Question

我有一张表格记录两个有效英国邮政编码之间的谷歌/必应之间的距离。在使用系统时，会添加此表，以便下一个距离查询变得更快，而且我不必调用Web服务来在线检索距离。

这是表结构：

    OID                  PostcodeA PostcodeB DistanceMeters                          DistanceMiles
    -------------------- --------- --------- --------------------------------------- ---------------------------------------
    1                    BR60PS    BT248DN   788847                                  490
    2                    BR60PS    CM201JA   64426                                   40
    3                    BR60PS    CM82AP    77640                                   48
    4                    BR60PS    CO123AX   131617                                  82
    5                    BR60PS    CT146EL   119366                                  74
    6                    BR60PS    DA110TA   29247                                   18
    7                    BR60PS    DE216AH   262570                                  163
    8                    BR60PS    DL81AB    397524                                  247
    9                    BR60PS    HG27JE    368802                                  229
    10                   BR60PS    IP121AL   144394                                  90
    11                   BR60PS    IP141AH   144183                                  90
    12                   BR60PS    IP209AH   172259                                  107

现在我有一个通过Linq-to-SQL使用的标量UDF，它的定义如下：

ALTER FUNCTION [dbo].[GetDistanceFromCache]
(@PostcodeA VARCHAR (MAX), @PostcodeB VARCHAR (MAX))
RETURNS DECIMAL
AS
BEGIN
    DECLARE @FoundDistance AS DECIMAL;
    SELECT @FoundDistance = DistanceMiles
    FROM   AddressInfoRecordedDistance
    WHERE  (PostcodeA = @PostcodeA
                    AND PostcodeB = @PostcodeB)
                 OR (PostcodeB = @PostcodeA
                         AND PostcodeA = @PostcodeB);
    RETURN ISNULL(@FoundDistance, -1);
END

我需要知道是否有更快的SQL语句或c＃linq来获取结果？如果我检索800名员工并针对50个工作运行它们，系统会暂停，如果我没有将DBContext.GetDistanceFromCache()添加到选择集中，则所花费的时间会大大减少。

这是停滞查询：

        var query =
                    from locum in DbContext.Locums
                    where
                     locum.IsActive == true &&
                     locum.IsAdminMarkedComplete == true &&
                     locum.IsLocumsExciteBan == false &&
                     locum.IsGPHCBan == false &&
                        filterID1.Contains(locum.OID) == false &&
                        filterID2.Contains(locum.OID) == false
                    select new {
                        LocumID = locum.OID,
                        LocumName = locum.FirstName + " " + locum.LastName,
                        locum.MobileNumber,
                        locum.Email,
                        Gender = locum.Gender ? "Male" : "Female",
                        locum.DateofBirth,
                        LocumType = locum.LocumType.Name,
                        **Distance** = DbContext.GetDistanceFromCache(_Postcode, locum.AddressInfo.Postcode),
                        Address = String.Format("{0} {1} {2} {3}",
                                             locum.AddressInfo.House.Length == 0 ? String.Empty : locum.AddressInfo.House + ", ",
                                             locum.AddressInfo.Street.Length == 0 ? String.Empty : locum.AddressInfo.Street + ", ",
                                             locum.AddressInfo.Area.Length == 0 ? String.Empty : locum.AddressInfo.Area + ", ",
                                             locum.AddressInfo.Postcode ?? String.Empty),
                        Postcode = locum.AddressInfo.Postcode,
                        City = locum.AddressInfo.City.Name,
                        County = locum.AddressInfo.City.County.Name,
                        locum.SystemUserID
                    };

Answer 1

我认为您的查询很慢，因为每次LINQ查询select new { ... }时都必须调用DB，这与结果行的次数相同。

在将结果集与之匹配之前，我会从表AddressInfoRecordedDistance下载数据。我在想这样的事情：

var query = from locum in DbContext.Locums
            where ...
            select new { ... }; // Don't include GetDistanceFromCache here

var airds = from a in DbContext.AddressInfoRecordedDistance
            select a;

foreach (var q in query)
    q.Distance = GetDistanceFromCache(q.PostcodeA, q.PostcodeB, airds);

你有它。当然，您还需要GetDistanceFromCache方法：

// The SQL UDF in C# code here (psuedo code, not tested, just "C blunt")
decimal GetDistanceFromCache(string PostcodeA, string PostcodeB, List<...> table)
{
    return (from t in table
            where
                (t.PostcodeA == PostcodeA && t.PostcodeB == PostcodeB) ||
                (t.PostcodeB == PostcodeA && t.PostcodeA == PostcodeB)
            select t).FirstOrDefault().DistanceMiles;
}

当然，如果您经常运行上述所有操作，则应缓存变量airds。

Answer 2

如果你满足于远程查询的脏读，你可以添加像Nolock这样的表提示。有关详细信息，请参阅MSDN's Table hints。

另一种选择是确保AddressInRecordedDatabse PostcodeA表上PostcodeB和{{1}}以及变量＆amp;的数据类型具有适当的索引。字段匹配，因此没有隐式数据类型转换。

快速查询以从元组获取结果

2 个答案: