执行计划收集
DAO_OBJECTS t1 , DAO_OBJECTS t2 WHERE deptno= t1.object_id AND t1.object_name=t2.object_name ;
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------------------------------------------+
| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 49793 | NULL |
| 1 | SIMPLE | EMP | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where; Using join buffer (Block Nested Loop) |
| 1 | SIMPLE | t2 | ALL | NULL | NULL | NULL | NULL | 49793 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------------------------------------------+
执行计划列详解
ID
ID列表明了该语句所在的层级 如果ID相同从上到下执行 如果ID不同则ID越大的越先执行 其作用类似于Oracle执行计划中缩进
mysql> explain SELECT * FROM EMP , DAO_OBJECTS1 t1 , DAO_OBJECTS2 t2 WHERE deptno= t1.object_id AND t1.object_name=t2.object_name ;
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------------------------------------------+
| 1 | SIMPLE | t1 | ALL | NULL | NULL | NULL | NULL | 49793 | NULL |
| 1 | SIMPLE | EMP | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where; Using join buffer (Block Nested Loop) |
| 1 | SIMPLE | t2 | ALL | NULL | NULL | NULL | NULL | 49793 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------+---------------+------+---------+------+-------+----------------------------------------------------+
- 语义等价查询
mysql> EXPLAIN
-> SELECT *
-> FROM (SELECT * FROM EMP , DAO_OBJECTS t1 WHERE DEPTNO=t1.object_id) V1 , DAO_OBJECTS t2
-> WHERE V1.object_id =t2.object_id ;
+----+-------------+------------+------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+
| 1 | PRIMARY | t2 | ALL | NULL | NULL | NULL | NULL | 49793 | Using where |
| 1 | PRIMARY | <derived2> | ref | <auto_key0> | <auto_key0> | 5 | dao.t2.OBJECT_ID | 11 | NULL |
| 2 | DERIVED | t1 | ALL | NULL | NULL | NULL | NULL | 49793 | NULL |
| 2 | DERIVED | EMP | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+------------+------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+
以上执行计划ID发生改变
整个执行计划顺序为
t1 与EMP关联 得到的结果集与t2关联 (t1 join EMP) join t2
不过执行计划有所差异 而就结果来说应该是完全相同的
select_type
SIMPLE
简单查询不包含子查询与union
mysql> explain SELECT * FROM EMP ;
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
| 1 | SIMPLE | EMP | ALL | NULL | NULL | NULL | NULL | 14 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)
PRIMARY
为复杂查询创建的首要表(也是最外层的表) 这种类型通常与DERIVED 或者union 混合使用见到
mysql> explain
-> SELECT *
-> FROM (SELECT * FROM EMP , DAO_OBJECTS t1 WHERE DEPTNO=t1.object_id) V1 , DAO_OBJECTS t2 ,DEPT t3
-> WHERE V1.object_id =t2.object_id and t3.DEPTNO= V1.DEPTNO ;
+----+-------------+------------+------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+
| 1 | PRIMARY | t2 | ALL | NULL | NULL | NULL | NULL | 49793 | Using where |
| 1 | PRIMARY | <derived2> | ref | <auto_key1> | <auto_key1> | 5 | dao.t2.OBJECT_ID | 16 | NULL |
| 1 | PRIMARY | t3 | ALL | PRIMARY | NULL | NULL | NULL | 4 | Using where; Using join buffer (Block Nested Loop) |
| 2 | DERIVED | t1 | ALL | NULL | NULL | NULL | NULL | 49793 | NULL |
| 2 | DERIVED | EMP | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where; Using join buffer (Block Nested Loop) |
+----+-------------+------------+------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+
5 rows in set (0.00 sec)
可以这么理解 primary 就是SQL在有子查询与union时 最外层的表 第一行(T4 )或者结果集 第二行(
UNION
当SELECT 之前的关键字为UNION 或 UNION ALL时 会出现UNION 关键字
mysql> EXPLAIN SELECT OBJECT_ID FROM DAO_OBJECTS1 UNION SELECT DEPTNO FROM EMP UNION SELECT OBJECT_ID FROM DAO_OBJECTS2
-> ;
+----+--------------+--------------+-------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+--------------+-------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 1 | NULL |
| 2 | UNION | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| 3 | UNION | DAO_OBJECTS2 | ALL | NULL | NULL | NULL | NULL | 1 | NULL |
| NULL | UNION RESULT | <union1,2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+--------------+-------+---------------+-----------+---------+------+------+-----------------+
4 rows in set (0.00 sec)
SELECT *
FROM DAO_OBJECTS1
UNION ALL
SELECT *
FROM DAO_OBJECTS2 ) V1
WHERE V1' at line 4
