Impatient

Ain't nobody got time for data

---

• Introduction
• Getting Started with Impatient.EntityFrameworkCore.SqlServer
• Versioning & Compatibility
• More About EF Core
  • Implementation Differences
  • Unsupported Features
• Sample Query Translations (coming soon)
• How Impatient Translates Queries
  • A Primer
  • The Query Processing Pipeline

Introduction

Impatient is a library that provides the infrastructure needed to build powerful LINQ query providers for SQL databases. It offers support for:

• Navigation properties

• Almost every standard LINQ query operator, including (but not limited to):

  • Select/Where and their 'index argument' variants
  • OrderBy/ThenBy and their Descending variants
  • Join/GroupJoin/SelectMany/GroupBy
  • Average/Count/LongCount/Max/Min/Sum
  • All/Any/Contains
  • First/Last/Single/ElementAt and their OrDefault variants
  • Concat/Except/Intersect/Union
  • SkipWhile/TakeWhile and their 'index argument' variants
  • Distinct
  • Reverse
  • SequenceEqual
  • Zip

• Database native JSON support (like SQL Server's FOR JSON):

  • Materialize nested collections and complex-type columns within results instead of issuing n+1 queries
  • Query against properties of JSON objects
  • Query against elements of JSON arrays, treating them as if they were any other queryable sequence

Impatient.EntityFrameworkCore.SqlServer is a project that takes Impatient and extends it to provide a substitution for EF Core's default IQueryCompiler. It is backed by EF Core's own specification test suites with over 5500 passing tests, many of which demonstrate Impatient's ability to translate some of the toughest LINQ queries that other query providers and ORMs cannot.

Getting Started with Impatient.EntityFrameworkCore.SqlServer

1. Install Impatient.EntityFrameworkCore.SqlServer

2. Add UseImpatientQueryCompiler() to your DbContextOptions:

 services.AddDbContext<NorthwindDbContext>(options =>
 {
     options
         .UseSqlServer(connectionString)
+        .UseImpatientQueryCompiler();
 });

3. Cross arms, tap foot, run queries

Versioning & Compatibility

The only currently supported database engine is SQL Server 2016 or newer but it is a goal of the project to be extensible enough to foster the development of support for other database engines.

The Impatient package is currently using an 'unstable' version number because it is expected that the API will see some significant changes yet before settling down.

The Impatient.EntityFrameworkCore.SqlServer package will be versioned according to the minor version of EF Core that is supports; that is, a 2.0.x version will support a 2.0.x version of EF Core, a 2.1.x version will support a 2.1.x version of EF Core, and so on. Until further notice, the UseImpatientQueryCompiler extension method is the only designated stable public API in this package.

More About EF Core

Implementation Differences

• Impatient currently provides a naive implementation of async queries that does not make use of the cancellation token argument. Improvements to this area are planned.

• Impatient takes a pessimistic approach to change tracking when client evaluation occurs. That means that when a selector is evaluated on the client where an entity is passed as an argument to some kind of expression like a method call or a constructor, Impatient is going to add that entity to the change tracker whereas EF Core's query compiler would not.

• The "manual left join" pattern of GroupBy/SelectMany/DefaultIfEmpty will not propagate nullability for navigations. This may or may not be supported in the future. For now, the recommended course of action would be to either use navigations all the way or manual joins all the way. (See: the EFCore unit test Manually_created_left_join_propagates_nullability_to_navigations.)

• Calling OrderBy two times in a row will not perform a stable sort. This is Queryable; we don't have to match the implementation of Enumerable here and it doesn't really make sense to. (See: the EFCore unit test OrderBy_Multiple.)

• Any exceptions thrown at runtime by expressions being applied to DbParameter values will be wrapped in an InvalidOperationException and rethrown with the original exception as the InnerException. This will probably not be an issue for anybody, but it is a difference nonetheless.

Unsupported Features

• Certain translations like System.Convert calls and others are not implemented yet but are definitely planned for the future. The section on sample translations will eventually be updated to enumerate all of the supported translations.

• Client evaluation warnings (and throwing behavior) are not supported but there are plans to implement support for it.

