LINQ Select Distinct By One Column Timeout Error

Question

I have a large table (~3 million rows) that I'm querying for just a few records using a distinct column (OperationTypeId). In SQL Management Studio, I'm using the following query which executes in 4 seconds and return 7 rows.

SELECT  *
FROM    (SELECT t.ReferenceNumber, t.OperationTypeId,
        ROW_NUMBER() OVER (PARTITION BY OperationTypeId     
         ORDER BY OperationTypeId) AS RowNumber
         FROM   TP_Transaction t) AS a
WHERE   a.RowNumber = 1

I'm trying to accomplish the same result in code using LINQ to SQL, and this is my code.

var temp = dbConn.TP_Transactions
                .GroupBy(x => x.OperationTypeId)
                .Select(g => new {OperationTypeId = g.Key, ReferenceNumber = g.FirstOrDefault().ReferenceNumber})
                .ToList();

However, this code results in an exception:

Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.

Please advise on what I can do to accomplish this query.

Answer 1

I ended up doing this instead. It still takes a significant amount of time to run (~2 minutes) but at least I'm getting some results back.

const string sqlQuery = @"SELECT  *
                    FROM    (SELECT t.ReferenceNumber,
                    ROW_NUMBER() OVER (PARTITION BY OperationTypeId ORDER BY OperationTypeId) AS RowNumber
                    FROM   TP_Transaction t) AS a
                    WHERE   a.RowNumber = 1";
var temp = dbConn.ExecuteQuery<Temp>(sqlQuery).ToList();