Mysql的减法低性能

我提出以下查询在我applicattion，检索行：Mysql的减法低性能

SELECT 
    title, 
    SUBSTRING(description, 1, 200) AS s_description, 
    refresh_time, (UNIX_TIMESTAMP()-refresh_time) AS since, 
    state, state_friendly, 
    city, city_friendly, 
    category, category_friendly, 
    basename, COUNT(ad_id) AS im_ocur 
FROM `ad` `a` 
JOIN `state` ON state_id = ad_state_id 
JOIN `city` ON city_id = ad_city_id 
JOIN `category` ON category_id = ad_category_id 
LEFT JOIN `ad_image` ON ad_image_ad_id = ad_id 
WHERE (ad_country_id = 195) 
GROUP BY `ad_id` 
ORDER BY `refresh_time` DESC, front DESC 
LIMIT 20

我已经在许多情况下测试，但指的是一个城市，有很多行的时候，表现它已经降低。经过几次更改后，我意识到问题是由于“（UNIX_TIMESTAMP（） - refresh_time）”表达式造成的，所以似乎Mysql Engine在行限制设置为20之前进行算术运算。

我的替代解决方案是post-用PHP处理数据，但我更喜欢完整的mysql方式。

可能吗？

来源

2011-11-28 Coda Tavern

与使用数学函数的返回列没有任何关系，只是由许多行上的GORUP BY造成的。 – ajreal

但是，当我删除该表达式时，尽管使用GROUP BY –

，但性能仍然非常出色您是否尝试对此查询使用EXPLAIN？ – Oyeme

我认为这不是一个减法，而是为每一行调用UNIX_TIMESTAMP（）（虽然我可能是错误的）。

如果您认为减法是性能低下的原因，您可以随时将其移至外部查询。

SELECT 
    title, 
    s_description, 
    refresh_time, (UNIX_TIMESTAMP()-refresh_time) AS since, 
    state, state_friendly, 
    city, city_friendly, 
    category, category_friendly, 
    basename, im_ocur 
FROM (
    SELECT 
    title, 
    SUBSTRING(description, 1, 200) AS s_description, 
    refresh_time, 
    state, state_friendly, 
    city, city_friendly, 
    category, category_friendly, 
    basename, COUNT(ad_id) AS im_ocur 
    FROM `ad` `a` 
    JOIN `state` ON state_id = ad_state_id 
    JOIN `city` ON city_id = ad_city_id 
    JOIN `category` ON category_id = ad_category_id 
    LEFT JOIN `ad_image` ON ad_image_ad_id = ad_id 
    WHERE (ad_country_id = 195) 
    GROUP BY `ad_id` 
    ORDER BY `refresh_time` DESC, front DESC 
    LIMIT 20 
) AS sq

来源

2011-11-28 12:02:59 Mchl

Mysql的减法低性能

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