某前臺sql語句,簡化後如下
SELECT products_name,products_viewed FROM `products_description` ORDER BY products_viewed DESC,products_name LIMIT 0,20;
該語句經常大批量出現在慢日誌中!
初步看改語句,非常簡單,根據products_viewed(產品被查看次數)倒序排序,再根據products_name(產品名字)排序!在products_viewed和products_name上分別建立有索引!
但是感覺products_name排序怪怪的!
explain後發現
+----+-------------+----------------------+------+---------------+------+---------+------+-------+----------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------------------+------+---------------+------+---------+------+-------+----------------+ | 1 | SIMPLE | products_description | ALL | NULL | NULL | NULL | NULL | 764370 | Using filesort | +----+-------------+----------------------+------+---------------+------+---------+------+-------+----------------+
改語句做竟然全表掃描!
mysql的order by語句,如果在where條件中沒有合適的索引選擇時,將會選擇order by col中的索引作爲條件,但是如果是多個order by組合,將會導致放棄使用索引!
和開發以及需求溝通,發現通過名字排序是可以不需要的!
我們去掉order by後面的 products_name!
再次explain後發現已經能夠使用索引:
explain SELECT products_name,products_viewed FROM `products_description` ORDER BY products_viewed LIMIT 0,20; +----+-------------+----------------------+-------+---------------+-----------------+---------+------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------------------+-------+---------------+-----------------+---------+------+------+-------+ | 1 | SIMPLE | products_description | index | NULL | products_viewed | 5 | NULL | 20 | | +----+-------------+----------------------+-------+---------------+-----------------+---------+------+------+-------+
再次對比兩次profiling(過程省略),發現第一次損壞大量io和cpu時間Sorting result上!因爲該語句爲前臺語句,有大量查詢,優化後,頁面打開速度明顯提升!
注意:
1. order by m,n 不要輕易寫這種語句,一般的order by前面的m纔是order by的重點,後面的n爲配角,如果沒有必要,儘量去掉
2. 參考我的另一篇博文http://qdjalone.blog.51cto.com/1222376/1360112