一、explain是什么？

1、 定義

EXPLAIN是mysql中的一個命令，可以模擬優化器執行SQL語句并返回執行計劃。通過執行計劃，我們可以分析查詢語句或表結構的性能瓶頸，從而進行SQL優化。

2、用法

mysql> explain select * from employee; +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)

通過上面的執行計劃我們可以了解：

表的執行順序

數據讀取操作的操作類型

哪些索引可以使用

哪些索引被實際使用

每張表有多少行被優化器查詢

看到這個表，也許你會兩眼一抹黑，這咋看呀？接著往下看，你一定可以從執行計劃中輕松得出以上結果。

二、名詞段解釋

1. id

定義：select查詢的序列號，包含一組數字，表示查詢中執行select子句或操作表的順序。

存在以下三種情況：

id相同，執行順序由上至下

mysql> desc select e.*,p.* from employee e,performance p where p.user_id = e.id; +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+ | 1 | SIMPLE | e | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 1 | SIMPLE | p | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where; Using join buffer (hash join) | +----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+--------------------------------------------+ 2 rows in set, 1 warning (0.00 sec)

id不同，如果是子查詢，id序號對遞增，id值越大優先級越高，越先執行

mysql> explain select e.* -> from employee e-> where id = ( select user_id-> from performance-> where performance.score > 80 and performance.year = 2020 and performance.quarter = 2); +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | 2 | SUBQUERY | performance | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where | +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ 2 rows in set, 1 warning (0.01 sec)

id相同而不同（不同的：id越大越先執行；相同的：由上至下按順序執行）

mysql> EXPLAIN SELECT-> e.*, d.depart_name-> FROM employee e,department d-> WHERE-> e.depart_id = d.id-> AND e.id = (-> SELECT user_id-> WHERE-> performance.score > 80-> AND performance. YEAR = 2020-> AND performance. QUARTER = 2-> ); +----+-------------+-------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+ | 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | 1 | PRIMARY | d | NULL | eq_ref | PRIMARY | PRIMARY | 4 | db.e.depart_id | 1 | 100.00 | NULL | | 2 | SUBQUERY | performance | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where | +----+-------------+-------------+------------+--------+---------------+---------+---------+----------------+------+----------+-------------+ 3 rows in set, 1 warning (0.00 sec)

2、select_type

定義：查詢的類型，主要用于區別普通查詢、聯合查詢和子查詢等復合查詢。

SIMPLE：簡單的select查詢，查詢中不包含子查詢和聯合查詢（union）

mysql> explain select * from employee; +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)

PRIMARY：若查詢中包含任何的子查詢，最外層查詢則標記為PRIMARY

SUBQUERY：在select或where列表中包含的子查詢

mysql> explain select e.* -> from employee e-> where id = ( select user_id-> from performance-> where performance.score > 80 and performance.year = 2020 and performance.quarter = 2); +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ | 1 | PRIMARY | e | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | 2 | SUBQUERY | performance | NULL | ALL | NULL | NULL | NULL | NULL | 2 | 50.00 | Using where | +----+-------------+-------------+------------+------+---------------+------+---------+------+------+----------+-------------+ 2 rows in set, 1 warning (0.01 sec)

DERIVED：在FROM列表中包含的子查詢被標記為（DERIVED）,Mysql會遞歸執行這些子查詢，把結果放在臨時表里。

mysql> EXPLAIN select employee.* FROM (SELECT distinct department.id FROM department) s1,employee WHERE s1.id = employee.depart_id; +----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------------+------+----------+-------------+ | 1 | PRIMARY | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where | | 1 | PRIMARY | <derived2> | NULL | ref | <auto_key0> | <auto_key0> | 4 | db.employee.depart_id | 2 | 100.00 | Using index | | 2 | DERIVED | department | NULL | index | PRIMARY | PRIMARY | 4 | NULL | 4 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+-------------+---------+-----------------------+------+----------+-------------+ 3 rows in set, 1 warning (0.00 sec)

UNION：如果第二個SELECT出現在UNION之后，則標記為UNION;如果UNION包含在FROM子句的子查詢中，外層SELECT被標記為DERIVED

UNION RESULT：從UNION表獲取結果的SELECT

mysql> explain select * from employee union select * from employee_asu; +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ | 1 | PRIMARY | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 2 | UNION | employee_asu | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary | +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ 3 rows in set, 1 warning (0.00 sec)mysql> explain select * from (select * from employee union select * from employee_asu) s; +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ | 1 | PRIMARY | <derived2> | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL | | 2 | DERIVED | employee | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 3 | UNION | employee_asu | NULL | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary | +----+--------------+--------------+------------+------+---------------+------+---------+------+------+----------+-----------------+ 4 rows in set, 1 warning (0.00 sec)