• Warnings for and forced client evaluation of exception-throwing aggregates used in subqueries are not supported nor are there plans to support them.

• ROW_NUMBER paging is not supported nor are there plans to support it. Only the 'modern' OFFSET/FETCH NEXT is supported.

• "Null reference protection" is not supported nor are there plans to support it.

• FromSql is not supported nor are there plans to support it. If you want to use FromSql you should check out Dapper.

Sample Query Translations

This section is coming soon! For now, here's a teaser taken from the test library:

        [TestMethod]
        public void SequenceEqual_simple()
        {
            var m1s = impatient.CreateQuery<MyClass1>(MyClass1QueryExpression).Select(m1 => new { m1.Prop1, m1.Prop2 });
            var m2s = impatient.CreateQuery<MyClass2>(MyClass2QueryExpression).Select(m2 => new { m2.Prop1, m2.Prop2 });

            var result = m1s.SequenceEqual(m2s);

            Assert.IsTrue(result);

            Assert.AreEqual(
                @"SELECT CAST((CASE WHEN EXISTS (
    SELECT 1
    FROM (
        SELECT [m1].[Prop1] AS [Prop1], [m1].[Prop2] AS [Prop2], ROW_NUMBER() OVER(ORDER BY (SELECT 1) ASC) AS [$rownumber]
        FROM [dbo].[MyClass1] AS [m1]
    ) AS [t]
    FULL JOIN (
        SELECT [m2].[Prop1] AS [Prop1], [m2].[Prop2] AS [Prop2], ROW_NUMBER() OVER(ORDER BY (SELECT 1) ASC) AS [$rownumber]
        FROM [dbo].[MyClass2] AS [m2]
    ) AS [t_0] ON [t].[$rownumber] = [t_0].[$rownumber]
    WHERE (([t].[$rownumber] IS NULL) OR ([t_0].[$rownumber] IS NULL)) OR (([t].[Prop1] <> [t_0].[Prop1]) OR ([t].[Prop2] <> [t_0].[Prop2]))
) THEN 0 ELSE 1 END) AS bit)",
                SqlLog);
        }

How Impatient Translates Queries

A Primer

Note: this section assumes some familiarity with IQueryable, Expression, and ExpressionVisitor.

A query begins with a RelationalQueryExpression which describes the resulting type of the query and contains a child SelectExpression. The SelectExpression is the Expression equivalent of a SQL SELECT statement. Calls to Queryable methods like Where, Select, GroupBy, and so forth are then interpreted by Impatient to compose upon the RelationalQueryExpression's SelectExpression.

For instance, let's look at the Northwind database. We have a Customers table that we want to query. The first thing we will do is define a basic SelectExpression representing the very basic SQL query SELECT [c].[CustomerID], [c].[CompanyName] /* and so on */ FROM [dbo].[Customers] AS [c]:

var table = new BaseTableExpression("dbo", "Customer", "c", typeof(Customer));

var materializer
    = Expression.MemberInit(
        Expression.New(typeof(Customer)),
        from property in GetWhateverProperties(typeof(Customer))
        let column = new SqlColumnExpression(table, property.Name, property.PropertyType)
        select Expression.Bind(property, column));
        
var queryExpression
  = new EnumerableRelationalQueryExpression(
      new SelectExpression(
        new ServerProjectionExpression(materializer),
        table));

Then, we get an instance of our query provider and use CreateQuery<Customer>(expression) to get our IQueryable<Customer>:

var customers 
    = services
        .GetRequiredService<ImpatientQueryProvider>()
        .CreateQuery<Customer>(queryExpression);

Now that we have our customer query, we can... well, query on it:

var customers = 
    (from c in customers 
     where c.City == "Berlin" 
     select new { c.CustomerID, c.CompanyName, c.ContactName }).ToList();

At this point, Impatient will translate and compile the query and execute it. Let's consider how it applies Where and Select. The expression tree contains a call to Queryable.Where(customers, c => c.City == "Berlin"). Impatient grabs the query expression from the first argument and the lambda expression from the second argument.

