Question

如果它有所作为，我使用的是Apache Derby 10.8。

我有一个非常简单的数据库，其中包含一个充满项目的表格以及一个充满这些项目出价的表格。我想选择加入该项目的出价最高的每件商品。以下是我第一次尝试它，性能很糟糕：

select
    item.id as item_id,
    item.name as item_name,
    item.retail_value as item_retail_value,
    item.vendor as item_vendor,
    bid.bid_amount as bid_amount,
    bid.bidder_name as bid_bidder_name,
    bid.bidder_phone as bid_bidder_phone,
    bid.operator_name as bid_operator_name
from item
    left outer join bid on bid.item_id = item.id and
    bid.bid_amount = (select max(bid.bid_amount) from bid where bid.item_id = item.id and bid.status = 'OK')

我创建了一组测试数据，使用了282个项目，每个项目有200个出价（总共56400个出价）。上述查询大约需要30-40秒才能运行。如果我选择每个项目并手动循环选择每个项目的高出价，则只需不到一秒钟。

我已尝试为bid.bid_amount和bid.status列编制索引，但它没有做任何明显的事情。 SQL不是我最强的领域，所以如果有人愿意解释为什么这个查询太慢了，我真的很感激。

Answer 1

查询速度很慢，因为您正在执行所谓的相关子查询 - 它为每一行运行max。

尝试这样的事情：

select
    item.id as item_id,
    item.name as item_name,
    item.retail_value as item_retail_value,
    item.vendor as item_vendor,
    bid.bid_amount as bid_amount,
    bid.bidder_name as bid_bidder_name,
    bid.bidder_phone as bid_bidder_phone,
    bid.operator_name as bid_operator_name
from 
    item
    left outer join (
        select 
            item_id, 
            MAX(bid_amount) maxamount 
        from 
            bid 
        where 
            status = 'OK' 
        group by 
            item_id
    ) b1 on
        item.id = b1.item_id
    left outer join bid on
        bid.item_id = item.id
        and bid.bid_amount = b1.maxamount

这个子查询只运行一次，速度会快得多。

Answer 2

您创建了一个同步（或相关）子查询。子查询是为外部表（项）的每一行执行的。

Answer 3

问题是您的嵌套子查询是否在JOIN操作的每一步上运行。难怪查询性能差，CPU和磁盘可能很难工作！假设您尝试对项目表格中的每个项目进行最高确定出价，则可能需要尝试此查询：

SELECT I.id AS item_id,
       I.name AS item_name,
       I.retail_value AS item_retail_value,
       I.vendor AS item_vendor,
       B.bid_amount AS bid_amount,
       B.bidder_name AS bid_bidder_name,
       B.bidder_phone AS bid_bidder_phone,
       B.operator_name AS bid_operator_name
FROM item AS I
     LEFT OUTER JOIN (SELECT item_id, MAX(bid_amount) AS bid_amount
                      FROM bid
                      WHERE STATUS = 'OK'
                      GROUP BY item_id) AS _TEMP ON _TEMP.item_id = B.item_id
     LEFT OUTER JOIN bid AS B ON B.item_id = _TEMP.item_id AND B.bid_amount = _TEMP.bid_amount;

Answer 4

您还可以通过在bid.item_id上应用索引来提高查询的效果，因为子查询会根据item_id选择记录。

加入使用select max的列时，为什么此SQL（连接）查询会变慢？

4 个答案: