从元组中获取结果的快速查询

Question

I have a table that records distance from Google/Bing between two valid UK postcodes. As the system is used, this table is added to so that the next distance query becomes fast and I do not have to call web services to retrieve distance(s) online.

Here is the table structure:

    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

Now I have a Scalar UDF that I use through Linq-to-SQL and it is defined as follows:

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

I need to know if there is a any faster SQL statement or c# linq to obtain the result? If I retrieve 800 employees and run them against 50 jobs, the system goes into a pause and if I don't add the DBContext.GetDistanceFromCache() to the set of selects, the time taken is reduced quite significantly.

And here is the stalling query:

        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

I think your query is slow because the DB has to be called each time the LINQ query does select new {... } , which is as many times as you have result rows.

I would download the data from the table AddressInfoRecordedDistance before matching the result set to it. I'm thinking something like this:

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);

And there you have it. Of course, you also need the GetDistanceFromCache method:

// 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;
}

Of course, if you are running all of the above a lot of times, you should cache the variable airds .

Answer 2

If you're content with dirty-reads for your distance query, you may be able to add table hints like Nolock. See MSDN's Table hints for more information.

Another option would be to ensure you have proper indexes on the AddressInRecordedDatabse table for PostcodeA and PostcodeB and the datatypes of variables & fields match so there are no implicit datatype conversions.