大量實例助你看懂Explain的輸出內容，輕松搞定慢查詢
EXPLAIN：查看SQL語句的執行計划
EXPLAIN命令可以幫助我們深入了解MySQL基於開銷的優化器，還可以獲得很多可能被優化器考慮到的訪問策略的細節，以及當運行SQL語句時哪種策略預計會被優化器采用，在優化慢查詢時非常有用
執行explain之后結果集包含如下信息
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+
下面將對每一個值進行解釋
1、id
id用來標識整個查詢中SELELCT語句的順序，在嵌套查詢中id越大的語句越先執行，該值可能為NULL
id如果相同，從上往下依次執行。id不同，id值越大，執行優先級越高，如果行引用其他行的並集結果，則該值可以為NULL
2、select_type
select_type表示查詢使用的類型，有下面幾種：
simple： 簡單的select查詢，沒有union或者子查詢
mysql> explain select * from test where id = 1000;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | test | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
primary： 最外層的select查詢
mysql> explain select * from (select * from test where id = 1000) a;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | NULL |
| 2 | DERIVED | test | const | PRIMARY | PRIMARY | 8 | const | 1 | NULL |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
union： union中的第二個或隨后的select查詢，不依賴於外部查詢的結果集
mysql> explain select * from test where id = 1000 union all select * from test2 ;
+----+--------------+------------+-------+---------------+---------+---------+-------+-------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+-------+---------------+---------+---------+-------+-------+-----------------+
| 1 | PRIMARY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | NULL |
| 2 | UNION | test2 | ALL | NULL | NULL | NULL | NULL | 67993 | NULL |
| NULL | UNION RESULT | <union1,2> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+-------+---------------+---------+---------+-------+-------+-----------------+
dependent union： union中的第二個或隨后的select查詢，依賴於外部查詢的結果集
mysql> explain select * from test where id in (select id from test where id = 1000 union all select id from test2) ;
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
| 1 | PRIMARY | test | ALL | NULL | NULL | NULL | NULL | 68505 | Using where |
| 2 | DEPENDENT SUBQUERY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | Using index |
| 3 | DEPENDENT UNION | test2 | eq_ref | PRIMARY | PRIMARY | 8 | func | 1 | Using index |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
subquery： 子查詢中的第一個select查詢，不依賴與外部查詢的結果集
mysql> explain select * from test where id = (select id from test where id = 1000);
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | PRIMARY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | NULL |
| 2 | SUBQUERY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
dependent subquery： 子查詢中的第一個select查詢，依賴於外部查詢的結果集
mysql> explain select * from test where id in (select id from test where id = 1000 union all select id from test2) ;
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
| 1 | PRIMARY | test | ALL | NULL | NULL | NULL | NULL | 68505 | Using where |
| 2 | DEPENDENT SUBQUERY | test | const | PRIMARY | PRIMARY | 8 | const | 1 | Using index |
| 3 | DEPENDENT UNION | test2 | eq_ref | PRIMARY | PRIMARY | 8 | func | 1 | Using index |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
derived： 用於from子句中有子查詢的情況，mysql會遞歸執行這些子查詢，此結果集放在臨時表中
mysql> explain select * from (select * from test2 where id = 1000)a;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | NULL |
| 2 | DERIVED | test2 | const | PRIMARY | PRIMARY | 8 | const | 1 | NULL |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
3、table
table用來表示輸出行所引用的表名
4、type（重要）
type表示訪問類型，下面依次解釋各種類型，類型順序從最好到最差排列
system： 表僅有一行，是const類型的一個特例
mysql> explain select * from (select * from test2 where id = 1000)a;
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
| 1 | PRIMARY | <derived2> | system | NULL | NULL | NULL | NULL | 1 | NULL |
| 2 | DERIVED | test2 | const | PRIMARY | PRIMARY | 8 | const | 1 | NULL |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
因為子查詢只有一行數據，模擬了單表只有一行數據，此時type為system
const： 確定只有一行匹配的時候，mysql優化器會在查詢前讀取它並且只讀取一次，速度非常快
mysql> explain select * from test where id =1 ;
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | test | const | PRIMARY | PRIMARY | 8 | const | 1 | NULL |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
eq_ref： 對於每個來自於前面的表的行組合，從該表中讀取一行，常用在一個索引是unique key或者primary key
mysql> explain select * from test,test2 where test.com_key=test2.com_key;
+----+-------------+-------+--------+---------------+--------------+---------+--------------------+-------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+--------------+---------+--------------------+-------+-------+
| 1 | SIMPLE | test2 | ALL | IDX(com_key) | NULL | NULL | NULL | 67993 | NULL |
| 1 | SIMPLE | test | eq_ref | IDX(com_key) | IDX(com_key) | 194 | test.test2.com_key | 1 | NULL |
+----+-------------+-------+--------+---------------+--------------+---------+--------------------+-------+-------+
ref： 對於來自前面的表的行組合，所有有匹配索引值的行都從這張表中讀取,如果聯接只使用鍵的最左邊的前綴，或如果鍵不是UNIQUE或PRIMARY KEY（換句話說，如果聯接不能基於關鍵字選擇單個行的話），則使用ref
ref可以用於使用=或<=>操作符的帶索引的列
mysql> explain select * from test ,test2 where test.bnet_id=test2.aid;
+----+-------------+-------+------+---------------+---------+---------+-------------------+-------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+---------+---------+-------------------+-------+-----------------------+
| 1 | SIMPLE | test | ALL | NULL | NULL | NULL | NULL | 68505 | Using where |
| 1 | SIMPLE | test2 | ref | idx_aid | idx_aid | 5 | test.test.bnet_id | 34266 | Using index condition |
+----+-------------+-------+------+---------------+---------+---------+-------------------+-------+-----------------------+
test表bnet_id不是索引，test2表aid為索引列
ref_or_null： 類似ref，但是添加了可以專門搜索null值的行
mysql> explain select * from test where bnet_id=1 or bnet_id is null;
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+-----------------------+
| 1 | SIMPLE | test | ref_or_null | idx_bnet | idx_bnet | 9 | const | 2 | Using index condition |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+-----------------------+
前提為bnet_id列為索引，且bnet_id列有null值
index_merge： 該訪問類型使用了索引合並優化方法，key列包含了使用的索引的清單，key_len包含了使用的索引的最長的關鍵元素
mysql> explain select * from test where id = 1 or bnet_id = 1;
+----+-------------+-------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
| 1 | SIMPLE | test | index_merge | PRIMARY,idx_bnet | PRIMARY,idx_bnet | 8,9 | NULL | 2 | Using union(PRIMARY,idx_bnet); Using where |
+----+-------------+-------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
前提條件為id列和bnet_id列都有單列索引。如果出現index_merge，並且這類SQL后期使用較頻繁，可以考慮把單列索引換為組合索引，這樣效率更高
range： 只檢索給定范圍的行，使用一個索引來選擇行。key列顯示使用了哪個索引。key_len包含所使用索引的最長關鍵元素。在該類型中ref列為NULL
當使用=、<>、>、>=、<、<=、IS NULL、<=>、BETWEEN或者IN操作符，用常量比較關鍵字列時，可以使用range
mysql> explain select * from test where bnet_id > 1000 and bnet_id < 10000;
+----+-------------+-------+-------+---------------+----------+---------+------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-----------------------+
| 1 | SIMPLE | test | range | idx_bnet | idx_bnet | 9 | NULL | 1 | Using index condition |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-----------------------+
前提條件為bnet_id列有索引
index： 在進行統計時非常常見，此聯接類型實際上會掃描索引樹
mysql> explain select count(*) from test;
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
| 1 | SIMPLE | test | index | NULL | idx_bnet | 9 | NULL | 68505 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
all： 對於每個來自於先前的表的行組合，進行完整的表掃描，通常可以增加更多的索引而不要使用ALL，使得行能基於前面的表中的常數值或列值被檢索出
mysql> explain select * from test where create_time = '0000-00-00 00:00:00';
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
| 1 | SIMPLE | test | ALL | NULL | NULL | NULL | NULL | 68505 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
5、possible_keys
possible_keys是指在這個SQL中，mysql可以使用這個索引去輔助查找記錄，當查詢涉及到的字段，都會被列出，但不一定被查詢使用.若為空則表示沒有可以使用的索引，此時可以通過檢查where語句看是否可以引用某些列或者新建索引來提高性能。
6、key（重要）
