Explain有什麼用
當Explain 與 SQL語句一起使用時，MySQL 會顯示來自優化器關於SQL執行的信息。也就是說，MySQL解釋了它將如何處理該語句，包括如何連接表以及什麼順序連接表等。
-
表的加載順序
-
sql的查詢類型 -
可能用到哪些索引，哪些索引又被實際使用
-
表與表之間的引用關係
-
一個表中有多少行被優化器查詢 .....
Explain有哪些信息
Explain 執行計劃包含字段信息如下：分別是 id、select_type、table、partitions、type、possible_keys、key、key_len、ref、rows、filtered、Extra 12個字段。
下邊我們會結合具體的SQL示例，詳細的解讀每個字段以及每個字段中不同參數的含義，以下所有示例數據庫版本爲 MySQL.5.7.17。
mysql> select version() from dual;
+------------+
| version() |
+------------+
| 5.7.17-log |
+------------+
我們創建三張表 one、two、three，表之間的關係 one.two_id = two.two_id AND two.three_id = three.three_id。
Explain執行計劃詳解
一、id
id： ：表示查詢中執行select子句或者操作表的順序，id的值越大，代表優先級越高，越先執行。 id大致會出現 3種情況：
1、id相同
看到三條記錄的id都相同，可以理解成這三個表爲一組，具有同樣的優先級，執行順序由上而下，具體順序由優化器決定。
mysql> EXPLAIN SELECT * FROM one o,two t, three r WHERE o.two_id = t.two_id AND t.three_id = r.three_id;
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | o | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL |
| 1 | SIMPLE | t | NULL | ALL | PRIMARY | NULL | NULL | NULL | 2 | 50 | Using where; Using join buffer (Block Nested Loop) |
| 1 | SIMPLE | r | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.t.three_id | 1 | 100 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+
2、id不同
如果我們的 SQL 中存在子查詢，那麼 id的序號會遞增，id值越大優先級越高，越先被執行 。當三個表依次嵌套，發現最裏層的子查詢 id最大，最先執行。
mysql> EXPLAIN select * from one o where o.two_id = (select t.two_id from two t where t.three_id = (select r.three_id from three r where r.three_name='我是第三表2'));
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | o | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 2 | SUBQUERY | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 3 | SUBQUERY | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
3、以上兩種同時存在
將上邊的 SQL 稍微修改一下，增加一個子查詢，發現 id的以上兩種同時存在。相同id劃分爲一組，這樣就有三個組，同組的從上往下順序執行，不同組 id值越大，優先級越高，越先執行。
mysql> EXPLAIN select * from one o where o.two_id = (select t.two_id from two t where t.three_id = (select r.three_id from three r where r.three_name='我是第三表2')) AND o.one_id in(select one_id from one where o.one_name="我是第一表2");
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+
| 1 | PRIMARY | o | NULL | ALL | PRIMARY | NULL | NULL | NULL | 2 | 50 | Using where |
| 1 | PRIMARY | one | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.o.one_id | 1 | 100 | Using index |
| 2 | SUBQUERY | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 3 | SUBQUERY | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
二、select_type
select_type：表示 select 查詢的類型，主要是用於區分各種複雜的查詢，例如：普通查詢、聯合查詢、子查詢等。
1、SIMPLE
SIMPLE：表示最簡單的 select 查詢語句，也就是在查詢中不包含子查詢或者 union交併差集等操作。
2、PRIMARY
PRIMARY：當查詢語句中包含任何複雜的子部分，最外層查詢則被標記爲PRIMARY。
3、SUBQUERY
SUBQUERY：當 select 或 where 列表中包含了子查詢，該子查詢被標記爲：SUBQUERY 。
4、DERIVED
DERIVED：表示包含在from子句中的子查詢的select，在我們的 from 列表中包含的子查詢會被標記爲derived 。
5、UNION
UNION：如果union後邊又出現的select 語句，則會被標記爲union；若 union 包含在 from 子句的子查詢中，外層 select 將被標記爲 derived。
6、UNION RESULT
UNION RESULT：代表從union的臨時表中讀取數據，而table列的<union1,4>表示用第一個和第四個select的結果進行union操作。
mysql> EXPLAIN select t.two_name, ( select one.one_id from one) o from (select two_id,two_name from two where two_name ='') t union (select r.three_name,r.three_id from three r);
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | two | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50 | Using where |
| 2 | SUBQUERY | one | NULL | index | NULL | PRIMARY | 4 | NULL | 2 | 100 | Using index |
| 4 | UNION | r | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL |
| NULL | UNION RESULT | <union1,4> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
三、table
查詢的表名，並不一定是真實存在的表，有別名顯示別名，也可能爲臨時表，例如上邊的DERIVED、 <union1,4>等。
四、partitions
查詢時匹配到的分區信息，對於非分區表值爲NULL，當查詢的是分區表時，partitions顯示分區表命中的分區情況。
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | p201801,p201802,p201803,p300012 | index | NULL | PRIMARY | 9 | NULL | 3 | 100 | Using index |