mysql> EXPLAIN
-> SELECT *
-> FROM DEPT D,
-> (SELECT *
-> FROM DAO_OBJECTS1
-> UNION ALL
-> SELECT *
-> FROM DAO_OBJECTS2 ) V1
-> WHERE V1.OBJECT_ID=D.DEPTNO ;
+----+--------------+--------------+------+---------------+-------------+---------+--------------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+--------------+------+---------------+-------------+---------+--------------+------+-----------------+
| 1 | PRIMARY | D | ALL | PRIMARY | NULL | NULL | NULL | 4 | NULL |
| 1 | PRIMARY | <derived2> | ref | <auto_key0> | <auto_key0> | 5 | dao.D.DEPTNO | 2 | NULL |
| 2 | DERIVED | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 1 | NULL |
| 3 | UNION | DAO_OBJECTS2 | ALL | NULL | NULL | NULL | NULL | 1 | NULL |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+--------------+------+---------------+-------------+---------+--------------+------+-----------------+
5 rows in set (0.00 sec)
DEPENDENT UNION
当子查询中存在UNION 时 UNION 后的SELECT TYPE 会出现 DEPENDENT UNION 而union 语句的第一行为 DEPENDENT SUBQUERY
mysql> EXPLAIN SELECT * FROM DEPT D WHERE D.DEPTNO IN (SELECT DEPTNO FROM EMP UNION SELECT OBJECT_ID FROM DAO_OBJECTS1) ;
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | D | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
| 2 | DEPENDENT SUBQUERY | EMP | ref | FK_DEPTNO | FK_DEPTNO | 5 | func | 2 | Using index |
| 3 | DEPENDENT UNION | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
4 rows in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM DEPT D WHERE D.DEPTNO IN ( SELECT OBJECT_ID FROM DAO_OBJECTS1 UNION SELECT DEPTNO FROM EMP ) ;
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | D | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
| 2 | DEPENDENT SUBQUERY | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| 3 | DEPENDENT UNION | EMP | ref | FK_DEPTNO | FK_DEPTNO | 5 | func | 2 | Using index |
| NULL | UNION RESULT | <union2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
4 rows in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM DEPT D WHERE D.DEPTNO IN ( SELECT OBJECT_ID FROM DAO_OBJECTS1 UNION SELECT DEPTNO FROM EMP UNION SELECT OBJECT_ID FROM DAO_OBJECTS2) ;
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | D | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
| 2 | DEPENDENT SUBQUERY | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| 3 | DEPENDENT UNION | EMP | ref | FK_DEPTNO | FK_DEPTNO | 5 | func | 2 | Using index |
| 4 | DEPENDENT UNION | DAO_OBJECTS2 | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| NULL | UNION RESULT | <union2,3,4> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
5 rows in set (0.00 sec)
UNION RESULT
mysql> EXPLAIN SELECT OBJECT_ID FROM DAO_OBJECTS1 UNION SELECT DEPTNO FROM EMP UNION SELECT OBJECT_ID FROM DAO_OBJECTS2 ;
+----+--------------+--------------+-------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+--------------+-------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 1 | NULL |
| 2 | UNION | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| 3 | UNION | DAO_OBJECTS2 | ALL | NULL | NULL | NULL | NULL | 1 | NULL |
| NULL | UNION RESULT | <union1,2,3> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+--------------+-------+---------------+-----------+---------+------+------+-----------------+
4 rows in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM DEPT D WHERE D.DEPTNO IN ( SELECT OBJECT_ID FROM DAO_OBJECTS1 UNION SELECT DEPTNO FROM EMP UNION SELECT OBJECT_ID FROM DAO_OBJECTS2) ;
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | D | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
| 2 | DEPENDENT SUBQUERY | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| 3 | DEPENDENT UNION | EMP | ref | FK_DEPTNO | FK_DEPTNO | 5 | func | 2 | Using index |
| 4 | DEPENDENT UNION | DAO_OBJECTS2 | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| NULL | UNION RESULT | <union2,3,4> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+--------------+------+---------------+-----------+---------+------+------+-----------------+
5 rows in set (0.00 sec)
