Retrieve only the first row from a table in Entity Framework

Question

Background:

Entity Framework 4, with SQL Server 2008

Problem:

I have a table Order. Each row has a column Timestamp.

The user can choose some time in past and I need to get the Order closest to the specified time, but that had occurred before the specified time. In other words, the last order before the specified time.

For example, if I have orders

2008-01-12
2009-04-17
2009-09-24
2010-11-02
2010-12-01
2011-05-16

and choose a date 2010-07-22, I should get the 2009-09-24 order, because that's the last order before the specified date.

var query = (from oData in db.OrderDatas
            where oData.Timestamp <= userTime
            orderby oData.Timestamp ascending
            select oData).Last();

This is closest to what I am trying. However, I am not sure how exactly does the Last operator work when translated to SQL, if it's translated at all.

Question:

Will this query fetch all data (earlier than userTime) and then take the last element, or will it be translated so that only one element will be returned from the database? My table can hold very large number of rows (100000+) so performance is an issue here.

Also, how would one retrieve the closest time in the database (not necessarily the earlier time)? In the example of 2010-07-22, one would get 2010-11-02, because it is closer to the date specified than 2009-09-24.

Answer 1

In general, if you're concerned about how LINQ behaves, you should check what does happen with the SQL. If you haven't worked out how to see how your LINQ queries are turned into SQL, that should be the very next thing you do.

As you noted in your comment, Last() isn't supported by LINQ to SQL so the same may be true for EF. Fortunately, it's easy to use First() instead:

var query = (from oData in db.OrderDatas
             where oData.Timestamp <= userTime
             orderby oData.Timestamp descending
             select oData).First();