HOW TO：控制 PLINQ 查詢中的順序
這些範例顯示如何使用 AsOrdered 擴充方法來控制 PLINQ 查詢中的順序。
.gif "注意事項") 警告 |
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這些範例主要是為了示範用法，執行速度不一定比對應的循序 LINQ to Objects 查詢更快。 |
範例
下列範例會保留來源序列的順序。 這有時候是必要的，例如某些查詢運算子需要已排序的來源序列，才能產生正確的結果。
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
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 v in parallelQuery)
Console.Write("{0} ", v);
// Some operators expect an ordered source sequence.
var lowValues = parallelQuery.Take(10);
下列範例顯示一些可能需要已排序來源序列的查詢運算子。 這些運算子對未排序的序列同樣能運作，但可能會產生未預期的結果。
' 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
// 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);
}
若要執行這個方法，請將方法貼至 PLINQ 資料範例專案中的 PLINQDataSample 類別，然後按 F5。
下列範例顯示如何保留查詢中第一個部分的順序，接著移除順序以增加 join 子句的效能，然後再將順序重新套用至最終結果序列。
' 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
// 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);
}
若要執行這個方法，請將方法貼至 PLINQ 資料範例專案中的 PLINQDataSample 類別，然後按 F5。