SUBQUERY
出现在复杂非相关子查询中 ,简单相关子查询MySQL会进行改写
mysql> EXPLAIN
-> SELECT *
-> FROM EMP
-> WHERE EMPNO IN ( SELECT COUNT(*) FROM DAO_OBJECTS1 ) ;
+----+-------------+--------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+------+------+-------------+
| 1 | PRIMARY | EMP | ALL | NULL | NULL | NULL | NULL | 14 | Using where |
| 2 | SUBQUERY | DAO_OBJECTS1 | index | NULL | PRIMARY | 32 | NULL | 1 | Using index |
+----+-------------+--------------+-------+---------------+---------+---------+------+------+-------------+
2 rows in set (0.03 sec)
mysql> EXPLAIN SELECT * FROM DEPT D WHERE D.DEPTNO IN (SELECT DEPTNO FROM EMP E ) ;
+----+-------------+-------+------+---------------+-----------+---------+--------------+------+----------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+-----------+---------+--------------+------+----------------------------+
| 1 | SIMPLE | D | ALL | PRIMARY | NULL | NULL | NULL | 4 | NULL |
| 1 | SIMPLE | E | ref | FK_DEPTNO | FK_DEPTNO | 5 | dao.D.DEPTNO | 2 | Using index; FirstMatch(D) |
+----+-------------+-------+------+---------------+-----------+---------+--------------+------+----------------------------+
2 rows in set (0.00 sec)
注意:这里发生了改写 变成了关联 不过是特殊关联 Extra 部分 FirstMatch(D) 只取一条
DEPENDENT SUBQUERY
出现在相关子查询中 而非相关子查询MySQL可以进行改写
mysql> EXPLAIN SELECT * FROM DEPT D WHERE EXISTS (SELECT DEPTNO FROM EMP E WHERE D.DEPTNO=E.DEPTNO) ;
+----+--------------------+-------+------+---------------+-----------+---------+--------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-------+------+---------------+-----------+---------+--------------+------+-------------+
| 1 | PRIMARY | D | ALL | NULL | NULL | NULL | NULL | 4 | Using where |
| 2 | DEPENDENT SUBQUERY | E | ref | FK_DEPTNO | FK_DEPTNO | 5 | dao.D.DEPTNO | 2 | Using index |
+----+--------------------+-------+------+---------------+-----------+---------+--------------+------+-------------+
2 rows in set (0.00 sec)
DERIVED
衍生表 当查询使用内联视图时 会出现此关键字
mysql> EXPLAIN
-> SELECT *
-> FROM EMP T1, (SELECT T2.DEPTNO, AVG(SAL) AVG_SAL FROM EMP T2 GROUP BY T2.DEPTNO) V1
-> WHERE T1.DEPTNO=V1.DEPTNO
-> AND T1.SAL < AVG_SAL ;
+----+-------------+------------+-------+---------------+-------------+---------+---------------+------+--------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+-------+---------------+-------------+---------+---------------+------+--------------------------+
| 1 | PRIMARY | T1 | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where |
| 1 | PRIMARY | <derived2> | ref | <auto_key0> | <auto_key0> | 5 | dao.T1.DEPTNO | 2 | Using where; Using index |
| 2 | DERIVED | T2 | index | FK_DEPTNO | FK_DEPTNO | 5 | NULL | 14 | NULL |
+----+-------------+------------+-------+---------------+-------------+---------+---------------+------+--------------------------+
3 rows in set (0.00 sec)
MATERIALIZED
子查询物化 当表出现在非相关子查询中 并且需要进行物化时会出现MATERIALIZED关键词
mysql> EXPLAIN
-> SELECT *
-> FROM EMP
-> WHERE EMPNO IN (SELECT OBJECT_ID
-> FROM DAO_OBJECTS1 T1
-> WHERE EXISTS (SELECT 1
-> FROM DEPT D
-> WHERE T1.OBJECT_ID = D.DEPTNO) );
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+-------------+
| 1 | PRIMARY | <subquery2> | ALL | NULL | NULL | NULL | NULL | NULL | Using where |
| 1 | PRIMARY | EMP | eq_ref | PRIMARY | PRIMARY | 4 | <subquery2>.OBJECT_ID | 1 | NULL |
| 2 | MATERIALIZED | T1 | ALL | NULL | NULL | NULL | NULL | 1 | Using where |
| 3 | DEPENDENT SUBQUERY | D | eq_ref | PRIMARY | PRIMARY | 4 | dao.T1.OBJECT_ID | 1 | Using index |
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+-------------+
4 rows in set (0.01 sec)
mysql> EXPLAIN EXTENDED
-> SELECT *
-> FROM EMP
-> WHERE EMPNO IN (SELECT T1.OBJECT_ID
-> FROM DAO_OBJECTS1 T1 ,DAO_OBJECTS1 T2
-> WHERE T1.OBJECT_ID =T2.OBJECT_ID
-> AND EXISTS (SELECT 1
-> FROM DEPT D
-> WHERE D.DEPTNO= T1.OBJECT_ID )
-> );
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+----------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+----------+----------------------------------------------------+
| 1 | PRIMARY | <subquery2> | ALL | NULL | NULL | NULL | NULL | NULL | 0.00 | Using where |
| 1 | PRIMARY | EMP | eq_ref | PRIMARY | PRIMARY | 4 | <subquery2>.OBJECT_ID | 1 | 100.00 | NULL |
| 2 | MATERIALIZED | T1 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
| 2 | MATERIALIZED | T2 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
| 3 | DEPENDENT SUBQUERY | D | eq_ref | PRIMARY | PRIMARY | 4 | dao.T1.OBJECT_ID | 1 | 100.00 | Using index |
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+----------+----------------------------------------------------+
5 rows in set, 2 warnings (0.00 sec)
UNCACHEABLE SUBQUERY
UNCACHEABLE SUBQUERY表示子查询不可被物化 需要逐次运行
mysql> EXPLAIN SELECT ( SELECT @rownum:= @rownum + sal+D.deptno from EMP limit 1 )
-> FROM DEPT D ;
+----+--------------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | PRIMARY | D | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
| 2 | DEPENDENT SUBQUERY | EMP | ALL | NULL | NULL | NULL | NULL | 14 | NULL |
+----+--------------------+-------+-------+---------------+---------+---------+------+------+-------------+
2 rows in set (0.00 sec)
UNCACHEABLE UNION
mysql> explain SELECT (select xx.c1 from ( SELECT @rownum c1 from EMP union SELECT @rownum from EMP ) xx limit 1 )
-> FROM DEPT D ;
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | D | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
| 2 | UNCACHEABLE SUBQUERY | <derived3> | ALL | NULL | NULL | NULL | NULL | 28 | NULL |
| 3 | DERIVED | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| 4 | UNCACHEABLE UNION | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| NULL | UNION RESULT | <union3,4> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
5 rows in set (0.00 sec)
table
table_name
访问的表,注意即使访问的是索引 这里也显示访问的表名 关于索引信息在 type key 中进行描述
mysql> EXPLAIN SELECT OBJECT_ID FROM DAO_OBJECTS1 ;
+----+-------------+--------------+-------+---------------+---------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+------+-------+-------------+
| 1 | SIMPLE | DAO_OBJECTS1 | index | NULL | PRIMARY | 4 | NULL | 52025 | Using index |
+----+-------------+--------------+-------+---------------+---------+---------+------+-------+-------------+
1 row in set (0.00 sec)
mysql> explain SELECT (select xx.c1 from ( SELECT @rownum c1 from EMP union SELECT @rownum from EMP ) xx limit 1 )
-> FROM DEPT D ;
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | D | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
| 2 | UNCACHEABLE SUBQUERY | <derived3> | ALL | NULL | NULL | NULL | NULL | 28 | NULL |
| 3 | DERIVED | **EMP** | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| 4 | UNCACHEABLE UNION | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| NULL | UNION RESULT | <union3,4> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
5 rows in set (0.00 sec)
(union m,n)
表示几个行源进行union
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | D | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
| 2 | UNCACHEABLE SUBQUERY | <derived3> | ALL | NULL | NULL | NULL | NULL | 28 | NULL |
| 3 | DERIVED | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| 4 | UNCACHEABLE UNION | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| NULL | UNION RESULT | **<union3,4>** | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
5 rows in set (0.00 sec)
表示使用内联视图
mysql> explain SELECT **(select xx.c1 from ( SELECT @rownum c1 from EMP union SELECT @rownum from EMP )** xx limit 1 )
-> FROM DEPT D ;
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
| 1 | PRIMARY | D | index | NULL | PRIMARY | 4 | NULL | 4 | Using index |
| 2 | UNCACHEABLE SUBQUERY | **<derived3>** | ALL | NULL | NULL | NULL | NULL | 28 | NULL |
| 3 | DERIVED | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| 4 | UNCACHEABLE UNION | EMP | index | NULL | FK_DEPTNO | 5 | NULL | 14 | Using index |
| NULL | UNION RESULT | <union3,4> | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+----------------------+------------+-------+---------------+-----------+---------+------+------+-----------------+
5 rows in set (0.00 sec)
type
type 表示的访问路径,及数据获取途径
