MySQL查询优化帮助

Question

希望你能帮助我在正确的轨道上开始优化我的查询。我以前从未考虑过太多关于优化的问题，但我有几个类似于下面的问题的查询，并希望开始专注于提高其效率。我非常需要优化查询的一个例子是如下：

SELECT COUNT(*) AS `records_found` 

FROM (`records_owners` AS `ro`, `records` AS `r`) 

WHERE r.reg_no = ro.contact_no 

AND `contacted_email` <> "0000-00-00" 

AND `contacted_post` <> "0000-00-00" 

AND `ro`.`import_date` BETWEEN "2010-01-01" AND "2010-07-11" AND `r`.`pa_date_of_birth` > "2010-01-01" AND EXISTS (SELECT `number` FROM `roles` WHERE `roles`.`number` = r.`reg_no`) 

运行解释其对上述产生以下：

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

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

| 1 | PRIMARY   | r | ALL | NULL   | NULL | NULL | NULL         | 21533 | Using where | 

| 1 | PRIMARY   | ro | eq_ref | PRIMARY  | PRIMARY | 4  |   r.reg_no |  1 | Using where | 

| 2 | DEPENDENT SUBQUERY | roles | ALL | NULL   | NULL | NULL | NULL         | 189 | Using where | 

Answer 1

正如你可以看到，你有一个从属子查询，其是MySQL中性能最差的事情之一。在这里看到提示：

http://dev.mysql.com/doc/refman/5.0/en/select-optimization.html

http://dev.mysql.com/doc/refman/5.0/en/in-subquery-optimization.html