Question

Let’s say i have a table in a database with 10k records. I dont need to actually use those 10k records anymore, but i still need to keep them in the database. That very table is now going to be used to store new data. So there’s gonna be more records coming on top of the 10K records already present in the table. As opposed to the “old” 10K records, i do need to work with the newly inserted data. Right now im doing this to get the data i need:

List<Stuff> l = (from x in db.Table
            where x.id > id
            select x).ToList();

My question now is: how does the where clause in LINQ (or in SQL in general) work under the covers? Is the ENTIRE table going to be searched until (x.id > id) is true? Because let’s say the table will increase from 10k records to 20K. It’d be a little silly to look through the entire 20 k records, if i know that i only have to start looking from a certain point.

I’ve had performance problems (not dramatic, but bad enough to be agitated by it) with this while using LINQ to entities, which i kinda don’t understand because it should be no problem at all for a modern computer to sift through a mere 20 k records. I’ve been advised to use a stored procedure instead of a LINQ query, but i dont know whether or not this will boost performance?

Any feedback will be appreciated.

Answer 1

It’s going to behave just like a similarly worded SQL query would. The question is whether the overhead you’re experiencing is happening in the query or in the conversion of the query to a list. The query itself as you’ve written should equate literally to:

Select ID, Column1, Column2, Column3, ... , Column(n+1)
From db.Table
Where ID > id

This query should be fairly fast depending on the nature of the data. The query itself will not be executed until it is acted upon, however. In this case, you’re converting it to a list, which is the equivalent of acting upon it. I can’t find the comment someone made to me about this practice, but I’ve found it too be quite helpful in keeping performance clean. Unless you have some very specific need, you should leave your queries as IQueryable. Converting them to lists doubles the effort because first the query must be executed and then the result set must be converted into an appropriate IEnumerable (List in this case).

So you have 2 potential bottlenecks. The simple query could be taking a long time to query a massive collection of data, or the number of records could be bottenecking at the poing where the List is created. Another possibility is the nature of ID in this case. If it is numeric, that will save you some time. If it’s performing a text-based search then it’s going to be heavier.

To answer your specific question, yes, it’s going to search every record in the database and return all of the records that match the expression. Edit: If the database has a proper index on the column in question, it will not search EVERY record but rather will use the index to perform the search. From comment from @Pleun.

As for using a stored procedure, that’s a load of hogwash, but it’s a perfectly acceptable alternative. I have several programs that routinely run similar queries against a database with over 40 million records, and the only performance issue I’ve run into so far has been CPU usage when multiple users are performing rapid firing queries. To solve your specific issue, I’d recommend that you tune it a little in SQL Management Studio until the query you want returns to your interface with an acceptable speed. Then you can convert that query into a compatible Linq statement. As long as you leave it as an IQueryable it should exhibit similar results.