昨天中午在食堂，和部門的技術大牛們坐在一桌吃飯，作為一個卑微技術渣仔默默的吃著飯，聽大佬們高談闊論，研究各種高階技術，我TM也想說話可實在插不上嘴。 聊著聊著突然說到他上午面試了一個工作6年的程式設計師，表情挺複雜，他說：我看他簡歷寫著熟悉`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，工作6年這麼基礎的東西都不瞭解！ 感受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~  ### 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執行的資訊。也就是說，`MySQL`解釋了它將如何處理該語句，包括如何連線表以及什麼順序連線表等。 - 表的載入順序 - `sql` 的查詢型別 - 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有哪些資訊 `Explain` 執行計劃包含欄位資訊如下：分別是 `id`、`select_type`、`table`、`partitions`、`type`、`possible_keys`、`key`、`key_len`、`ref`、`rows`、`filtered`、`Extra` 12個欄位。  下邊我們會結合具體的`SQL`示例，詳細的解讀每個欄位以及每個欄位中不同引數的含義，以下所有示例資料庫版本為 `MySQL.5.7.17`。 ```sql mysql> select version() from dual; +------------+ | version() | +------------+ | 5.7.17-log | +------------+ ``` 我們建立三張表 `one`、`two`、`three`，表之間的關係 `one.two_id = two.two_id AND two.three_id = three.three_id`。 ### 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，代表優先順序越高，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為一組，具有同樣的優先順序，執行順序由上而下，具體順序由優化器決定。 ```sql 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`值越大優先順序越高，越先被執行 。當三個表依次巢狀，發現最裡層的子查詢 `id`最大，最先執行。 ```sql 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`劃分為一組，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，優先順序越高，越先執行。 ```sql 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` 查詢的型別，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為`PRIMARY`。 ##### 3、SUBQUERY `SUBQUERY`：當 `select` 或 `where` 列表中包含了子查詢，該子查詢被標記為：`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為`derived` 。 ##### 5、UNION `UNION`：如果`union`後邊又出現的`select` 語句，則會被標記為`union`；若 `union` 包含在 `from` 子句的子查詢中，外層 `select` 將被標記為 `derived`。 ##### 6、UNION RESULT `UNION RESULT`：代表從`union`的臨時表中讀取資料，而`table`列的``表示用第一個和第四個`select`的結果進行`union`操作。 ```sql 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 | | 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，也可能為臨時表，例如上邊的`DERIVED`、 ``等。 #### 四、partitions 查詢時匹配到的分割槽資訊，對於非分割槽表值為`NULL`，當查詢的是分割槽表時，`partitions`顯示分割槽表命中的分割槽情況。 ```sql +----+-------------+----------------+---------------------------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 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`：查詢使用了何種型別，它在 `SQL`優化中是一個非常重要的指標，以下效能從好到壞依次是：`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)，資料量很少，往往不需要進行磁碟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，或者被連線的部分是一個常量(`const`)值。這類掃描效率極高，返回資料量少，速度非常快。 ```sql 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`。 ```sql 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。 ```sql 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`：這種連線型別類似於 ref，區別在於 `MySQL`會額外搜尋包含`NULL`值的行。 ```sql 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 欄位`one_name`的`idx_name` 索引 。 ```sql 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。 ```sql 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。 ```sql 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欄位，給定範圍檢索資料。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`為唯一主鍵，`user_id`普通欄位未建索引。 ```sql 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定了索引的欄位，做範圍檢索 `type` 才是 `range`。 ```sql 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`是從硬碟中讀取。 下邊示例：`three_id` 為主鍵，不帶 `where` 條件全表查詢 ，`type`結果為`index` 。 ```sql 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，效能最差。 ```sql 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欄位上存在索引，則索引將被列出，**但這個索引並不定一會是最終查詢資料時所被用到的索引**。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，顯示為`NULL`。具體請參考上邊的例子。 >當 `type` 為 `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（位元組數），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`，欄位名。 - 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`關聯欄位` - 如果查詢條件使用了`表示式`、`函式`，或者條件列發生內部隱式轉換，可能顯示為`func` - 其他情況`null` #### 十、rows `rows`：以表的統計資訊和索引使用情況，估算要找到我們所需的記錄，需要讀取的行數。 這是評估`SQL` 效能的一個比較重要的資料，`mysql`需要掃描的行數，很直觀的顯示 `SQL` 效能的好壞，一般情況下 `rows` 值越小越好。 ```sql 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，表裡符合條件的記錄數的百分比。簡單點說，這個欄位表示儲存引擎返回的資料在經過過濾後，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，預設`explain`直接顯示`partitions`和`filtered`的資訊。 #### 十二、Extra `Extra` ：不適合在其他列中顯示的資訊，`Explain` 中的很多額外的資訊會在 `Extra` 欄位顯示。 ##### 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，我們所需要查詢的資料（一個或幾個欄位），而不必通過二級索引，查到主鍵之後再通過主鍵查詢整行資料（`select *` ）。 `one_id`表為主鍵 ```sql 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` 時只取出需要的欄位，不可`select *`，而且該欄位建了索引。 ```sql 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`條件過濾獲取所需資料，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` 為 `ALL`，即`MySQL`通過全表掃描後再按`where`條件篩選資料。 ```sql 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儲存，一般在排序或者分組查詢時用到。 ```sql 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`的欄位沒有索引，通常這樣的SQL都是需要優化的。 ```sql 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`欄位有索引就會用到覆蓋索引，相比執行速度快很多。 ```sql 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，如果表的連線條件沒有用到索引，需要有一個連線緩衝區來儲存中間結果。 先看一下有索引的情況：連線條件 `one_name` 、`two_name` 都用到索引。 ```sql 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 連線條件 `one_name` 、`two_name` 的欄位索引。發現`Extra` 列變成 `Using join buffer`，`type`均為全表掃描，這也是`SQL`優化中需要注意的地方。 ```sql 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。 ```sql 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。 ```sql 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`列的資訊非常非常多，這裡就不再一一列舉了，詳見 `MySQL`官方文件 ：https://dev.mysql.com/doc/refman/5.7/en/explain-output.html#jointype_index_merge  #### 總結 上邊只是簡單介紹了下 `Explain` 執行計劃各個列的含義，瞭解它不僅僅是要應付面試，在實際開發中也經常會用到。比如對慢`SQL`進行分析，如果連執行計劃結果都不會看，那還談什麼`SQL`優化呢？ --- 整理了幾百本各類技術電子書和視訊課程 ，噓~，「**免費**」 送給小夥伴們。關注公號回覆【**666**】自行領取。和一些小夥伴們建了一個技術交流群，一起探討技術、分享技術資料，旨在共同學習進步，如果感興趣就掃碼加入我們吧！