[英]Fast query to obtain a result from a tuple
我有一張表格,記錄了兩個有效的英國郵政編碼之間與 Google/Bing 的距離。 隨着系統的使用,這個表被添加,以便下一個距離查詢變得更快,我不必調用 web 服務來在線檢索距離。
這是表結構:
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
現在我有一個通過 Linq-to-SQL 使用的標量 UDF,它的定義如下:
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 語句或 c# linq 以獲得結果? 如果我檢索 800 名員工並針對 50 個作業運行它們,系統會暫停,如果我不將DBContext.GetDistanceFromCache()
添加到選擇集,則所用時間會大大減少。
這是停滯的查詢:
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
};
我認為您的查詢很慢,因為每次 LINQ 查詢執行select new {... }
時都必須調用數據庫,這與您的結果行數一樣多。
在將結果集與之匹配之前,我會從表AddressInfoRecordedDistance
下載數據。 我在想這樣的事情:
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。 當然,你還需要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;
}
當然,如果您多次運行上述所有操作,則應該緩存變量airds
。
如果您對距離查詢的臟讀感到滿意,您可以添加像 Nolock 這樣的表提示。 有關詳細信息,請參閱MSDN 的表格提示。
另一種選擇是確保您在PostcodeA
和PostcodeB
的AddressInRecordedDatabse
表上有正確的索引,並且變量和字段的數據類型匹配,因此沒有隱式數據類型轉換。
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