MySQL EXPLAIN結果集分析 - 附帶大量案例

時間 2019-11-06

原文原文鏈接

大量實例助你看懂Explain的輸出內容，輕鬆搞定慢查詢mysql

EXPLAIN：查看SQL語句的執行計劃算法

EXPLAIN命令能夠幫助咱們深刻了解MySQL基於開銷的優化器，還能夠得到不少可能被優化器考慮到的訪問策略的細節，以及當運行SQL語句時哪一種策略預計會被優化器採用，在優化慢查詢時很是有用sql

執行explain以後結果集包含以下信息bash

+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows   | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+--------+----------+-------+
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下面將對每個值進行解釋mysql優化

一、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，該值可能爲NULL運維

id若是相同，從上往下依次執行。id不一樣，id值越大，執行優先級越高，若是行引用其餘行的並集結果，則該值能夠爲NULLoop

二、select_type

select_type表示查詢使用的類型，有下面幾種：性能

simple： 簡單的select查詢，沒有union或者子查詢優化

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  |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
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primary： 最外層的select查詢ui

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  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
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union： union中的第二個或隨後的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 |
+----+--------------+------------+-------+---------------+---------+---------+-------+-------+-----------------+
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dependent union： union中的第二個或隨後的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 |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
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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 |
+----+-------------+-------+-------+---------------+---------+---------+-------+------+-------------+
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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 |
+----+--------------------+------------+--------+---------------+---------+---------+-------+-------+-----------------+
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derived： 用於from子句中有子查詢的狀況，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  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
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三、table

table用來表示輸出行所引用的表名

四、type（重要）

type表示訪問類型，下面依次解釋各類類型，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  |
+----+-------------+------------+--------+---------------+---------+---------+-------+------+-------+
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由於子查詢只有一行數據，模擬了單表只有一行數據，此時type爲system

const： 肯定只有一行匹配的時候，mysql優化器會在查詢前讀取它而且只讀取一次，速度很是快

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)
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eq_ref： 對於每一個來自於前面的表的行組合，從該表中讀取一行，經常使用在一個索引是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  |
+----+-------------+-------+--------+---------------+--------------+---------+--------------------+-------+-------+
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ref： 對於來自前面的表的行組合，全部有匹配索引值的行都從這張表中讀取,若是聯接只使用鍵的最左邊的前綴，或若是鍵不是UNIQUE或PRIMARY KEY（換句話說，若是聯接不能基於關鍵字選擇單個行的話），則使用ref

ref能夠用於使用=或<=>操做符的帶索引的列

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 |
+----+-------------+-------+------+---------------+---------+---------+-------------------+-------+-----------------------+
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test表bnet_id不是索引，test2表aid爲索引列

ref_or_null： 相似ref，可是添加了能夠專門搜索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 |
+----+-------------+-------+-------------+---------------+----------+---------+-------+------+-----------------------+
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前提爲bnet_id列爲索引，且bnet_id列有null值

index_merge： 該訪問類型使用了索引合併優化方法，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 |
+----+-------------+-------+-------------+------------------+------------------+---------+------+------+--------------------------------------------+
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前提條件爲id列和bnet_id列都有單列索引。若是出現index_merge，而且這類SQL後期使用較頻繁，能夠考慮把單列索引換爲組合索引，這樣效率更高

range： 只檢索給定範圍的行，使用一個索引來選擇行。key列顯示使用了哪一個索引。key_len包含所使用索引的最長關鍵元素。在該類型中ref列爲NULL

當使用=、<>、>、>=、<、<=、IS NULL、<=>、BETWEEN或者IN操做符，用常量比較關鍵字列時，可使用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 |
+----+-------------+-------+-------+---------------+----------+---------+------+------+-----------------------+
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前提條件爲bnet_id列有索引

index： 在進行統計時很是常見，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 |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
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all： 對於每一個來自於先前的表的行組合，進行完整的表掃描，一般能夠增長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 |
+----+-------------+-------+------+---------------+------+---------+------+-------+-------------+
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五、possible_keys

possible_keys是指在這個SQL中，mysql可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，但不必定被查詢使用.若爲空則表示沒有可使用的索引，此時能夠經過檢查where語句看是否能夠引用某些列或者新建索引來提升性能。

六、key（重要）

key列顯示的是當前表實際使用的索引，若是沒有選擇索引，則此列爲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

七、key_len

key_len列顯示MySQL決定使用的鍵長度。若是KEY鍵是NULL，則長度爲NULL。在不損失精確性的狀況下，長度越短越好

key len的長度還和字符集有關,latin1一個字符佔用1個字節,gbk一個字符佔用2個字節,utf8一個字符佔用3個字節。key_len的計算法方法：

列類型	KEY_LEN	備註
id int	key_len = 4+1	int爲4bytes,容許爲NULL,加1byte
id bigint not null	key_len=8	bigint爲8bytes
user char(30) utf8	key_len=30*3+1	utf8每一個字符爲3bytes,容許爲NULL,加1byte
user varchar(30) not null utf8	key_len=30*3+2	utf8每一個字符爲3bytes,變長數據類型,加2bytes
user varchar(30) utf8	key_len=30*3+2+1	utf8每一個字符爲3bytes,容許爲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,被視爲動態列類型。

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

八、ref

ref列用來顯示使用哪一個列或常數與key一塊兒從表中選擇相應的行。它顯示的列的名字（或const），此列多數時候爲null

九、rows

rows列顯示的是mysql解析器認爲執行此SQL時必須掃描的行數。此數值爲一個預估值，不是具體值，一般比實際值小

十、filtered

此參數爲mysql 5.7 新加參數，指的是返回結果的行數所佔須要讀到的行（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

十一、extra（重要）

extra表示不在其餘列而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（覆蓋索引是指能夠直接在索引列中獲得想要的結果，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 |
+----+-------------+-------+-------+---------------+----------+---------+------+-------+-------------+
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這個例子中id字段爲主鍵，可是key那裏顯示走的並非主鍵索引，這個是由於mysql的全部二級索引中都會包含全部的主鍵信息，而mysql沒有單獨的存儲主鍵索引，因此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後進行了後過濾(POST-FILTER)，若是查詢未能使用索引，using where的做用只是提醒咱們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 |
+----+-------------+-------+-------+---------------+---------+---------+------+-------+-------------+
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using temporary 表示mysql須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 |
+----+-------------+-------+-------+-----------------------------------------+---------+---------+------+------+----------------------------------------------+
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using filesort： 是指mysql沒法利用索引直接完成排序（排序的字段不是索引字段），此時會用到緩衝空間來進行排序

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 |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------+
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using join buffer： 強調在獲取鏈接條件時沒有用到索引，而且須要鏈接緩衝區來存儲中間結果。（性能能夠經過添加索引或者修改鏈接字段改進）

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)
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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中的記錄作比較，從而減小內層循環的次數.

impossible where： 表示where條件致使沒有返回的行

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 |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
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using index condition： 是mysql 5.6 以後新加的特性，結合mysql的ICP（Index Condition Pushdown）特性使用。主要是優化了能夠在索引（僅限二級索引）上進行 like 查找

若是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