MySQL执行计划初探
2016-01-22 13:53
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-Mysql执行计划总结–
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
关于索引信息在 type key 中进行描述
null->system->const->eq-ref->ref->fulltext->ref_or_null->index_merge->unique_subquery->index_subquery->range->index->ALL
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
1 执行计划概述
先看看一个执行计划 mysql> explain SELECT * FROM EMP , 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) | +----+-------------+-------+------+---------------+------+---------+------+-------+----------------------------------------------------+
2 执行计划列详解
2.1 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) | +----+-------------+-------+------+---------------+------+---------+------+-------+----------------------------------------------------+ 可以看到三个操作ID相同 那么其执行顺序就是 表t1 做全表扫描与EMP关联 所得到的结果集于表t2关联 (t1 join EMP) join t2 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
2.2 select_type
2.2.1 SIMPLE
简单查询不包含子查询与unionmysql> 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)
2.2.2 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FROM (SELECT * FROM EMP , DAO_OBJECTS t1 WHERE DEPTNO=t1.object' at line 1 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) +----+-------------+------------+--------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+ | 1 | PRIMARY | T4 | ALL | NULL | NULL | NULL | NULL | 49793 | Using where | | 1 | PRIMARY | <derived2> | ref | <auto_key0> | <auto_key0> | 5 | dao.T4.OBJECT_ID | 16 | NULL | | 2 | DERIVED | T2 | ALL | NULL | NULL | NULL | NULL | 49793 | Using where | | 2 | DERIVED | <derived3> | ref | <auto_key1> | <auto_key1> | 5 | dao.T2.OBJECT_ID | 16 | Using where | | 2 | DERIVED | T3 | eq_ref | PRIMARY | PRIMARY | 4 | V1.DEPTNO | 1 | Using index | | 3 | DERIVED | T1 | ALL | NULL | NULL | NULL | NULL | 49793 | NULL | | 3 | DERIVED | EMP | ALL | FK_DEPTNO | NULL | NULL | NULL | 14 | Using where; Using join buffer (Block Nested Loop) | +----+-------------+------------+--------+---------------+-------------+---------+------------------+-------+----------------------------------------------------+ 7 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( <derived2>)
2.2.3 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)
2.2.4 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 SUBQUERYmysql> 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)
2.2.5 UNION RESULT
出现在UNION 或UNION ALL 语句中 代表把所有结果集联合起来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)
2.2.6 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
2.2.7 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)
2.2.8 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)
2.2.9 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)
2.2.10 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)
2.2.11 UNCACHEABLE UNION
同上 子查询中出现union 并且不可被缓存 在union 后的select 语句出现此关键词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)
2.3 table
2.3.1 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)
2.3.2 (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)
2.3.3 (union m,n)
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)
2.4 type
type 表示的访问路径,及数据获取途径2.4.1 type 优先级
参考官档 http://dev.mysql.com/doc/refman/5.7/en/explain-output.html#jointype_refnull->system->const->eq-ref->ref->fulltext->ref_or_null->index_merge->unique_subquery->index_subquery->range->index->ALL
2.4.2 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)
2.4.3 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)
2.4.4 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)
2.4.5 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)
2.4.6 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)
2.4.7 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)2.4.8 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)
2.4.9 index_merge
当谓词分别用到多个索引的首列,并且谓词连接为或时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)
2.4.10 unique_subquery
不知道是不是我使用percona 版本的问题 模拟不出来 改天再研究
2.4.11 index_subquery
不知道是不是我使用percona 版本的问题 模拟不出来 改天再研究
2.4.12 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)
2.4.13 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)
2.4.14 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)
2.5 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
2.6 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)
2.7 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)
2.8 ref
关联谓词信息或常量
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)
2.9 rows
rows 如果是驱动行源则为该行源大小 如果是被驱动行源,则是单次操作的行源大小 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
2.10 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
2.10.1 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)
2.10.2 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)
2.10.3 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)
2.10.4 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)
2.10.5 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)
2.10.6 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)
2.10.7 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)
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