Question

现在需要很长时间才能运行。

查询是：

select count(id), variety_id, name 
from tblItem 
where order_id IN (
    select order_id 
    from tblItem 
    where variety_id=4005 
    order by order_id DESC) 
AND variety_id != 4005 
GROUP BY variety_id 
order by count(id) DESC
LIMIT 5;

我在variety_id和order_id上有索引。我基本上是在尝试构建一个推荐引擎。该查询正在寻找人们在购买variety_id 4005时购买的前5个项目。但就像我说它需要长时间运行。

有没有人可以优化此查询？

Answer 1

试试这个：

select count(t1.id), t1.variety_id, t1.name 
from tblItem t1
inner join tblItem t2 ON t2.order_id = t1.order_id and t2.variety_id = 4005
where t1.variety_id != 4005 
GROUP BY t1.variety_id, t1.name
ORDER BY count(t1.id) DESC 
LIMIT 5;

Answer 2

我经常发现MySQL不能很好地优化WHERE ... IN (SELECT ...)，而JOIN效果更好;我已经读过最近的MySQL版本更好，所以它可能依赖于版本。此外，您应该使用COUNT(*)，除非该列可以是NULL，并且您需要忽略计数中的空值。

SELECT COUNT(*) count, variety_id, name
FROM tblItem AS t1
JOIN (SELECT DISTINCT order_id
      FROM tblItem
      WHERE variety_id = 4005) AS t2
ON t1.order_id = t2.order_id
WHERE t1.variety_id != 4005
GROUP BY variety_id
ORDER BY count DESC
LIMIT 5

需要使用DISTINCT的子查询来防止将计数乘以交叉积中的匹配行数。

如何优化以下SQL查询

2 个答案: