+-------+------+---------------+------+---------+-------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

e. eq_ref：類似ref，區別就在使用的索引是唯一索引，對於每個索引鍵值，表中只有一條記錄匹配，簡單來說，就是多表連接中使用primary key或者 unique key作為關聯條件

(root@yayun-mysql-server) [test]>explain select t1.name from t1, t2 where t1.id=t2.id;
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref        | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
|  1 | SIMPLE      | t1    | index  | PRIMARY       | name    | 63      | NULL       |    4 | Using index |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t1.id |    1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
2 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

f. const、system：當MySQL對查詢某部分進行優化，並轉換為一個常量時，使用這些類型訪問。如將主鍵置於where列表中，MySQL就能將該查詢轉換為一個常量

(root@yayun-mysql-server) [test]>explain select * from ( select * from t1 where id=1)b1;
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------+
| id | select_type | table      | type   | possible_keys | key     | key_len | ref  | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL    | NULL    | NULL |    1 |       |
|  2 | DERIVED     | t1         | const  | PRIMARY       | PRIMARY | 4       |      |    1 |       |
+----+-------------+------------+--------+---------------+---------+---------+------+------+-------+
2 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

註：system是const類型的特例，當查詢的表只有一行的情況下，使用system

g. NULL：MySQL在優化過程中分解語句，執行時甚至不用訪問表或索引，例如從一個索引列裏選取最小值可以通過單獨索引查找完成。

(root@yayun-mysql-server) [test]>explain select * from t1 where id = (select min(id) from t2);
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref   | rows | Extra                        |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
|  1 | PRIMARY     | t1    | const | PRIMARY       | PRIMARY | 4       | const |    1 |                              |
|  2 | SUBQUERY    | NULL  | NULL  | NULL          | NULL    | NULL    | NULL  | NULL | Select tables optimized away |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+------------------------------+
2 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

4. possible_keys
指出MySQL能使用哪個索引在表中找到記錄，查詢涉及到的字段上若存在索引，則該索引將被列出，但不一定被查詢使用

5. key
顯示MySQL在查詢中實際使用的索引，若沒有使用索引，顯示為NULL

Example

(root@yayun-mysql-server) [test]>explain select id,age from t1;
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

6. key_len
表示索引中使用的字節數，可通過該列計算查詢中使用的索引的長度（key_len顯示的值為索引字段的最大可能長度，並非實際使用長度，即key_len是根據表定義計算而得，不是通過表內檢索出的）

7. ref
表示上述表的連接匹配條件，即哪些列或常量被用於查找索引列上的值

8. rows
表示MySQL根據表統計信息及索引選用情況，估算的找到所需的記錄所需要讀取的行數

Example

(root@yayun-mysql-server) [test]>explain select * from t1 , t2 where t1.id=t2.id and t2.name=‘atlas‘;
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref        | rows | Extra       |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
|  1 | SIMPLE      | t2    | ref    | PRIMARY,name  | name    | 63      | const      |    1 | Using where |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 4       | test.t2.id |    1 |             |
+----+-------------+-------+--------+---------------+---------+---------+------------+------+-------------+
2 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

9. Extra
包含不適合在其他列中顯示但十分重要的額外信息
a. Using index
該值表示相應的select操作中使用了覆蓋索引（Covering Index）

Example

(root@yayun-mysql-server) [test]>explain select id from t1;
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

覆蓋索引（Covering Index）
MySQL可以利用索引返回select列表中的字段，而不必根據索引再次讀取數據文件
包含所有滿足查詢需要的數據的索引稱為覆蓋索引（Covering Index）
註意：如果要使用覆蓋索引，一定要註意select列表中只取出需要的列，不可select *，因為如果將所有字段一起做索引會導致索引文件過大，查詢性能下降

b. Using where
表示mysql服務器將在存儲引擎檢索行後再進行過濾。許多where條件裏涉及索引中的列，當（並且如果）它讀取索引時，就能被存儲引擎檢驗，因此不是所有帶where字句的查詢都會顯示"Using where"。有時"Using where"的出現就是一個暗示：查詢可受益與不同的索引。