type 优先级
null
不访问任何表
mysql> explain select current_timestamp ;
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
+----+-------------+-------+------+---------------+------+---------+------+------+----------------+
1 row in set (0.00 sec)
mysql> EXPLAIN
-> SELECT *
-> FROM DAO_OBJECTS1
-> WHERE OBJECT_NAME ='DBA_TABLES'
-> and OBJECT_ID='9527';
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
1 row in set (0.01 sec)
system
根据官档所说 这种方式属于一种特殊的const 要求表中只有一条记录
mysql> analyze table TYPE_SYSTEM ;
+-----------------+---------+----------+----------+
| Table | Op | Msg_type | Msg_text |
+-----------------+---------+----------+----------+
| dao.TYPE_SYSTEM | analyze | status | OK |
+-----------------+---------+----------+----------+
1 row in set (0.02 sec)
mysql> use dao ;
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
mysql> create table TYPE_SYSTEM (c1 int primary key ) ;
Query OK, 0 rows affected (0.05 sec)
mysql> insert into TYPE_SYSTEM values(1) ;
Query OK, 1 row affected (0.02 sec)
mysql> explain select 1 from TYPE_SYSTEM where c1=1 ;
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | SIMPLE | TYPE_SYSTEM | const | PRIMARY | PRIMARY | 4 | const | 1 | Using index |
+----+-------------+-------------+-------+---------------+---------+---------+-------+------+-------------+
1 row in set (0.00 sec)
const
此种类型查询最多返回一行记录,由于只有一行记录,后续优化器会把它当作常量. 基本可以认为表进行了索引唯一性扫描,并且唯一性索引只返回一行记录
mysql> explain select object_id from dao.DAO_OBJECTS1 WHERE OBJECT_ID =2 ;
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | SIMPLE | DAO_OBJECTS1 | const | PRIMARY | PRIMARY | 4 | const | 1 | Using index |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
1 row in set (0.00 sec)
mysql> explain
-> select *
-> from DAO_OBJECTS2
-> where object_name in
-> (select object_name
-> from dao.DAO_OBJECTS1
-> WHERE OBJECT_ID = 9527);
+----+-------------+--------------+-------+---------------+---------+---------+-------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+-------+-------+-------------+
| 1 | SIMPLE | DAO_OBJECTS1 | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
| 1 | SIMPLE | DAO_OBJECTS2 | ALL | NULL | NULL | NULL | NULL | 51976 | Using where |
+----+-------------+--------------+-------+---------------+---------+---------+-------+-------+-------------+
2 rows in set (0.00 sec)
mysql> explain
-> select *
-> from DAO_OBJECTS2
-> where object_id in
-> (select object_id
-> from dao.DAO_OBJECTS1
-> WHERE OBJECT_ID = 9527);
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
| 1 | SIMPLE | DAO_OBJECTS2 | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
| 1 | SIMPLE | DAO_OBJECTS1 | const | PRIMARY | PRIMARY | 4 | const | 1 | Using index |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------------+
2 rows in set (0.00 sec)
mysql> explain select * from DAO_OBJECTS2 where object_id =9527 ;
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | DAO_OBJECTS2 | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
eq_ref
关联时进行多次索引唯一性扫描
mysql> EXPLAIN
-> select *
-> from DAO_OBJECTS1 T1 ,EMP E
-> WHERE E.DEPTNO= T1.OBJECT_ID ;
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
| 1 | SIMPLE | E | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where |
| 1 | SIMPLE | T1 | eq_ref | PRIMARY | PRIMARY | 4 | dao.E.DEPTNO | 1 | NULL |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
2 rows in set (0.00 sec)
ref
索引非唯一性扫描
mysql> EXPLAIN
-> SELECT *
-> FROM DAO_OBJECTS1
-> WHERE OBJECT_NAME = 'DBA_TABLES' ;
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | ref | IDX2_OBJECTS2 | IDX2_OBJECTS2 | 131 | const | 2 | Using index condition |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.02 sec)
fulltext
使用全文索引时出现
mysql> ALTER TABLE DAO_OBJECTS2 ADD FULLTEXT( OBJECT_NAME );
Query OK, 0 rows affected, 1 warning (2.15 sec)
Records: 0 Duplicates: 0 Warnings: 1
mysql> EXPLAIN
-> SELECT *
-> FROM DAO_OBJECTS2
-> WHERE match(OBJECT_NAME) AGAINST('TABLES') ;
+----+-------------+--------------+----------+---------------+-------------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+----------+---------------+-------------+---------+------+------+-------------+
