MySQL查詢優化幫助

Question

希望你能幫助我在正確的軌道上開始優化我的查詢。我以前從未考慮過太多關於優化的問題，但我有幾個類似於下面的問題的查詢，並希望開始專注於提高其效率。我非常需要優化查詢的一個例子是如下：

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`) 

運行解釋其對上述產生以下：

| 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，你有一個從屬子查詢，其是MySQL中性能最差的事情之一。在這裏看到提示：

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