如何：在 PLINQ 查询中控制排序

  • 项目

这些示例展示了如何使用 AsOrdered 扩展方法控制 PLINQ 查询中的顺序。

警告

这些示例主要用于演示用法，可能会或可能不会比相当的顺序 LINQ to Objects 查询快。

示例 1

下面的示例暂留了源序列的顺序。 有时，这样做是有必要的；例如，一些查询运算符需要有序的源序列，才能生成正确结果。

var source = Enumerable.Range(9, 10000);

// Source is ordered; let's preserve it.
var parallelQuery =
    from num in source.AsParallel().AsOrdered()
    where num % 3 == 0
    select num;

// Use foreach to preserve order at execution time.
foreach (var item in parallelQuery)
{
    Console.Write($"{item} ");
}

// Some operators expect an ordered source sequence.
var lowValues = parallelQuery.Take(10);

Sub OrderedQuery()

    Dim source = Enumerable.Range(9, 10000)

    ' Source is ordered let's preserve it.
    Dim parallelQuery = From num In source.AsParallel().AsOrdered()
                        Where num Mod 3 = 0
                        Select num

    ' Use For Each to preserve order at execution time.
    For Each item In parallelQuery
        Console.Write("{0} ", item)
    Next

    ' Some operators expect an ordered source sequence.
    Dim lowValues = parallelQuery.Take(10)

End Sub

示例 2

下面的示例展示了一些可能要求源序列应为有序的查询运算符。 虽然这些运算符可以处理无序序列，但可能会生成意外结果。

// Paste into PLINQDataSample class.
static void SimpleOrdering()
{

    var customers = GetCustomers();

    // Take the first 20, preserving the original order
    var firstTwentyCustomers = customers
                                .AsParallel()
                                .AsOrdered()
                                .Take(20);

    foreach (var c in firstTwentyCustomers)
        Console.Write("{0} ", c.CustomerID);

    // All elements in reverse order.
    var reverseOrder = customers
                        .AsParallel()
                        .AsOrdered()
                        .Reverse();

    foreach (var v in reverseOrder)
        Console.Write("{0} ", v.CustomerID);

    // Get the element at a specified index.
    var cust = customers.AsParallel()
                        .AsOrdered()
                        .ElementAt(48);

    Console.WriteLine("Element #48 is: {0}", cust.CustomerID);
}

' Paste into PLINQDataSample class
Shared Sub SimpleOrdering()
    Dim customers As List(Of Customer) = GetCustomers().ToList()

    ' Take the first 20, preserving the original order

    Dim firstTwentyCustomers = customers _
                                .AsParallel() _
                                .AsOrdered() _
                                .Take(20)

    Console.WriteLine("Take the first 20 in original order")
    For Each c As Customer In firstTwentyCustomers
        Console.Write(c.CustomerID & " ")
    Next

    ' All elements in reverse order.
    Dim reverseOrder = customers _
                        .AsParallel() _
                        .AsOrdered() _
                        .Reverse()

    Console.WriteLine(vbCrLf & "Take all elements in reverse order")
    For Each c As Customer In reverseOrder
        Console.Write("{0} ", c.CustomerID)
    Next
    ' Get the element at a specified index. 
    Dim cust = customers.AsParallel() _
                        .AsOrdered() _
                        .ElementAt(48)

    Console.WriteLine("Element #48 is: " & cust.CustomerID)

End Sub

若要运行此方法，请将它粘贴到 PLINQ 数据样本项目的 PLINQDataSample 类中，再按 F5。

示例 3

下面的示例展示了如何暂留查询第一部分的顺序，再删除顺序以提升 join 子句的性能，并对最终结果序列重新应用顺序。

// Paste into PLINQDataSample class.
static void OrderedThenUnordered()
{

    var orders = GetOrders();
    var orderDetails = GetOrderDetails();

    var q2 = orders.AsParallel()
       .Where(o => o.OrderDate < DateTime.Parse("07/04/1997"))
       .Select(o => o)
       .OrderBy(o => o.CustomerID) // Preserve original ordering for Take operation.
       .Take(20)
       .AsUnordered()  // Remove ordering constraint to make join faster.
       .Join(
              orderDetails.AsParallel(),
              ord => ord.OrderID,
              od => od.OrderID,
              (ord, od) =>
              new
              {
                  ID = ord.OrderID,
                  Customer = ord.CustomerID,
                  Product = od.ProductID
              }
             )
       .OrderBy(i => i.Product); // Apply new ordering to final result sequence.

    foreach (var v in q2)
        Console.WriteLine("{0} {1} {2}", v.ID, v.Customer, v.Product);
}

' Paste into PLINQDataSample class
Sub OrderedThenUnordered()
    Dim Orders As IEnumerable(Of Order) = GetOrders()
    Dim orderDetails As IEnumerable(Of OrderDetail) = GetOrderDetails()

    ' Sometimes it's easier to create a query
    ' by composing two subqueries
    Dim query1 = From ord In Orders.AsParallel()
                 Where ord.OrderDate < DateTime.Parse("07/04/1997")
                 Select ord
                 Order By ord.CustomerID
                 Take 20

    Dim query2 = From ord In query1.AsUnordered()
                 Join od In orderDetails.AsParallel() On ord.OrderID Equals od.OrderID
                 Order By od.ProductID
                 Select New With {ord.OrderID, ord.CustomerID, od.ProductID}


    For Each item In query2
        Console.WriteLine("{0} {1} {2}", item.OrderID, item.CustomerID, item.ProductID)
    Next
End Sub

若要运行此方法，请将它粘贴到 PLINQ 数据样本项目的 PLINQDataSample 类中，再按 F5。

请参阅