NOIP 模擬 $14\; \text{隊長快跑}$
阿新 • • 發佈:2021-07-14
慢查詢sql分析器explain
explain select * from student;
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 9795274 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
1 row in set, 1 warning (0.00 sec)
欄位描述:
id:選擇識別符號
select_type:表示查詢的型別。
table:輸出結果集的表
partitions:匹配的分割槽
type:表示表的連線型別
possible_keys:表示查詢時,可能使用的索引
key:表示實際使用的索引
key_len:索引欄位的長度
ref:列與索引的比較
rows:掃描出的行數(估算的行數)
filtered:按表條件過濾的行百分比
Extra:執行情況的描述和說明
id
SELECT識別符 1 id相同時,執行順序由上至下 2 如果是子查詢,id的序號會遞增,id值越大優先順序越高,越先被執行 3 id如果相同,可以認為是一組,從上往下順序執行;在所有組中,id值越大,優先順序越高,越先執行
explain select * from student left join class on class.id=student.class_id where class.id=1;
+----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------+ | 1 | SIMPLE | class | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL | | 1 | SIMPLE | student | NULL | ref | class_age | class_age | 9 | const | 200 | 100.00 | NULL | +----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------+ 2 rows in set, 1 warning (0.00 sec)
select_type
select子句的型別
1 SIMPLE(簡單SELECT,不使用UNION或子查詢等)
2 PRIMARY(子查詢中最外層查詢,查詢中若包含任何複雜的子部分,最外層的select被標記為PRIMARY)
3 UNION(UNION中的第二個或後面的SELECT語句)
4 DEPENDENT UNION(UNION中的第二個或後面的SELECT語句,取決於外面的查詢)
5 UNION RESULT(UNION的結果,union語句中第二個select開始後面所有select)
6 SUBQUERY(子查詢中的第一個SELECT,結果不依賴於外部查詢)
7 DEPENDENT SUBQUERY(子查詢中的第一個SELECT,依賴於外部查詢)
8 DERIVED(派生表的SELECT, FROM子句的子查詢)
9 UNCACHEABLE SUBQUERY(一個子查詢的結果不能被快取,必須重新評估外連結的第一行)
explain select * from student;
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 9795274 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
1 row in set, 1 warning (0.00 sec)
explain select * from student where id in (1,2,3) union select * from student where id in(7,8,9);
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | student | NULL | range | PRIMARY | PRIMARY | 8 | NULL | 3 | 100.00 | Using where |
| 2 | UNION | student | NULL | range | PRIMARY | PRIMARY | 8 | NULL | 3 | 100.00 | Using where |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
explain select * from student where class_id=(select id from class where id=1);
+----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | student | NULL | ref | class_age | class_age | 9 | const | 200 | 100.00 | Using where |
| 2 | SUBQUERY | class | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
table
所訪問資料庫中表名稱
explain select * from student where id in (1,2);
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | student | NULL | range | PRIMARY | PRIMARY | 8 | NULL | 2 | 100.00 | Using where |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
type
訪問型別
常用的型別有: ALL、index、range、 ref、eq_ref、const、system、NULL(從左到右,效能從差到好)
ALL:Full Table Scan, MySQL將遍歷全表以找到匹配的行
index: Full Index Scan,index與ALL區別為index型別只遍歷索引樹
range:只檢索給定範圍的行,使用一個索引來選擇行
ref: 表示上述表的連線匹配條件,即哪些列或常量被用於查詢索引列上的值
eq_ref: 類似ref,區別就在使用的索引是唯一索引,對於每個索引鍵值,表中只有一條記錄匹配,簡單來說,就是多表連線中使用primary key或者 unique key作為關聯條件
const、system: 當MySQL對查詢某部分進行優化,並轉換為一個常量時,使用這些型別訪問。如將主鍵置於where列表中,MySQL就能將該查詢轉換為一個常量,system是const型別的特例,當查詢的表只有一行的情況下,使用system
NULL: MySQL在優化過程中分解語句,執行時甚至不用訪問表或索引,例如從一個索引列裡選取最小值可以通過單獨索引查詢完成
explain select * from student;
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
| 1 | SIMPLE | student | NULL | ALL | NULL | NULL | NULL | NULL | 9795274 | 100.00 | NULL |
+----+-------------+---------+------------+------+---------------+------+---------+------+---------+----------+-------+
1 row in set, 1 warning (0.00 sec)
explain select id from student;
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+---------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+---------+----------+-------------+
| 1 | SIMPLE | student | NULL | index | NULL | class_age | 11 | NULL | 9795274 | 100.00 | Using index |
+----+-------------+---------+------------+-------+---------------+-----------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
explain select * from student where id <10;
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | student | NULL | range | PRIMARY | PRIMARY | 8 | NULL | 9 | 100.00 | Using where |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
explain select * from student left join class on class.id=student.class_id where class.id=1;
+----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | class | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
| 1 | SIMPLE | student | NULL | ref | class_age | class_age | 9 | const | 200 | 100.00 | NULL |
+----+-------------+---------+------------+-------+---------------+-----------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
explain select * from student inner join class on class.id=student.class_id where student.id in(1,2);
+----+-------------+---------+------------+--------+-------------------+---------+---------+------------------------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+--------+-------------------+---------+---------+------------------------------+------+----------+-------------+
| 1 | SIMPLE | student | NULL | range | PRIMARY,class_age | PRIMARY | 8 | NULL | 2 | 100.00 | Using where |
| 1 | SIMPLE | class | NULL | eq_ref | PRIMARY | PRIMARY | 8 | school_info.student.class_id | 1 | 100.00 | Using where |
+----+-------------+---------+------------+--------+-------------------+---------+---------+------------------------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
explain select * from student where id=1;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | student | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
possible_keys
可能使用到的索引
explain select * from student where id=1;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | student | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
Key
實際使用到的索引
explain select * from student where id=1;
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | student | NULL | const | PRIMARY | PRIMARY | 8 | const | 1 | 100.00 | NULL |
+----+-------------+---------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
ref
列與索引的比較,表示上述表的連線匹配條件,即哪些列或常量被用於查詢索引列上的值
rows
估算出結果集行數,表示MySQL根據表統計資訊及索引選用情況,估算的找到所需的記錄所需要讀取的行數
Extra
額外重要資訊
Using where:不用讀取表中所有資訊,僅通過索引就可以獲取所需資料,這發生在對錶的全部的請求列都是同一個索引的部分的時候,表示mysql伺服器將在儲存引擎檢索行後再進行過濾
Using temporary:表示MySQL需要使用臨時表來儲存結果集,常見於排序和分組查詢,常見 group by ; order by
Using filesort:當Query中包含 order by 操作,而且無法利用索引完成的排序操作稱為“檔案排序”
Using join buffer:改值強調了在獲取連線條件時沒有使用索引,並且需要連線緩衝區來儲存中間結果。如果出現了這個值,那應該注意,根據查詢的具體情況可能需要新增索引來改進能。
explain select * from student where id < 10 order by name;
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------------+
| 1 | SIMPLE | student | NULL | range | PRIMARY | PRIMARY | 8 | NULL | 9 | 100.00 | Using where; Using filesort |
+----+-------------+---------+------------+-------+---------------+---------+---------+------+------+----------+-----------------------------+
1 row in set, 1 warning (0.00 sec)
explain select count(*) from student where id<200 group by class_id order by class_id;
+----+-------------+---------+------------+-------+-------------------+---------+---------+------+------+----------+----------------------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+---------+------------+-------+-------------------+---------+---------+------+------+----------+----------------------------------------------+
| 1 | SIMPLE | student | NULL | range | PRIMARY,class_age | PRIMARY | 8 | NULL | 199 | 100.00 | Using where; Using temporary; Using filesort |
+----+-------------+---------+------------+-------+-------------------+---------+---------+------+------+----------+----------------------------------------------+
1 row in set, 1 warning (0.00 sec)