实体框架为每种类型的记录选择最近的记录

Question

I have a table called values that looks like this:

+-------+------------+-----------+----------+
|Id     |DateTime    |SensorId   |Value     |
+-------+------------+-----------+----------+

SensorId is a foreign key to a table of the sensor details. There will be 10m+ records in this values table.

I can run this sql command to return the most recent record for each SensorId and it runs in about 0.3 seconds.

SELECT a.*
    FROM Values as a
    INNER JOIN (
        SELECT SensorId, MAX(ID) maxId 
        FROM Values
        GROUP BY SensorId
    ) b ON a.SensorId = b.SensorId 
        AND a.Id = b.maxId
ORDER BY a.SensorId ASC

How can I achieve the same output with entity framework in ac# application while maintaining (or improving) the performance?

Answer 1

With LINQ to Entities and lambdas you can do it like this:

dataContext.Values.GroupBy(p => p.SensorId)
     .Select(p => p.FirstOrDefault(w => w.Id == p.Max(m => m.Id)))  
     .OrderBy(p => p.SensorId).ToList()

where dataContext is your instance of ObjectContext class. ToList() compiles the query.

Answer 2

1. I guess it would not be possible reach better performance than pure SQl query because by using EF you are adding abstraction layer to the process.
2. EF is usually very slow with GroupBy command. I suggest try sql query in EF directly (this code is for EF Core )

context.Values.FromSqlRaw<Values>("SELECT a.*
    FROM Values as a
    INNER JOIN (
        SELECT SensorId, MAX(ID) maxId 
        FROM Values
        GROUP BY SensorId
    ) b ON a.SensorId = b.SensorId 
        AND a.Id = b.maxId
ORDER BY a.SensorId ASC").ToList<Values>();

FromSqlRaw is faster than a normal Linq Query. for EF you can try context.ExecuteQuery<Values> by the same way.