3、table

定義：顧名思義，表名。

4、partitions（分區）

定義：顯示表分區的分區名

mysql> explain select * from tk; +----+-------------+-------+------------+------+---------------+------+---------+--- | id | select_type | table | partitions | type | possible_keys | key | key_len | re +----+-------------+-------+------------+------+---------------+------+---------+--- | 1 | SIMPLE | tk | p0,p1,p2 | ALL | NULL | NULL | NULL | NU +----+-------------+-------+------------+------+---------------+------+---------+--- 1 row in set, 1 warning (0.00 sec)

5、type

定義：type顯示的是訪問類型，是較為重要的一個指標，結果值從最好到最壞依次為system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > all，常見的有system > const > eq_ref > ref > range > index > all

system：表只有一行記錄（等于系統表），這是const類型的特例，平時不會出現，可忽略不計。

mysql> EXPLAIN select * FROM (SELECT distinct department.id FROM department where id=2) s1; +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ | 1 | PRIMARY | <derived2> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 2 | DERIVED | department | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index | +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)

const：表示通過索引一次就找到了，const用于比較primary key或者unique索引，只匹配一行數據，速度很快。

mysql> explain SELECT distinct department.id FROM department where id=2; +----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | department | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)

eq_ref：唯一性索引掃描，對于每個索引鍵，表中只有一條數據與之匹配，常見于主鍵或唯一索引掃描。

mysql> explain SELECT * from employee,department where employee.depart_id = department.id ; +----+-------------+------------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ALL | NULL | NULL | NULL | NULL | 7 | 100.00 | Using where | | 1 | SIMPLE | department | NULL | eq_ref | PRIMARY | PRIMARY | 4 | db.employee.depart_id | 1 | 100.00 | NULL | +----+-------------+------------+------------+--------+---------------+---------+---------+-----------------------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)

ref：非唯一性索引掃描，返回匹配某個單獨值的所有行

mysql> show index from employee; #查看表索引 +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment | Visible | Expression | +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ | employee | 0 | PRIMARY | 1 | id | A | 0 | NULL | NULL | | BTREE | | | YES | NULL | | employee | 1 | idx_employee_username | 1 | username | A | 0 | NULL | NULL | YES | BTREE | | | YES | NULL | +----------+------------+-----------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+---------+------------+ 2 rows in set (0.00 sec)mysql> explain select * from employee where username = "john"; +----+-------------+----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ | 1 | SIMPLE | employee | NULL | ref | idx_employee_username | idx_employee_username | 83 | const | 1 | 100.00 | NULL | +----+-------------+----------+------------+------+-----------------------+-----------------------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.01 sec)

range：只檢索給定范圍的行，使用一個索引來選擇行。key列顯示使用了那個索引，一般是where語句中出現了between、<、>或in等查詢。

mysql> explain select * from employee where id between 1 and 3; +----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 3 | 100.00 | Using where | +----+-------------+----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)

index：full index scan,全索引樹掃描，通常比all快，因為索引文件比數據文件小很多。

mysql> explain select id from department; +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ | 1 | SIMPLE | department | NULL | index | NULL | PRIMARY | 4 | NULL | 4 | 100.00 | Using index | +----+-------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)

all：全表掃描，當數據量達到百萬級別，性能會下降明顯，需要建立索引；當然小數據量的全表掃描是沒有問題的。

mysql> explain select * from department; +----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | SIMPLE | department | NULL | ALL | NULL | NULL | NULL | NULL | 4 | 100.00 | NULL | +----+-------------+------------+------------+------+---------------+------+---------+------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)

6、possible_keys

定義：顯示可能應用在這張表中的索引，一個或多個（查詢涉及到的字段若存在索引，則該索引將被列出，但不一定被查詢實際使用）

mysql> explain select id,username,age,salary from employee where username='a'; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)

補充說明：從執行計劃關鍵字possible_keys中可以看出，sql語句執行可能用到idx_username和idx_username_age_salary兩個索引。如果為null，就是沒有可能使用的索引。

7、key

定義：實際使用的索引。如果為null，則沒有使用索引。

常規舉例

mysql> explain select username,age,salary from employee where username = 'a' group by age,salary; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)

補充說明：possible_keys，可能使用的所以為idx_username,idx_username_age_salary; key,實際使用的索引為idx_username_age_salary

覆蓋索引

看到覆蓋索引也許你兩眼一抹黑，別急，先記住這么一個名詞，繼續往下看。

mysql> explain select username,age,salary from employee; +----+-------------+----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | index | NULL | idx_username_age_salary | 93 | NULL | 7 | 100.00 | Using index | +----+-------------+----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)