| 1 | SIMPLE | DAO_OBJECTS2 | fulltext | OBJECT_NAME | OBJECT_NAME | 0 | NULL | 1 | Using where |
+----+-------------+--------------+----------+---------------+-------------+---------+------+------+-------------+
1 row in set (0.00 sec)
ref_or_null
当谓词出现索引列等于某值或为空时出现
mysql> EXPLAIN
-> SELECT *
-> FROM DAO_OBJECTS1
-> WHERE OBJECT_NAME ='DBA_TABLES' OR OBJECT_NAME is NULL ;
+----+-------------+--------------+-------------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------------+---------------+---------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | ref_or_null | IDX2_OBJECTS2 | IDX2_OBJECTS2 | 131 | const | 3 | Using index condition |
+----+-------------+--------------+-------------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
index_merge
mysql> EXPLAIN
-> SELECT *
-> FROM DAO_OBJECTS1
-> WHERE OBJECT_NAME ='DBA_TABLES' OR OBJECT_ID='9527';
+----+-------------+--------------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | index_merge | PRIMARY,IDX2_OBJECTS2 | IDX2_OBJECTS2,PRIMARY | 131,4 | NULL | 3 | Using union(IDX2_OBJECTS2,PRIMARY); Using where |
+----+-------------+--------------+-------------+-----------------------+-----------------------+---------+------+------+-------------------------------------------------+
1 row in set (0.01 sec)
range
通过范围方式访问索引
mysql> explain
-> SELECT * FROM DAO_OBJECTS1 WHERE OBJECT_ID BETWEEN 2000 AND 4000 ;
+----+-------------+--------------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | DAO_OBJECTS1 | range | PRIMARY | PRIMARY | 4 | NULL | 4012 | Using where |
+----+-------------+--------------+-------+---------------+---------+---------+------+------+-------------+
1 row in set (0.05 sec)
index
索引全扫描 正常情况下应该在在没有过滤谓词时出现
mysql> EXPLAIN SELECT OBJECT_NAME FROM DAO_OBJECTS1 ORDER BY OBJECT_NAME DESC ;
+----+-------------+--------------+-------+---------------+---------------+---------+------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------------+---------+------+-------+-------------+
| 1 | SIMPLE | DAO_OBJECTS1 | index | NULL | IDX2_OBJECTS2 | 131 | NULL | 52025 | Using index |
+----+-------------+--------------+-------+---------------+---------------+---------+------+-------+-------------+
1 row in set (0.00 sec)
ALL
全表扫描
mysql> EXPLAIN SELECT OBJECT_TYPE FROM DAO_OBJECTS1
-> ;
+----+-------------+--------------+------+---------------+------+---------+------+-------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+------+---------+------+-------+-------+
| 1 | SIMPLE | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 52025 | NULL |
+----+-------------+--------------+------+---------------+------+---------+------+-------+-------+
1 row in set (0.00 sec)
possible_keys
显示可能被使用的索引 ,并不是所有的索引
mysql> SHOW INDEXES FROM DAO_OBJECTS1 ;
+--------------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+--------------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| DAO_OBJECTS1 | 0 | PRIMARY | 1 | OBJECT_ID | A | 52025 | NULL | NULL | | BTREE | | |
| DAO_OBJECTS1 | 1 | IDX2_OBJECTS2 | 1 | OBJECT_NAME | A | 52025 | NULL | NULL | YES | BTREE | | |
| DAO_OBJECTS1 | 1 | IDX3_OBJECTS1 | 1 | OWNER | A | 54 | NULL | NULL | | BTREE | | |
+--------------+------------+---------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 WHERE OBJECT_NAME = 'DBA_TABLES' ;
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | ref | IDX2_OBJECTS2 | IDX2_OBJECTS2 | 131 | const | 2 | Using index condition |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 WHERE OBJECT_NAME = 'DBA_TABLES' AND OWNER='DAO' ;
+----+-------------+--------------+-------------+-----------------------------+-----------------------------+---------+------+------+-----------------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------------+-----------------------------+-----------------------------+---------+------+------+-----------------------------------------------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | index_merge | IDX2_OBJECTS2,IDX3_OBJECTS1 | IDX2_OBJECTS2,IDX3_OBJECTS1 | 131,32 | NULL | 1 | Using intersect(IDX2_OBJECTS2,IDX3_OBJECTS1); Using where |
