When is it OK to utilize a CompiledQuery?

asp.net c# compiled-query entity-framework


I have a table:

-- Tag

ID  | Name
1   | c#
2   | linq
3   | entity-framework

I have a class that will have the following methods:

IEnumerable<Tag> GetAll();
IEnumerable<Tag> GetByName();

Should I use a compiled query in this case?

static readonly Func<Entities, IEnumerable<Tag>> AllTags =
    CompiledQuery.Compile<Entities, IEnumerable<Tag>>
        e => e.Tags

Then my GetByName method would be:

IEnumerable<Tag> GetByName(string name)
    using (var db = new Entities())
        return AllTags(db).Where(t => t.Name.Contains(name)).ToList();

Which generates a SELECT ID, Name FROM Tag and execute Where on the code. Or should I avoid CompiledQuery in this case?

Basically I want to know when I should use compiled queries. Also, on a website they are compiled only once for the entire application?

2/8/2011 12:04:45 PM

Accepted Answer

You should use a CompiledQuery when all of the following are true:

  • The query will be executed more than once, varying only by parameter values.
  • The query is complex enough that the cost of expression evaluation and view generation is "significant" (trial and error)
  • You are not using a LINQ feature like IEnumerable<T>.Contains() which won't work with CompiledQuery.
  • You have already simplified the query, which gives a bigger performance benefit, when possible.
  • You do not intend to further compose the query results (e.g., restrict or project), which has the effect of "decompiling" it.

CompiledQuery does its work the first time a query is executed. It gives no benefit for the first execution. Like any performance tuning, generally avoid it until you're sure you're fixing an actual performance hotspot.

2012 Update: EF 5 will do this automatically (see "Entity Framework 5: Controlling automatic query compilation") . So add "You're not using EF 5" to the above list.

10/18/2012 7:51:29 PM

Popular Answer

Compiled queries save you time, which would be spent generating expression trees. If the query is used often and you'll save the compiled query, you should definitely use it. I had many cases when the query parsing took more time than the actual round trip to the database.

In your case, if you are sure that it would generate SELECT ID, Name FROM Tag without the WHERE case (which I doubt, as your AllQueries function should return IQueryable and the actual query should be made only after calling ToList) - you shouldn't use it.

As someone already mentioned, on bigger tables SELECT * FROM [someBigTable] would take very long and you'll spend even more time filtering that on the client side. So you should make sure that your filtering is made on the database side, no matter if you are using compiled queries or not.

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