Example

(root@yayun-mysql-server) [test]>explain select id,name from t1 where id<4;
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra                    |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
|  1 | SIMPLE      | t1    | index | PRIMARY       | name | 63      | NULL |    4 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+--------------------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

c. Using temporary
表示MySQL需要使用臨時表來存儲結果集，常見於排序和分組查詢

這個值表示使用了內部臨時(基於內存的)表。一個查詢可能用到多個臨時表。有很多原因都會導致MySQL在執行查詢期間創建臨時表。兩個常見的原因是在來自不同表的上使用了DISTINCT,或者使用了不同的ORDER BY和GROUP BY列。可以強制指定一個臨時表使用基於磁盤的MyISAM存儲引擎。這樣做的原因主要有兩個：
1)內部臨時表占用的空間超過min(tmp_table_size，max_heap_table_size)系統變量的限制
2)使用了TEXT/BLOB 列

Example

(root@yayun-mysql-server) [test]>explain select id from t1 where id in (1,2) group by age,name;
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
| id | select_type | table | type  | possible_keys | key     | key_len | ref  | rows | Extra                                        |
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
|  1 | SIMPLE      | t1    | range | PRIMARY       | PRIMARY | 4       | NULL |    2 | Using where; Using temporary; Using filesort |
+----+-------------+-------+-------+---------------+---------+---------+------+------+----------------------------------------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

d. Using filesort
MySQL中無法利用索引完成的排序操作稱為“文件排序”

Example

(root@yayun-mysql-server) [test]>explain select id,age from t1 order by name; 
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra          |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>explain select id,age from t1 order by age; 
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | t1    | index | NULL          | age  | 5       | NULL |    4 | Using index |
+----+-------------+-------+-------+---------------+------+---------+------+------+-------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

e. Using join buffer
改值強調了在獲取連接條件時沒有使用索引，並且需要連接緩沖區來存儲中間結果。如果出現了這個值，那應該註意，根據查詢的具體情況可能需要添加索引來改進能。

Example

(root@yayun-mysql-server) [test]>explain select t1.name from t1 inner join t2 on t1.name=t2.name;
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
| id | select_type | table | type  | possible_keys | key  | key_len | ref          | rows | Extra                    |
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
|  1 | SIMPLE      | t1    | index | name          | name | 63      | NULL         |    4 | Using index              |
|  1 | SIMPLE      | t2    | ref   | name          | name | 63      | test.t1.name |    2 | Using where; Using index |
+----+-------------+-------+-------+---------------+------+---------+--------------+------+--------------------------+
2 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>alter table t1 drop key name;                                   
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

(root@yayun-mysql-server) [test]>alter table t2 drop key name; 
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

(root@yayun-mysql-server) [test]>explain select t1.name from t1 inner join t2 on t1.name=t2.name;
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                          |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    4 |                                |
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    4 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
2 rows in set (0.00 sec)

(root@yayun-mysql-server) [test]>

f. Impossible where
這個值強調了where語句會導致沒有符合條件的行。

Example

(root@yayun-mysql-server) [test]>EXPLAIN SELECT * FROM t1 WHERE 1=2;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra            |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
|  1 | SIMPLE      | NULL  | NULL | NULL          | NULL | NULL    | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

h. Select tables optimized away
這個值意味著僅通過使用索引，優化器可能僅從聚合函數結果中返回一行.

Example

(root@yayun-mysql-server) [test]>explain select max(id) from t1;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
| 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 |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------------------+
1 row in set (0.00 sec)

(root@yayun-mysql-server) [test]>

I. Index merges
當MySQL 決定要在一個給定的表上使用超過一個索引的時候，就會出現以下格式中的一個，詳細說明使用的索引以及合並的類型。
Using sort_union(...)
Using union(...)
Using intersect(...)

總結：
? EXPLAIN不會告訴你關於觸發器、存儲過程的信息或用戶自定義函數對查詢的影響情況
? EXPLAIN不考慮各種Cache
? EXPLAIN不能顯示MySQL在執行查詢時所作的優化工作
? 部分統計信息是估算的，並非精確值
? EXPALIN只能解釋SELECT操作，其他操作要重寫為SELECT後查看執行計劃。

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test cacheable sel exp 打開慢一起 select語句方式命令詳解在工作中，我們用於捕捉性能問題最常用的就是打開慢查詢，定位執行效率差的SQL，那麽當我們定位到一個SQL以後還不算完事，我們還需要知道該SQL的執行計劃，比如是全表掃描，還是索引掃

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