+----+-------------+--------------+-------------+-----------------------------+-----------------------------+---------+------+------+-----------------------------------------------------------+
1 row in set (0.01 sec)
注意:这里possible_keys中并没有出现PRIMARY 但是我仍然感叹与MySQL中index merge 出现的频率.这可能innodb 存储结构有关系
key
被使用的索引
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 WHERE OBJECT_NAME = 'DBA_TABLES' ;
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | ref | IDX2_OBJECTS2 | IDX2_OBJECTS2 | 131 | const | 2 | Using index condition |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
key_len
key_len 计算公式为 可变长度*字符集位数+(如果可为null+1 否则+0)+2 声明变长字段 固定字段 int 4 +(not null +1 else 0 )
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 WHERE OBJECT_NAME = 'DBA_TABLES' ;
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | ref | IDX2_OBJECTS2 | IDX2_OBJECTS2 | 131 | const | 2 | Using index condition |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
CREATE TABLE `DAO_OBJECTS1` (
`OWNER` varchar(30) NOT NULL,
`OBJECT_NAME` varchar(128) DEFAULT NULL,
`SUBOBJECT_NAME` varchar(30) DEFAULT NULL,
`OBJECT_ID` int(11) NOT NULL DEFAULT '0',
`DATA_OBJECT_ID` int(11) DEFAULT NULL,
`OBJECT_TYPE` varchar(19) DEFAULT NULL,
`CREATED` datetime DEFAULT NULL,
`LAST_DDL_TIME` datetime DEFAULT NULL,
`TIMESTAMP` varchar(19) DEFAULT NULL,
`STATUS` varchar(7) DEFAULT NULL,
`TEMPORARY` varchar(1) DEFAULT NULL,
`GENERATED` varchar(1) DEFAULT NULL,
`SECONDARY` varchar(1) DEFAULT NULL,
PRIMARY KEY (`OBJECT_ID`),
KEY `IDX2_OBJECTS2` (`OBJECT_NAME`),
KEY `IDX3_OBJECTS1` (`OWNER`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |
key_len=128+1(null)+2(var变长)
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 WHERE OBJECT_ID=9527 ;
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------+
| 1 | SIMPLE | DAO_OBJECTS1 | const | PRIMARY | PRIMARY | 4 | const | 1 | NULL |
+----+-------------+--------------+-------+---------------+---------+---------+-------+------+-------+
1 row in set (0.00 sec)
key_len=4(int)+0(primary key not null 0 ) +0 (固定长度)
ref
mysql> EXPLAIN
-> select *
-> from DAO_OBJECTS1 T1 ,EMP E
-> WHERE E.DEPTNO= T1.OBJECT_ID ;
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
| 1 | SIMPLE | E | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where |
| 1 | SIMPLE | T1 | eq_ref | PRIMARY | PRIMARY | 4 | dao.E.DEPTNO | 1 | NULL |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
2 rows in set (0.00 sec)
mysql> EXPLAIN SELECT COUNT(*) FROM DAO_OBJECTS1 T1 ,DAO_OBJECTS2 T2 WHERE T1.OBJECT_ID=T2.OBJECT_ID
-> ;
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
| 1 | SIMPLE | T2 | index | PRIMARY | PRIMARY | 4 | NULL | 50887 | Using index |
| 1 | SIMPLE | T1 | eq_ref | PRIMARY | PRIMARY | 4 | dao.T2.OBJECT_ID | 1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
2 rows in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 WHERE OBJECT_NAME = 'DBA_TABLES' ;
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | ref | IDX2_OBJECTS2 | IDX2_OBJECTS2 | 131 | const | 2 | Using index condition |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
rows
mysql> EXPLAIN SELECT COUNT(*) FROM DAO_OBJECTS1 T1 ,DAO_OBJECTS2 T2 WHERE T1.OBJECT_ID=T2.OBJECT_ID
-> ;
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
| 1 | SIMPLE | T2 | index | PRIMARY | PRIMARY | 4 | NULL | 50887 | Using index |
| 1 | SIMPLE | T1 | eq_ref | PRIMARY | PRIMARY | 4 | dao.T2.OBJECT_ID | 1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
2 rows in set (0.00 sec)
第一行 row = 50887 为T2表总大小 第二行 row = 1 代表从T2表取一行 到T1表中遍历可获取的行数的大小
Extra
Extra 显示了一些不适合在其他列显示 但是很重要的信息
下面只列举了一些我遇见过的 后面有的再补充
Using where
相当于Oracle 谓词信息中的filter 先取出数据之后再比对
mysql> EXPLAIN
-> select *
-> from DAO_OBJECTS1 T1 ,EMP E
-> WHERE E.DEPTNO= T1.OBJECT_ID ;
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
| 1 | SIMPLE | E | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where |