The projection expression from the query looks like this:

new Customer()
{
    CustomerID = [SqlColumnExpression],
    CompanyName = [SqlColumnExpression],
    ContactName = [SqlColumnExpression],
    City = [SqlColumnExpression],
    // and so on.
}

In order to interpret the Where predicate, Impatient replaces all instances of the parameter c with the current projection. So we get this:

new Customer()
{
    CustomerID = [SqlColumnExpression],
    CompanyName = [SqlColumnExpression],
    ContactName = [SqlColumnExpression],
    City = [SqlColumnExpression],
    // and so on.
}.City == "Berlin"

Impatient then simplifies everything it can in the expression. There is a member access on the Customer to its City property, and because the Customer expression is a MemberInitExpression with City as a binding, we reduce the member access down to the bound value:

[SqlColumnExpression] == "Berlin"

This is translatable to SQL, so it gets added to the SelectExpression and the call to Where has now been replaced by a RelationalQueryExpression. The same process takes place for the call to Select:

new { c.CustomerID, c.CompanyName, c.ContactName }

becomes

new { [SqlColumnExpression], [SqlColumnExpression], [SqlColumnExpression] }

which is translatable, so it replaces the projection on the SelectExpression and the call to Select has now been replaced with a RelationalQueryExpression.

This is how all of the query operators are translated, although many are more involved, like GroupBy or SelectMany.

The Query Processing Pipeline

There are three main stages to processing a query expression.

1. Preparation

   The query expression is parameterized, meaning all constant expressions that are not literal constants are swapped out for parameter expressions. Literal constants are constants like numeric literals, string literals, and so forth. Basically, we look for closure instances and swap them out.

   The query expression is then inlined, meaning we look into all of the subtrees of type IQueryable, re-swap the constants back into place, and see if new query subtrees are produced. Those appear in lambda expressions, such as calls to SelectMany. If we successfully inline a query subtree, we will parameterize that subtree because it may reference new closures or other constants that were not present prior to inlining.

   Finally, the query expression is hashed, meaning we visit the entire tree and build a hash code based on the structure and semantics of the tree. This hash code is used to look up a previously compiled 'execution plan' for the query so it can just be run using the parameterized constants instead of having to complete the rest of the pipeline.

2. Composition

   The query expression is composed by a sequence of composing expression visitors. The default sequence of composing visitors consists of one that rewrites navigation properties into calls to the appropriate Join/GroupJoin/ etc. method calls, one that visits each call to a Queryable or Enumerable method with a lambda expression and uses the lambda's parameter name(s) to set the table aliases in the corresponding SelectExpression, and one that interprets calls to Queryable or Enumerable methods and uses them to compose (build) SelectExpressions.

   That last visitor is in a sense the composing expression visitor, and it employs the use of another sequence of expression visitors -- the rewriting expression visitors. They are named so because they rewrite certain types of expressions like DateTime.Now or string.IsNullOrEmpty into translatable forms using things like SqlFunctionExpression, SqlAggregateExpression, or SqlInExpression. These visitors are used every time a lambda's parameters are expanded, and afterward, the resulting expression is analyzed for translatability. The composing visitor then uses the outcome of that analysis to determine whether or not it needs to fall back to client evaluation or in some cases take a hybrid approach.

   There is also a sequence of visitors known as the optimizing expression visitors. This sequence of visitors is applied before the composing visitors, between each of the composing visitors, and after all of the composing visitors. They consist of simple expression tree optimizations, like reducing or inverting some boolean-typed binary expressions, rewriting comparisons between conditional expressions into a binary expression tree, or removing unnecessary type conversions. These visitors are run when they are run so that each composing visitor only needs to worry about applying such optimizations as needed for its own purposes.

3. Compilation

   The query expression is compiled by a sequence of compiling expression visitors. This is where the SelectExpressions are translated into SQL and the projection expressions are converted into materializer delegates that read from a DbDataReader to produce results.

   The compiled query expression is then used as the body of a lambda expression which is given the parameters discovered in the preparation stage. The lambda expression is then compiled into an executable delegate and cached using the hash code from the preparation stage.