LINQ to SQL*编译*查询以及何时执行

Question

我有以下编译查询.

private static Func<Db, int, IQueryable<Item>> func =
        CompiledQuery.Compile((Db db, int id) => 
            from i in db.Items
            where i.ID == id
            select i
            );

当我这样做时,它立即在数据库上执行

var db = new Db()
var query = func(db, 5);  // Query hits the database here

就像在做之前一样

var result = query.SingleOrDefault(); // Happens in memory

但是如果没有编译这个查询,就像在

var query = from i in db.Items
            where i.ID == id
            select i

然后它在完成后在数据库上执行

   var result = query.SingleOrDefault();

这是预期的行为吗？

注意:这是返回IQueryable执行的编译查询何时重复？,但那里的所有答案似乎都不同意我的发现.我已经在那里发布了我的答案,但我不知道如何让人们关注它,因为它已经超过2年了.

Answer 1

有趣的问题.将它带到反编译源,编译查询时,会发生以下情况:

public static Func<TArg0, TArg1, TResult> Compile<TArg0, TArg1, TResult>(Expression<Func<TArg0, TArg1, TResult>> query) where TArg0 : DataContext
{
  if (query == null)
    System.Data.Linq.Error.ArgumentNull("query");
  if (CompiledQuery.UseExpressionCompile((LambdaExpression) query))
    return query.Compile();
  else
    return new Func<TArg0, TArg1, TResult>(new CompiledQuery((LambdaExpression) query).Invoke<TArg0, TArg1, TResult>);
}

UseExpressionCompile方法定义如下:

private static bool UseExpressionCompile(LambdaExpression query)
{
  return typeof (ITable).IsAssignableFrom(query.Body.Type);
}

对于您定义的表达式,此计算结果为false,因此使用else案例.

Invoke是这样的:

private TResult Invoke<TArg0, TArg1, TResult>(TArg0 arg0, TArg1 arg1) where TArg0 : DataContext
{
  return (TResult) this.ExecuteQuery((DataContext) arg0, new object[2]
  {
    (object) arg0,
    (object) arg1
  });
}

ExecuteQuery就像:

private object ExecuteQuery(DataContext context, object[] args)
{
  if (context == null)
    throw System.Data.Linq.Error.ArgumentNull("context");
  if (this.compiled == null)
  {
    lock (this)
    {
      if (this.compiled == null)
        this.compiled = context.Provider.Compile((Expression) this.query);
    }
  }
  return this.compiled.Execute(context.Provider, args).ReturnValue;
}

在这种情况下,我们的提供程序是SqlProvider类,SqlProvider.CompiledQuery是实现ICompiledQuery的类.执行该类的执行:

  public IExecuteResult Execute(IProvider provider, object[] arguments)
  {
    if (provider == null)
      throw System.Data.Linq.SqlClient.Error.ArgumentNull("provider");
    SqlProvider sqlProvider = provider as SqlProvider;
    if (sqlProvider == null)
      throw System.Data.Linq.SqlClient.Error.ArgumentTypeMismatch((object) "provider");
    if (!SqlProvider.CompiledQuery.AreEquivalentShapes(this.originalShape, sqlProvider.services.Context.LoadOptions))
      throw System.Data.Linq.SqlClient.Error.CompiledQueryAgainstMultipleShapesNotSupported();
    else
      return sqlProvider.ExecuteAll(this.query, this.queryInfos, this.factory, arguments, this.subQueries);
  }

SqlProvider.ExecuteAll调用SqlProvider.Execute,这是一个非常大的方法,所以我将发布亮点:

private IExecuteResult Execute(Expression query, SqlProvider.QueryInfo queryInfo, IObjectReaderFactory factory, object[] parentArgs, object[] userArgs, ICompiledSubQuery[] subQueries, object lastResult)
{
  this.InitializeProviderMode();
  DbConnection dbConnection = this.conManager.UseConnection((IConnectionUser) this);
  try
  {
    DbCommand command = dbConnection.CreateCommand();
    command.CommandText = queryInfo.CommandText;
    command.Transaction = this.conManager.Transaction;
    command.CommandTimeout = this.commandTimeout;
    this.AssignParameters(command, queryInfo.Parameters, userArgs, lastResult);
    this.LogCommand(this.log, command);
    ++this.queryCount;
    switch (queryInfo.ResultShape)
    {
      case SqlProvider.ResultShape.Singleton:
        DbDataReader reader1 = command.ExecuteReader();
...
      case SqlProvider.ResultShape.Sequence:
        DbDataReader reader2 = command.ExecuteReader();
...
      default:
        return (IExecuteResult) new SqlProvider.ExecuteResult(command, queryInfo.Parameters, (IObjectReaderSession) null, (object) command.ExecuteNonQuery(), true);
    }
  }
  finally
  {
    this.conManager.ReleaseConnection((IConnectionUser) this);
  }
}

在获取和释放连接之间,它除了sql命令之外.所以我说你是对的.与普遍看法相反,编译查询在延迟执行时的行为与未编译查询的行为不同.

我很确定你可以从MS下载实际的源代码,但我没有它的方便,Resharper 6有一个很棒的反编译功能,所以我只是用它.