MySQL查询极慢

Question

以下查询花费太多时间（此刻为0.8169sec）来显示20个条目，但我找不到原因...

SELECT
        `Order`.ID,
        Order_Type.Description as OrderTypeDescription,
        `Order`.OrderType as OrderType,
        DATE_FORMAT(`Order`.InsertDate, '%d.%m.%Y') as OrderInsertDate,
        round(sum(Item_Price.Amount),2) as OrderAmount,
        c1.ID as buyerCustomerId,
        c2.ID as sellerCustomerId,
        c1.CompanyName as BuyerCompany,
        c2.CompanyName as SellerCompany,
        c1.ID as BuyerCustomer,
        c2.ID as SellerCustomer
    FROM `Order`
        INNER JOIN Order_Type ON Order_Type.ID=`Order`.OrderType
        INNER JOIN Customer as c1 ON c1.ID=`Order`.BuyerCustomer
        INNER JOIN Customer as c2 ON c2.ID=`Order`.SellerCustomer
        INNER JOIN Item ON Item.`Order`=`Order`.ID
        INNER JOIN Item_Price ON Item_Price.Item=Item.ID
    GROUP BY `Order`.ID,OrderType,OrderTypeDescription,buyerCustomerId,sellerCustomerId,BuyerCustomer,SellerCustomer
    ORDER BY `Order`.ID DESC
    LIMIT 20

EXPLAIN显示以下输出： http : //pastebin.com/5f9QYizq

我并不是非常擅长优化查询，但是我认为性能不佳的原因可能是项目和item_price表上的联接（和总和），因为两个表中都有很多行（项目：16974， item_price：23981），因为每个项目都有一个或多个item_prices，它们总计为订单金额。

任何想法如何使此查询更快？

Answer 1

您可以尝试使用相关子查询，而不要使用group by ：

SELECT o.ID, ot.Description as OrderTypeDescription, ot.OrderType as OrderType,
       DATE_FORMAT(o.InsertDate, '%d.%m.%Y') as OrderInsertDate,
       (SELECT round(sum(ip.Amount), 2)
        FROM Item i INNER JOIN 
             Item_Price ip
             ON ip.Item = i.ID
        WHERE i.`Order` = o.ID
       ) as OrderAmount,
       c1.ID as buyerCustomerId,
       c2.ID as sellerCustomerId,
       c1.CompanyName as BuyerCompany,
       c2.CompanyName as SellerCompany,
       c1.ID as BuyerCustomer,
       c2.ID as SellerCustomer
FROM `Order` o INNER JOIN
      Order_Type ot
      ON ot.ID = o.OrderType INNER JOIN
      Customer c1
      ON c1.ID = o.BuyerCustomer INNER JOIN
      Customer c2
      ON c2.ID = `Order`.SellerCustomer
ORDER BY o.ID DESC
LIMIT 20;

这样可以节省GROUP BY开销。

您甚至可以将LIMIT移至子查询，假设联接未过滤任何行：

SELECT o.ID, ot.Description as OrderTypeDescription, ot.OrderType as OrderType,
       DATE_FORMAT(o.InsertDate, '%d.%m.%Y') as OrderInsertDate,
       (SELECT round(sum(ip.Amount), 2)
        FROM Item i INNER JOIN 
             Item_Price ip
             ON ip.Item = i.ID
        WHERE i.`Order` = o.ID
       ) as OrderAmount,
       c1.ID as buyerCustomerId,
       c2.ID as sellerCustomerId,
       c1.CompanyName as BuyerCompany,
       c2.CompanyName as SellerCompany,
       c1.ID as BuyerCustomer,
       c2.ID as SellerCustomer
FROM (SELECT o.*
      FROM `Order` o 
      ORDER BY o.id DESC
     ) o INNER JOIN
     Order_Type ot
     ON ot.ID = o.OrderType INNER JOIN
     Customer c1
     ON c1.ID = o.BuyerCustomer INNER JOIN
     Customer c2
     ON c2.ID = `Order`.SellerCustomer
ORDER BY o.ID DESC
LIMIT 20;