| 1 | SIMPLE | T1 | eq_ref | PRIMARY | PRIMARY | 4 | dao.E.DEPTNO | 1 | NULL |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------+
2 rows in set (0.00 sec)
Using index
使用索引来访问数据而避免回表
mysql> EXPLAIN SELECT COUNT(*) FROM DAO_OBJECTS1 T1 ,DAO_OBJECTS2 T2 WHERE T1.OBJECT_ID=T2.OBJECT_ID
-> ;
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
| 1 | SIMPLE | T2 | index | PRIMARY | PRIMARY | 4 | NULL | 50887 | Using index |
| 1 | SIMPLE | T1 | eq_ref | PRIMARY | PRIMARY | 4 | dao.T2.OBJECT_ID | 1 | Using index |
+----+-------------+-------+--------+---------------+---------+---------+------------------+-------+-------------+
2 rows in set (0.00 sec)
Using index conditio
使用索引过滤数据
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 WHERE OBJECT_NAME = 'DBA_TABLES' ;
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
| 1 | SIMPLE | DAO_OBJECTS1 | ref | IDX2_OBJECTS2 | IDX2_OBJECTS2 | 131 | const | 2 | Using index condition |
+----+-------------+--------------+------+---------------+---------------+---------+-------+------+-----------------------+
1 row in set (0.00 sec)
Using temporary
使用了临时表
mysql> explain select distinct object_type from DAO_OBJECTS ;
+----+-------------+-------------+------+---------------+------+---------+------+-------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------------+------+---------------+------+---------+------+-------+-----------------+
| 1 | SIMPLE | DAO_OBJECTS | ALL | NULL | NULL | NULL | NULL | 50235 | Using temporary |
+----+-------------+-------------+------+---------------+------+---------+------+-------+-----------------+
1 row in set (0.00 sec)
Using filesort
无法利用索引来完成的排序
mysql> EXPLAIN select * FROM DAO_OBJECTS1 ORDER BY OBJECT_TYPE ;
+----+-------------+--------------+------+---------------+------+---------+------+-------+----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+--------------+------+---------------+------+---------+------+-------+----------------+
| 1 | SIMPLE | DAO_OBJECTS1 | ALL | NULL | NULL | NULL | NULL | 52025 | Using filesort |
+----+-------------+--------------+------+---------------+------+---------+------+-------+----------------+
1 row in set (0.00 sec)
Using join buffer
将之前访问的数据存放于join buffer中
mysql> EXPLAIN EXTENDED
-> SELECT *
-> FROM EMP
-> WHERE EMPNO IN (SELECT T1.OBJECT_ID
-> FROM DAO_OBJECTS1 T1 ,DAO_OBJECTS1 T2
-> WHERE T1.OBJECT_ID =T2.OBJECT_ID
-> AND EXISTS (SELECT 1
-> FROM DEPT D
-> WHERE D.DEPTNO= T1.OBJECT_ID )
-> );
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+----------+----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+----------+----------------------------------------------------+
| 1 | PRIMARY | <subquery2> | ALL | NULL | NULL | NULL | NULL | NULL | 0.00 | Using where |
| 1 | PRIMARY | EMP | eq_ref | PRIMARY | PRIMARY | 4 | <subquery2>.OBJECT_ID | 1 | 100.00 | NULL |
| 2 | MATERIALIZED | T1 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where |
| 2 | MATERIALIZED | T2 | ALL | NULL | NULL | NULL | NULL | 1 | 100.00 | Using where; Using join buffer (Block Nested Loop) |
| 3 | DEPENDENT SUBQUERY | D | eq_ref | PRIMARY | PRIMARY | 4 | dao.T1.OBJECT_ID | 1 | 100.00 | Using index |
+----+--------------------+-------------+--------+---------------+---------+---------+-----------------------+------+----------+----------------------------------------------------+
5 rows in set, 2 warnings (0.00 sec)
Impossible WHERE
谓词不成立
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 T1 LIMIT 0 ;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
1 row in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM DAO_OBJECTS1 T1 WHERE 1=2 ;
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE |
+----+-------------+-------+------+---------------+------+---------+------+------+------------------+
1 row in set (0.00 sec)
mysql> EXPLAIN
-> SELECT *
-> FROM DAO_OBJECTS1
-> WHERE OBJECT_NAME ='DBA_TABLES'
-> and OBJECT_ID='9527';
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after reading const tables |
+----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+
1 row in set (0.01 sec)