一次mysql order by 优化案例

标签：mysql

1. desc SELECT ActionID, UserID, CreateUser, ActionType, ActionName, ActionComment, CreateDate, PointsRulesID,Reason,ObjectID,ByUserID,ByUserName,SubjectID FROM  wikiuseractionlog Where CreateDate>‘0001-01-01 00:00:00 ‘  and ActionType in(10,9,19,20)  ORDER BY CreateDate DESC limit 93060,20;

| id | select_type | table             | type | possible_keys          | key  | key_len | ref  | rows  | Extra                       |

+----+-------------+-------------------+------+------------------------+------+---------+------+-------+-----------------------------+

|  1 | SIMPLE      | wikiuseractionlog | ALL  | idx_date_type,idx_date | NULL | NULL    | NULL | 91859 | Using where; Using filesort

看执行计划，没有用上索引，虽然我建了一个复合索引idx_date_type(createdate,actiontype)和一个列索引idx_date(createdate)，要使上面的语句走索引只要有下面几点：

1，需要将日期‘0001-01-01 00：00：00’改成2014-01-01 00:00:00  (2013以下都不行，不知道为什么)

2，需将createdate和actiontype做复合索引！一般情况下order by的字段如果要走索引优化，就需要将where条件下条件字段与order by的字段做联合索引！

3，Limit limit 93060,20  之前一直以后这句Limit会影响扫描的行数！不管有没索引都必须扫描93060行数据！实际不然！只要用上了索引，便只是取数据了！

mysql> desc SELECT ActionID, UserID, CreateUser, ActionType, ActionName, ActionComment, CreateDate, PointsRulesID,Reason,ObjectID,ByUserID,ByUserName,SubjectID FROM  wikiuseractionlog  ORDER BY CreateDate;

+----+-------------+-------------------+------+---------------+------+---------+------+-------+----------------+

| id | select_type | table             | type | possible_keys | key  | key_len | ref  | rows  | Extra          |

+----+-------------+-------------------+------+---------------+------+---------+------+-------+----------------+

|  1 | SIMPLE      | wikiuseractionlog | ALL  | NULL          | NULL | NULL    | NULL | 95220 | Using filesort |

+----+-------------+-------------------+------+---------------+------+---------+------+-------+----------------+

1 row in set (0.00 sec)

mysql> desc SELECT ActionID, UserID, CreateUser, ActionType, ActionName, ActionComment, CreateDate, PointsRulesID,Reason,ObjectID,ByUserID,ByUserName,SubjectID FROM  wikiuseractionlog Where CreateDate>‘2014-01-01 00:00:00 ‘  ORDER BY CreateDate DESC  limit 93060,20;

+----+-------------+-------------------+-------+------------------------+---------------+---------+------+------+-------------+

| id | select_type | table             | type  | possible_keys          | key           | key_len | ref  | rows | Extra       |

+----+-------------+-------------------+-------+------------------------+---------------+---------+------+------+-------------+

|  1 | SIMPLE      | wikiuseractionlog | range | idx_date_type,idx_date | idx_date_type | 8       | NULL | 3737 | Using where 

此种情况下虽然有单独的索引idx_date(createdate)，但发现依旧用不上索引，原因是order by的字段也必须出现在where 条件里，此处的原因大概也就是，排序操作是在最后的，首先提取数据的时候，如果where条件里的字段有索引，在提取出来的时候就已经做了排序操作了，此后再order by的时候就不用filesort了，不然则，在最后提取出来的数据里order by的时候，自然就要做filesort了！！

mysql> desc SELECT ActionID, UserID, CreateUser, ActionType, ActionName, ActionComment, PointsRulesID,Reason,ObjectID,ByUserID,ByUserName,SubjectID FROM  wikiuseractionlog Where ActionType in(10,9,19,20)  ORDER BY CreateDate;

+----+-------------+-------------------+------+---------------+------+---------+------+-------+-----------------------------+

| id | select_type | table             | type | possible_keys | key  | key_len | ref  | rows  | Extra                       |

+----+-------------+-------------------+------+---------------+------+---------+------+-------+-----------------------------+

|  1 | SIMPLE      | wikiuseractionlog | ALL  | idx_type_date | NULL | NULL    | NULL | 90747 | Using where; Using filesort |

+----+-------------+-------------------+------+---------------+------+---------+------+-------+-----------------------------+

1 row in set (0.00 sec)

上面的例子用不上索引！下面的则可以！将ActionType in(10,9,19,20) 改成or 也不行！

mysql> desc SELECT ActionID, UserID, CreateUser, ActionType, ActionName, ActionComment, PointsRulesID,Reason,ObjectID,ByUserID,ByUserName,SubjectID FROM  wikiuseractionlog Where ActionType =‘10‘  ORDER BY CreateDate;

+----+-------------+-------------------+------+---------------+---------------+---------+-------+-------+-------------+

| id | select_type | table             | type | possible_keys | key           | key_len | ref   | rows  | Extra       |

+----+-------------+-------------------+------+---------------+---------------+---------+-------+-------+-------------+

|  1 | SIMPLE      | wikiuseractionlog | ref  | idx_type_date | idx_type_date | 3       | const | 45373 | Using where |

+----+-------------+-------------------+------+---------------+---------------+---------+-------+-------+-------------+

1 row in set (0.00 sec)

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一次mysql order by 优化案例

标签：mysql

原文地址：http://fucheng.blog.51cto.com/2404495/1441534