LINQ query optimization by query rewriting using a custom IQueryable LINQ provider

In my SkyLinq open source project so far, I tried to make LINQ better and created several extension methods (for example, see GroupBy and TopK) that are more memory efficient than the standard methods in some scenarios. However, I do not necessarily want people to bind into these extensions methods directly for the following reasons:

  1. Good ideas evolve. New methods may come and existing methods may change.
  2. There is a parallel set of methods in IEnumerable<T>, IQueryable<T> and to some degree, in IObservable<T>. Using custom LINQ extension breaks the parallelism.

A better approach would be to write the LINQ queries normally using the standard library methods and then optimize the queries using a custom query provider. Query rewriting is not a new idea. SQL server has been doing this for years. I would like to bring this idea into LINQ.

If we examine the IQueryable<T> interface, we will find there is a parallel set of LINQ methods that accepts Expression<Func> instead of <Func>.


There is a little C# compiler trick here. If C# see a method that accepts Expression<Func>, it would create Expressions of Lamda instead of compiled Lambda expression. At run time, these expressions are passed to an IQueryable implementation and then passed to the underlying IQueryProvider implementation. The query provider is responsible for executing the expression tree and return the results. This is how the magic of LINQ to SQL and Entity Frameworks works.

The .net framework already has a class called EnumerableQuery<T>. It is a query provider than turns IQueryable class into LINQ to objects queries. In this work, I am going one step further by creating an optimizing query provider.

A common perception of writing a custom LINQ provider is that it requires a lot of code. The reason is that even for the most trivial provider we need to visit the every possible expressions in the System.Linq.Expressions namespace. (Note that we only need to handle expressions as of .net framework 3.5 but do not need to handle the new expressions added as part of dynamic language runtime in .net 4.x). There are reusable framework such as IQToolkit project that makes it easier to create a custom LINQ provider.

In contrast, creating an optimizing query provider is fairly easy. The ExpressionVistor class already has the framework that I need. I only needed to create a subclass of ExpressionVisitor called SkyLinqRewriter to override a few methods. The query rewriter basically replaces all call to the Queryable class with equivalent calls to the Enumerable class and rewrite queries for optimization when an opportunity presents.

It is fairly easy to consume the optimizing query provider. All we need is to call AsSkyLinqQueryable() to convert IEnumerable<T> to IQueryable<T> and remaining code can stay intact:


An end-to-end example can be found at

To conclude this post, I would recommend that we always code to IQueryable<T> instead of IEnumerable<T>. This way, the code has the flexibility to be optimized by an optimizing query provider, or be converted from pull based query to push based query using Reactive Extension without rewriting any code.

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