key列顯示的是當前表實際使用的索引，如果沒有選擇索引，則此列為null，要想強制MySQL使用或忽視possible_keys列中的索引，在查詢中使用FORCE INDEX、USE INDEX或者IGNORE INDEX
7、key_len
key_len列顯示MySQL決定使用的鍵長度。如果KEY鍵是NULL，則長度為NULL。在不損失精確性的情況下，長度越短越好
key len的長度還和字符集有關,latin1一個字符占用1個字節,gbk一個字符占用2個字節,utf8一個字符占用3個字節。key_len的計算法方法：
| 列類型 | KEY_LEN | 備注 |
|---|---|---|
| id int | key_len = 4+1 | int為4bytes,允許為NULL,加1byte |
| id bigint not null | key_len=8 | bigint為8bytes |
| user char(30) utf8 | key_len=30*3+1 | utf8每個字符為3bytes,允許為NULL,加1byte |
| user varchar(30) not null utf8 | key_len=30*3+2 | utf8每個字符為3bytes,變長數據類型,加2bytes |
| user varchar(30) utf8 | key_len=30*3+2+1 | utf8每個字符為3bytes,允許為NULL,加1byte,變長數據類型,加2bytes |
| detail text(10) utf8 | key_len=30*3+2+1 | TEXT截取部分,被視為動態列類型。 |
key_len只指示了where中用於條件過濾時被選中的索引列，是不包含order by或group by這一部分被選中的索引列
8、ref
ref列用來顯示使用哪個列或常數與key一起從表中選擇相應的行。它顯示的列的名字（或const），此列多數時候為null
9、rows
rows列顯示的是mysql解析器認為執行此SQL時必須掃描的行數。此數值為一個預估值，不是具體值，通常比實際值小
10、filtered
此參數為mysql 5.7 新加參數，指的是返回結果的行數所占需要讀到的行（rows的值）的比例
對於使用join時，前一個表的結果集大小直接影響了循環的行數
11、extra（重要）
extra表示不在其他列並且也很重要的額外信息
using index： 該值表示這個SQL語句使用了覆蓋索引（覆蓋索引是指可以直接在索引列中得到想要的結果，而不用去回表），此時效率最高
mysql> explain select id from test;
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
| 1 | SIMPLE | test | index | NULL | idx_bnet | 9 | NULL | 68505 | Using index |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
這個例子中id字段為主鍵，但是key那里顯示走的並不是主鍵索引，這個是因為mysql的所有二級索引中都會包含所有的主鍵信息，而mysql沒有單獨的存儲主鍵索引，所以掃描二級索引的開銷比全表掃描更快
using where： 表示存儲引擎搜到記錄后進行了后過濾(POST-FILTER)，如果查詢未能使用索引，using where的作用只是提醒我們mysql要用where條件過濾結果集
mysql> explain select * from test where id > 1;
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
| 1 | SIMPLE | test | range | PRIMARY | PRIMARY | 8 | NULL | 34252 | Using where |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
using temporary 表示mysql需要使用臨時表來存儲結果集，常見於排序和分組查詢
mysql> explain select * from test where id in (1,2) group by bnet_id;
+----+-------------+-------+-------+-----------------------------------------+---------+---------+------+------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+-----------------------------------------+---------+---------+------+------+----------------------------------------------+
| 1 | SIMPLE | test | range | PRIMARY,IDX(event_key-bnet_Id),idx_bnet | PRIMARY | 8 | NULL | 2 | Using where; Using temporary; Using filesort |
+----+-------------+-------+-------+-----------------------------------------+---------+---------+------+------+----------------------------------------------+
using filesort： 是指mysql無法利用索引直接完成排序（排序的字段不是索引字段），此時會用到緩沖空間來進行排序
mysql> explain select * from test order by bnet_id;
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------+
| 1 | SIMPLE | test | ALL | NULL | NULL | NULL | NULL | 68505 | Using filesort |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------+
using join buffer： 強調在獲取連接條件時沒有用到索引，並且需要連接緩沖區來存儲中間結果。（性能可以通過添加索引或者修改連接字段改進）
mysql> explain select * from test left join test2 on test.create_time = test2.create_time;
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+
| 1 | SIMPLE | test | NULL | ALL | NULL | NULL | NULL | NULL | 959692 | 100.00 | NULL |
| 1 | SIMPLE | test2 | NULL | ALL | NULL | NULL | NULL | NULL | 958353 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+----------------------------------------------------+
2 rows in set, 1 warning (0.00 sec)
Block Nested Loop是指Block Nested-Loop Join算法：將外層循環的行/結果集存入join buffer, 內層循環的每一行與整個buffer中的記錄做比較，從而減少內層循環的次數.
impossible where： 表示where條件導致沒有返回的行
mysql> explain select * from test where id is null;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
using index condition： 是mysql 5.6 之后新加的特性，結合mysql的ICP（Index Condition Pushdown）特性使用。主要是優化了可以在索引（僅限二級索引）上進行 like 查找
如果extra中出現多個上面結果，則表示順序使用上面的方法進行解析查詢
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