Why does the Contains() operator degrade Entity Framework's performance so dramatically?

UPDATE: With the addition of InExpression in EF6, the performance of processing Enumerable.Contains improved dramatically. The approach described in this answer is no longer necessary.

You are right that most of the time is spent processing the translation of the query. EF's provider model doesn't currently include an expression that represents an IN clause, therefore ADO.NET providers can't support IN natively. Instead, the implementation of Enumerable.Contains translates it to a tree of OR expressions, i.e. for something that in C# looks like like this:

new []{1, 2, 3, 4}.Contains(i)

... we will generate a DbExpression tree that could be represented like this:

((1 = @i) OR (2 = @i)) OR ((3 = @i) OR (4 = @i))

(The expression trees have to be balanced because if we had all the ORs over a single long spine there would be more chances that the expression visitor would hit a stack overflow (yes, we actually did hit that in our testing))

We later send a tree like this to the ADO.NET provider, which can have the ability to recognize this pattern and reduce it to the IN clause during SQL generation.

When we added support for Enumerable.Contains in EF4, we thought it was desirable to do it without having to introduce support for IN expressions in the provider model, and honestly, 10,000 is much more than the number of elements we anticipated customers would pass to Enumerable.Contains. That said, I understand that this is an annoyance and that the manipulation of expressions trees makes things too expensive in your particular scenario.

I discussed this with one of our developers and we believe that in the future we could change the implementation by adding first-class support for IN. I will make sure this is added to our backlog, but I cannot promise when it will make it given there are many other improvements we would like to make.

To the workarounds already suggested in the thread I would add the following:

Consider creating a method that balances the number of database roundtrips with the number of elements you pass to Contains. For instance, in my own testing I observed that computing and executing against a local instance of SQL Server the query with 100 elements takes 1/60 of a second. If you can write your query in such a way that executing 100 queries with 100 different sets of ids would give you equivalent result to the query with 10,000 elements, then you can get the results in aproximately 1.67 seconds instead of 18 seconds.

Different chunk sizes should work better depending on the query and the latency of the database connection. For certain queries, i.e. if the sequence passed has duplicates or if Enumerable.Contains is used in a nested condition you may obtain duplicate elements in the results.

Here is a code snippet (sorry if the code used to slice the input into chunks looks a little too complex. There are simpler ways to achieve the same thing, but I was trying to come up with a pattern that preserves streaming for the sequence and I couldn't find anything like it in LINQ, so I probably overdid that part :) ):

Usage:

var list = context.GetMainItems(ids).ToList();

Method for context or repository:

public partial class ContainsTestEntities
{
    public IEnumerable<Main> GetMainItems(IEnumerable<int> ids, int chunkSize = 100)
    {
        foreach (var chunk in ids.Chunk(chunkSize))
        {
            var q = this.MainItems.Where(a => chunk.Contains(a.Id));
            foreach (var item in q)
            {
                yield return item;
            }
        }
    }
}

Extension methods for slicing enumerable sequences:

public static class EnumerableSlicing
{

    private class Status
    {
        public bool EndOfSequence;
    }

    private static IEnumerable<T> TakeOnEnumerator<T>(IEnumerator<T> enumerator, int count, 
        Status status)
    {
        while (--count > 0 && (enumerator.MoveNext() || !(status.EndOfSequence = true)))
        {
            yield return enumerator.Current;
        }
    }

    public static IEnumerable<IEnumerable<T>> Chunk<T>(this IEnumerable<T> items, int chunkSize)
    {
        if (chunkSize < 1)
        {
            throw new ArgumentException("Chunks should not be smaller than 1 element");
        }
        var status = new Status { EndOfSequence = false };
        using (var enumerator = items.GetEnumerator())
        {
            while (!status.EndOfSequence)
            {
                yield return TakeOnEnumerator(enumerator, chunkSize, status);
            }
        }
    }
}

Hope this helps!

If you find a performance problem which is blocking for you don't try to spend ages on solving it because you will most probably don't success and you will have to communicate it with MS directly (if you have premium support) and it takes ages.

Use workaround and workaround in case of performance issue and EF means direct SQL. There is nothing bad about it. Global idea that using EF = not using SQL anymore is a lie. You have SQL Server 2008 R2 so:

• Create stored procedure accepting table valued parameter to pass your ids
• Let your stored procedure return multiple result sets to emulate Include logic in optimal way
• If you need some complex query building use dynamic SQL inside stored procedure
• Use SqlDataReader to get results and construct your entities
• Attach them to context and work with them as if they were loaded from EF

If the performance is critical for you you will not find better solution. This procedure cannot be mapped and executed by EF because current version doesn't support either table valued parameters or multiple result sets.

We were able to solve the EF Contains problem by adding an intermediate table and joining on that table from LINQ query that needed to use Contains clause. We were able to get amazing results with this approach. We have a large EF model and as "Contains" is not allowed when pre-compiling EF queries we were getting very poor performance for queries that use "Contains" clause.

An overview:

• Create a table in SQL Server - for example HelperForContainsOfIntType with HelperID of Guid data-type and ReferenceID of int data-type columns. Create different tables with ReferenceID of differing data-types as needed.

• Create an Entity / EntitySet for HelperForContainsOfIntType and other such tables in EF model. Create different Entity / EntitySet for different data-types as needed.

• Create a helper method in .NET code which takes the input of an IEnumerable<int> and returns an Guid. This method generates a new Guid and inserts the values from IEnumerable<int> into HelperForContainsOfIntType along with the generated Guid. Next, the method returns this newly generated Guid to the caller. For fast inserting into HelperForContainsOfIntType table, create a stored-procedure which takes input of an list of values and does the insertion. See Table-Valued Parameters in SQL Server 2008 (ADO.NET). Create different helpers for different data-types or create a generic helper method to handle different data-types.

• Create a EF compiled query which is similar to something like below:

  static Func<MyEntities, Guid, IEnumerable<Customer>> _selectCustomers =
      CompiledQuery.Compile(
          (MyEntities db, Guid containsHelperID) =>
              from cust in db.Customers
              join x in db.HelperForContainsOfIntType on cust.CustomerID equals x.ReferenceID where x.HelperID == containsHelperID
              select cust 
      );
  

• Call the helper method with values to be used in the Contains clause and get the Guid to use in the query. For example:

  var containsHelperID = dbHelper.InsertIntoHelperForContainsOfIntType(new int[] { 1, 2, 3 });
  var result = _selectCustomers(_dbContext, containsHelperID).ToList();