不會看 Explain執行計劃，勸你簡歷別寫熟悉 SQL優化

時間 2020-05-21

標籤不會 explain 執行計劃簡歷熟悉 sql 優化欄目快樂工作简体版

原文原文鏈接

昨天中午在食堂，和部門的技術大牛們坐在一桌吃飯，做爲一個卑微技術渣仔默默的吃着飯，聽大佬們高談闊論，研究各類高端技術，我TM也想說話可實在插不上嘴。html

聊着聊着忽然說到他上午面試了一個工做6年的程序員，表情挺複雜，他說：我看他簡歷寫着熟悉SQL語句調優，就問了下 Explain 執行計劃怎麼看？結果這老哥一問三不知，工做6年這麼基礎的東西都不瞭解！mysql

感覺到了大佬的王之鄙視，回到工位我就開始默默寫這個，哎~ 我TM也不太懂 Explain ，老哥你這是針對我啊！哭唧唧~
程序員

Explain有什麼用

當Explain 與 SQL語句一塊兒使用時，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，包括如何鏈接表以及什麼順序鏈接表等。面試

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以下：分別是 id、select_type、table、partitions、type、possible_keys、key、key_len、ref、rows、filtered、Extra 12個字段。

下邊咱們會結合具體的SQL示例，詳細的解讀每一個字段以及每一個字段中不一樣參數的含義，如下全部示例數據庫版本爲 MySQL.5.7.17。sql

mysql> select version() from dual;
+------------+
| version()  |
+------------+
| 5.7.17-log |
+------------+

咱們建立三張表 one、two、three，表之間的關係 one.two_id = two.two_id AND two.three_id = three.three_id。數據庫

Explain執行計劃詳解

1、id

id： ：表示查詢中執行select子句或者操做表的順序，id的值越大，表明優先級越高，越先執行。 id大體會出現 3種狀況：函數

一、`id`相同

看到三條記錄的id都相同，能夠理解成這三個表爲一組，具備一樣的優先級，執行順序由上而下，具體順序由優化器決定。oop

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                                               |
+----+-------------+-------+------------+--------+---------------+---------+---------+----------------------+------+----------+----------------------------------------------------+

二、`id`不一樣

若是咱們的 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，最早執行。性能

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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

三、以上兩種同時存在

將上邊的 SQL 稍微修改一下，增長一個子查詢，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，不一樣組 id值越大，優先級越高，越先執行。學習

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 |
+----+-------------+-------+------------+--------+---------------+---------+---------+--------------------+------+----------+-------------+

2、select_type

select_type：表示 select 查詢的類型，主要是用於區分各類複雜的查詢，例如：普通查詢、聯合查詢、子查詢等。

一、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交併差集等操做。

二、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。

三、SUBQUERY

SUBQUERY：當 select 或 where 列表中包含了子查詢，該子查詢被標記爲：SUBQUERY 。

四、DERIVED

DERIVED：表示包含在from子句中的子查詢的select，在咱們的 from 列表中包含的子查詢會被標記爲derived 。

五、UNION

UNION：若是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。

六、UNION RESULT

UNION RESULT：表明從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操做。

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 |
+------+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+

3、table

查詢的表名，並不必定是真實存在的表，有別名顯示別名，也可能爲臨時表，例如上邊的DERIVED、 <union1,4>等。

4、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狀況。

+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 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 |
+----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+

5、type

type：查詢使用了何種類型，它在 SQL優化中是一個很是重要的指標，如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

一、system

system：當表僅有一行記錄時(系統表)，數據量不多，每每不須要進行磁盤IO，速度很是快。

二、const

const：表示查詢時命中 primary key 主鍵或者 unique 惟一索引，或者被鏈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，速度很是快。

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  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+

三、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 |
+----+-------------+-------+------------+--------+---------------+----------+---------+--------------------+------+----------+-------------+

四、ref

ref：區別於eq_ref ，ref表示使用非惟一性索引，會找到不少個符合條件的行。

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 |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+

五、ref_or_null

ref_or_null：這種鏈接類型相似於 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 |
+----+-------------+-------+------------+-------------+---------------+----------+---------+-------+------+----------+--------------------------+

六、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 |
+----+-------------+-------+------------+-------------+------------------+------------------+---------+------+------+----------+------------------------------------------------+

七、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)

八、index_subquery

index_subquery：區別於unique_subquery，用於非惟一索引，能夠返回重複值。

value IN (SELECT key_column FROM single_table WHERE some_expr)

九、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爲惟一主鍵，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，作範圍檢索 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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

十、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 |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

十一、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  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

6、possible_keys

possible_keys：表示在MySQL中經過哪些索引，能讓咱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。

7、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。

8、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，那麼須要將整個索引長度算進去；
多列索引，不是全部列都能用到，須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，而排序和分組即使是用到了索引，也不會計算到key_len中。

9、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
若是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
其餘狀況null

10、rows

rows：以表的統計信息和索引使用狀況，估算要找到咱們所需的記錄，須要讀取的行數。

這是評估SQL 性能的一個比較重要的數據，mysql須要掃描的行數，很直觀的顯示 SQL 性能的好壞，通常狀況下 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  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

11、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通過過濾後，剩下知足條件的記錄數量的比例。

在MySQL.5.7版本之前想要顯示filtered須要使用explain extended命令。MySQL.5.7後，默認explain直接顯示partitions和filtered的信息。

12、Extra

Extra ：不適合在其餘列中顯示的信息，Explain 中的不少額外的信息會在 Extra 字段顯示。

一、Using index

Using index：咱們在相應的 select 操做中使用了覆蓋索引，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很是快，SQl優化中理想的狀態。

什麼又是覆蓋索引?

一條 SQL只須要經過索引就能夠返回，咱們所須要查詢的數據（一個或幾個字段），而沒必要經過二級索引，查到主鍵以後再經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，咱們在 select 時只取出須要的字段，不可select *，並且該字段建了索引。

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  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+

二、Using where

Using where：查詢時未找到可用的索引，進而經過where條件過濾獲取所需數據，但要注意的是並非全部帶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經過全表掃描後再按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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

三、Using temporary

Using temporary：表示查詢後結果須要使用臨時表來存儲，通常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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+

四、Using filesort

Using filesort：表示沒法利用索引完成的排序操做，也就是ORDER BY的字段沒有索引，一般這樣的SQL都是須要優化的。

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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+

若是ORDER BY字段有索引就會用到覆蓋索引，相比執行速度快不少。

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 |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+

五、Using join buffer

Using join buffer：在咱們聯表查詢的時候，若是表的鏈接條件沒有用到索引，須要有一個鏈接緩衝區來存儲中間結果。

先看一下有索引的狀況：鏈接條件 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              |
+----+-------------+-------+------------+-------+---------------+----------+---------+----------------------+------+----------+--------------------------+

接下來刪掉鏈接條件 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優化中須要注意的地方。

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) |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------------------------------------------+

六、Impossible where

Impossible where：表示在咱們用不太正確的where語句，致使沒有符合條件的行。

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 |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------+

七、No tables used

No tables used：咱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列的信息很是很是多，這裏就再也不一一列舉了，詳見 MySQL官方文檔：https://dev.mysql.com/doc/refman/5.7/en/explain-output.html#jointype_index_merge