補充說明：如果查詢中使用了覆蓋索引，則該索引只會出現在key中，不會出現在possible_keys中。

8、key_len

定義：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長度，在不損失精度的情況下，key_len越短越好。換句話說，key_len顯示的值為索引字段的最大可能長度，即key_len是通過表定義計算而得，不是通過表內檢索出來的，不過具體怎么算出來的沒有研究。

9、ref

定義：顯示索引哪一列別使用了，如果可能的話，是一個常數。指明哪些列或常量被用于查找索引列上的值。

mysql> EXPLAIN SELECT employee.username, employee.age, employee.salary FROM employee, department WHERE employee.id = department.id; +----+-------------+------------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+ | 1 | SIMPLE | department | NULL | index | PRIMARY | PRIMARY | 4 | NULL | 4 | 100.00 | Using index | | 1 | SIMPLE | employee | NULL | eq_ref | PRIMARY | PRIMARY | 4 | db.department.id | 1 | 100.00 | NULL | +----+-------------+------------+------------+--------+---------------+---------+---------+------------------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)

補充說明：ref對呀的值為db.department.id,表示數據庫db中的department表中的id列。

mysql> EXPLAIN SELECT username,age,salary FROM employee where username = 'john'; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)

補充說明：username對應的值為john,所以是常量const

10、rows

定義：根據表的統計信息及索引使用情況，大致估算出找到所需的記錄需要讀取的行數。

11、filtered

定義：這個字段表示存儲引擎返回的數據在server層過濾后，剩下多少滿足查詢的記錄數量的比例，注意是百分比，不是具體記錄數。值越大越好。

12、Extra

定義：包含不適合在其它列中顯示但十分重要的的額外信息

Using filesort：說明Mysql使用一個外部索引排序，而不是按照表內的索引進行排序（Mysql中無法使用索引完成的排序操作稱為“文件排序”）。

mysql> EXPLAIN SELECT username,age,salary FROM employee where username = 'john' order by id; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-----------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-----------------------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index; Using filesort | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-----------------------------+ 1 row in set, 1 warning (0.00 sec)

補充說明：雖然使用了idx_username_age_salary索引，但是排序規則不是按照這個索引來的，而是索引文件外排序。因此sql語句的排序規則盡量按照索引的字段來排序，避免文件外排序，降低性能。

using temporary：表示使用了臨時表保存了中間結果，Mysql在對查詢結果排序時使用臨時表，常見于排序order by和分組gruop by。

mysql> explain select age from employee where username in ('john','asd') group by age; +----+-------------+----------+------------+-------+------------------------------------------------------------+-------------------------+---------+------+------+----------+-------------------------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+-------+------------------------------------------------------------+-------------------------+---------+------+------+----------+-------------------------------------------+ | 1 | SIMPLE | employee | NULL | range | idx_username_age_salary,idx_username,idx_username_departid | idx_username_age_salary | 83 | NULL | 2 | 100.00 | Using where; Using index; Using temporary | +----+-------------+----------+------------+-------+------------------------------------------------------------+-------------------------+---------+------+------+----------+-------------------------------------------+ 1 row in set, 1 warning (0.00 sec)

補充說明：產生臨時表很消耗性能

using index：表示相應的select操作中使用了覆蓋索引，避免訪問了數據行，效率不錯。如果同時出現了using where，表示在索引中查找；如果沒有，表明是從索引中讀取數據，而非查找。

覆蓋索引/索引覆蓋(Covering index)：select數據列只需要從索引中就可以全部獲取，不必讀取數據行，換句話說查詢列要被所建的索引覆蓋。

mysql> explain select username,age,salary from employee where username ='john' order by age,salary; +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | employee | NULL | ref | idx_username_age_salary,idx_username | idx_username_age_salary | 83 | const | 1 | 100.00 | Using index | +----+-------------+----------+------------+------+--------------------------------------+-------------------------+---------+-------+------+----------+-------------+ 1 row in set, 1 warning (0.00 sec)

注意：要想使用覆蓋索引，一定要從select列表中獲取自己想要的列，不可使用select *;如果使用所有列做索引會導致索引過于龐大。

using where：sql語句中使用了where過濾

using join buffer：使用了連接緩存

impossible where：where子句中總是false，不可能查到任何數據

select table optimized away：在沒有group by的情況下，基于索引優化MIN/MAX操作或者對于MyISAM存儲優化count(*)操作，不必等到執行階段在進行計算，執行計劃完成的階段就完成優化。

distinct：優化distinct操作，再找到第一匹配的元組后就停止找同樣值的操作。

總結

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