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
五、type
type：查詢使用了何種類型，它在 SQL優化中是一個非常重要的指標，以下性能從好到壞依次是：system > const > eq_ref > ref > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
1、system
system： 當表僅有一行記錄時(系統表)，數據量很少，往往不需要進行磁盤IO，速度非常快。
2、const
const：表示查詢時命中 primary key 主鍵或者 unique 唯一索引，或者被連接的部分是一個常量(const)值。這類掃描效率極高，返回數據量少，速度非常快。
mysql> EXPLAIN SELECT * from three where three_id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | three | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
3、eq_ref
eq_ref：查詢時命中主鍵primary key 或者 unique key索引， type 就是 eq_ref。
mysql> EXPLAIN select o.one_name from one o ,two t where o.one_id = t.two_id ;
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
| 1 | SIMPLE | o | NULL | index | PRIMARY | idx_name | 768 | NULL | 2 | 100 | Using index |
| 1 | SIMPLE | t | NULL | eq_ref | PRIMARY | PRIMARY | 4 | xin-slave.o.one_id | 1 | 100 | Using index |
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+
4、ref
ref：區別於eq_ref ，ref表示使用非唯一性索引，會找到很多個符合條件的行。
mysql> select o.one_id from one o where o.one_name = "xin" ;
+--------+
| one_id |
+--------+
| 1 |
| 3 |
+--------+```
```sql
mysql> EXPLAIN select o.one_id from one o where o.one_name = "xin" ;
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | o | NULL | ref | idx_name | idx_name | 768 | const | 1 | 100 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
5、ref_or_null
ref_or_null：這種連接類型類似於 ref，區別在於 MySQL會額外搜索包含NULL值的行。
mysql> EXPLAIN select o.one_id from one o where o.one_name = "xin" OR o.one_name IS NULL;
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
| 1 | SIMPLE | o | NULL | ref_or_null | idx_name | idx_name | 768 | const | 3 | 100 | Using where; Using index |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+
6、index_merge
index_merge：使用了索引合併優化方法，查詢使用了兩個以上的索引。
下邊示例中同時使用到主鍵one_id 和 字段one_name的idx_name 索引 。
mysql> EXPLAIN select * from one o where o.one_id >1 and o.one_name ='xin';
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
| 1 | SIMPLE | o | NULL | index_merge | PRIMARY,idx_name | idx_name,PRIMARY | 772,4 | NULL | 1 | 100 | Using intersect(idx_name,PRIMARY); Using where |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+
7、unique_subquery
unique_subquery：替換下面的 IN子查詢，子查詢返回不重複的集合。
value IN (SELECT primary_key FROM single_table WHERE some_expr)
8、index_subquery
index_subquery：區別於unique_subquery，用於非唯一索引，可以返回重複值。
value IN (SELECT key_column FROM single_table WHERE some_expr)
9、range
range：使用索引選擇行，僅檢索給定範圍內的行。簡單點說就是針對一個有索引的字段，給定範圍檢索數據。在where語句中使用 bettween...and、<、>、<=、in 等條件查詢 type 都是 range。
舉個栗子：three表中three_id爲唯一主鍵，user_id普通字段未建索引。
mysql> EXPLAIN SELECT * from three where three_id BETWEEN 2 AND 3;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | three | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 1 | 100 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
從結果中看到只有對設置了索引的字段，做範圍檢索 type 纔是 range。
mysql> EXPLAIN SELECT * from three where user_id BETWEEN 2 AND 3;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | three | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
10、index
index：Index 與ALL 其實都是讀全表，區別在於index是遍歷索引樹讀取，而ALL是從硬盤中讀取。
下邊示例：three_id 爲主鍵，不帶 where 條件全表查詢 ，type結果爲index 。
mysql> EXPLAIN SELECT three_id from three ;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | three | NULL | index | NULL | PRIMARY | 4 | NULL | 1 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
11、ALL
ALL：將遍歷全表以找到匹配的行，性能最差。
mysql> EXPLAIN SELECT * from two ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | two | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
六、possible_keys
possible_keys：表示在MySQL中通過哪些索引，能讓我們在表中找到想要的記錄，一旦查詢涉及到的某個字段上存在索引，則索引將被列出，但這個索引並不定一會是最終查詢數據時所被用到的索引。具體請參考上邊的例子。
七、key
key：區別於possible_keys，key是查詢中實際使用到的索引，若沒有使用索引，顯示爲NULL。具體請參考上邊的例子。
當
type爲index_merge時，可能會顯示多個索引。
八、key_len
key_len：表示查詢用到的索引長度（字節數），原則上長度越短越好 。
-
單列索引，那麼需要將整個索引長度算進去；
-
多列索引，不是所有列都能用到，需要計算查詢中實際用到的列。
注意：
key_len只計算where條件中用到的索引長度，而排序和分組即便是用到了索引，也不會計算到key_len中。
九、ref
ref：常見的有：const，func，null，字段名。
-
當使用常量等值查詢，顯示
const， -
當關聯查詢時，會顯示相應關聯表的
關聯字段 -
如果查詢條件使用了
表達式、函數，或者條件列發生內部隱式轉換，可能顯示爲func -
其他情況
null
十、rows
rows：以表的統計信息和索引使用情況，估算要找到我們所需的記錄，需要讀取的行數。
這是評估SQL 性能的一個比較重要的數據，mysql需要掃描的行數，很直觀的顯示 SQL 性能的好壞，一般情況下 rows 值越小越好。
mysql> EXPLAIN SELECT * from three;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | three | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
十一、filtered
filtered 這個是一個百分比的值，表裏符合條件的記錄數的百分比。簡單點說，這個字段表示存儲引擎返回的數據在經過過濾後，剩下滿足條件的記錄數量的比例。
在MySQL.5.7版本以前想要顯示filtered需要使用explain extended命令。MySQL.5.7後，默認explain直接顯示partitions和filtered的信息。
十二、Extra
Extra ：不適合在其他列中顯示的信息，Explain 中的很多額外的信息會在 Extra 字段顯示。
1、Using index
Using index：我們在相應的 select 操作中使用了覆蓋索引，通俗一點講就是查詢的列被索引覆蓋，使用到覆蓋索引查詢速度會非常快，SQl優化中理想的狀態。
什麼又是覆蓋索引?
一條 SQL只需要通過索引就可以返回，我們所需要查詢的數據（一個或幾個字段），而不必通過二級索引，查到主鍵之後再通過主鍵查詢整行數據（select * ）。
one_id表爲主鍵
mysql> EXPLAIN SELECT one_id from one ;
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | NULL | index | NULL | idx_two_id | 5 | NULL | 3 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+------------+---------+------+------+----------+-------------+
注意：想要使用到覆蓋索引，我們在 select 時只取出需要的字段，不可select *，而且該字段建了索引。
mysql> EXPLAIN SELECT * from one ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
2、Using where
Using where：查詢時未找到可用的索引，進而通過where條件過濾獲取所需數據，但要注意的是並不是所有帶where語句的查詢都會顯示Using where。
下邊示例create_time 並未用到索引，type 爲 ALL，即MySQL通過全表掃描後再按where條件篩選數據。
mysql> EXPLAIN SELECT one_name from one where create_time ='2020-05-18';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
3、Using temporary
Using temporary：表示查詢後結果需要使用臨時表來存儲，一般在排序或者分組查詢時用到。
mysql> EXPLAIN SELECT one_name from one where one_id in (1,2) group by one_name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | NULL | range| NULL | NULL | NULL | NULL | 3 | 33.33 | Using where; Using temporary; Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
4、Using filesort
Using filesort：表示無法利用索引完成的排序操作，也就是ORDER BY的字段沒有索引，通常這樣的SQL都是需要優化的。
mysql> EXPLAIN SELECT one_id from one ORDER BY create_time;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | one | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
如果ORDER BY字段有索引就會用到覆蓋索引，相比執行速度快很多。
mysql> EXPLAIN SELECT one_id from one ORDER BY one_id;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | one | NULL | index | NULL | PRIMARY | 4 | NULL | 3 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
5、Using join buffer
Using join buffer：在我們聯表查詢的時候，如果表的連接條件沒有用到索引，需要有一個連接緩衝區來存儲中間結果。
先看一下有索引的情況：連接條件 one_name 、two_name 都用到索引。
mysql> EXPLAIN SELECT one_name from one o,two t where o.one_name = t.two_name;
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
| 1 | SIMPLE | o | NULL | index | idx_name | idx_name | 768 | NULL | 3 | 100 | Using where; Using index |
| 1 | SIMPLE | t | NULL | ref | idx_name | idx_name | 768 | xin-slave.o.one_name | 1 | 100 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+
接下來刪掉 連接條件 one_name 、two_name 的字段索引。發現Extra 列變成 Using join buffer，type均爲全表掃描，這也是SQL優化中需要注意的地方。
mysql> EXPLAIN SELECT one_name from one o,two t where o.one_name = t.two_name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
| 1 | SIMPLE | t | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 100 | NULL |
| 1 | SIMPLE | o | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+
6、Impossible where
Impossible where：表示在我們用不太正確的where語句，導致沒有符合條件的行。
mysql> EXPLAIN SELECT one_name from one WHERE 1=2;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+
7、No tables used
No tables used：我們的查詢語句中沒有FROM子句，或者有 FROM DUAL子句。
mysql> EXPLAIN select now();
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
Extra列的信息非常非常多，這裏就不再一一列舉了，詳見 MySQL官方文檔 ：https://dev.mysql.com/doc/refman/5.7/en/explain-output.html#jointype_index_merge