Question

我知道下面的查询非常糟糕，需要2分钟才能获得10条记录（虽然列表中有超过1M的记录）但是我不确定最好的方法是写这个

我只是想让所有列出表格的国家/地区连接到国家/地区的列表是省..

 ALTER VIEW [dbo].[CountriesWithListings]
 AS
 SELECT     distinct    
    cn.CountryID,
    cn.Code as CountryCode,
    cn.Name as CountryName
FROM         dbo.Countries AS cn

              INNER JOIN dbo.Provinces AS p ON p.CountryID = cn.CountryID
              INNER JOIN dbo.Cities c on c.ProvinceID = p.ProvinceID
              INNER JOIN dbo.Listings AS l ON l.CityID = c.CityID

WHERE l.IsActive = 1 AND l.IsApproved = 1

Answer 1

假设您有适当的索引，使用distinct是很昂贵的。您应该可以使用exists：

获得更好的性能

SELECT     
    cn.CountryID,
    cn.Code as CountryCode,
    cn.Name as CountryName
FROM dbo.Countries AS cn
WHERE EXISTS (
    SELECT 1
    FROM dbo.Provinces AS p
              INNER JOIN dbo.Cities c on c.ProvinceID = p.ProvinceID
              INNER JOIN dbo.Listings AS l ON l.CityID = c.CityID
    WHERE p.CountryID = cn.CountryID
        AND l.IsActive = 1 
        AND l.IsApproved = 1
)

Answer 2

列表是否已编入索引？如果它有1M个条目，那么最好先将其编入索引并在此之后检查性能。您的查询并不复杂

Answer 3

查询的性能还取决于您环境中存在的索引以及您正在使用的列。

试试这个......

SELECT cn.CountryID
    ,MIN(cn.Code) AS CountryCode
    ,MIN(cn.NAME) AS CountryName
FROM dbo.Countries AS cn
INNER JOIN dbo.Provinces AS p ON p.CountryID = cn.CountryID
INNER JOIN dbo.Cities c ON c.ProvinceID = p.ProvinceID
INNER JOIN dbo.Listings AS l ON l.CityID = c.CityID
WHERE l.IsActive = 1
    AND l.IsApproved = 1
GROUP BY cn.CountryID

Answer 4

如果不添加索引或其他性能调优，此查询应该运行得更快：

ALTER VIEW [dbo].[CountriesWithListings]
AS
    SELECT cn.CountryID, cn.Code as CountryCode, cn.Name as CountryName
    FROM dbo.Countries AS cn
    WHERE cn.CountryID IN
    (
        SELECT p.CountryId FROM dbo.Provinces
        INNER JOIN dbo.Cities c on c.ProvinceID = p.ProvinceID
        INNER JOIN dbo.Listings AS l ON l.CityID = c.CityID
        WHERE l.IsActive = 1 AND l.IsApproved = 1
    )

内部连接具有明显的缓慢

4 个答案: