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哪个查询具有更好的性能来返回具有特定属性最大值的对象？

 var i = from item in listOfItems
         orderby item.Number descending
         select item;

或者：

 var i = from item in listOfItems
          where item.Number== (from l in listOfItems select item).Max(l => l.Number)
          select item;
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2 回答 2

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在 SQL 中，这两个查询被转换为（至少，如果您在生成的 IEnumerables 中执行 .FirstOrDefault() 以选择想要的对象）：

SELECT TOP (1) [t0].[Number]
FROM [Item] AS [t0]
ORDER BY [t0].[Number] DESC

SELECT TOP (1) [t0].[Number]
FROM [Item] AS [t0]
WHERE [t0].[Number] = ((
    SELECT MAX([t1].[Number])
    FROM [Item] AS [t1]
    ))

根据任何事实，我不能说哪个会更快，但我会选择第一个顺序解决方案。

-* 编辑地址查询数据库 *-

如果您正在查询内存中的集合，则差异应该非常小。

然而，这是一个完全不科学的测试结果，它以两种不同的方式查询 100.000 个半随机数，每一个查询 10 次：

By desc (0): 00:00:00.0173879
By .Max (0): 00:00:00.0132833

By desc (1): 00:00:00.0250781
By .Max (1): 00:00:00.0140374

By desc (2): 00:00:00.0073955
By .Max (2): 00:00:00.0111658

By desc (3): 00:00:00.0066200
By .Max (3): 00:00:00.0115127

By desc (4): 00:00:00.0071220
By .Max (4): 00:00:00.0119572

By desc (5): 00:00:00.0070341
By .Max (5): 00:00:00.0114320

By desc (6): 00:00:00.0066670
By .Max (6): 00:00:00.0111127

By desc (7): 00:00:00.0071905
By .Max (7): 00:00:00.0116715

By desc (8): 00:00:00.0065414
By .Max (8): 00:00:00.0118076

By desc (9): 00:00:00.0071662
By .Max (9): 00:00:00.0131962

这是整个 LINQPad 脚本：

void Main()
{
    var listOfItems = new List<Item>();

    // Make 100000 Items with semirandom numbers
    for(int i=0; i<100000; i++)
    {
        listOfItems.Add(new Item { Number = i * DateTime.Now.Ticks });
    }

    for(int i=0; i<10; i++)
    {
        RunTest(i, listOfItems);
    }
}

void RunTest(int count, List<Item> listOfItems)
{
    var timer = new System.Diagnostics.Stopwatch();
    timer.Start();
    // Find by descending
    for(int i=0; i<100000; i++)
    {
        var itemWithLargestNumber = 
            from item in listOfItems
            orderby item.Number descending
            select item;
    }
    timer.Stop();
    string.Format("By desc ({0}): {1}", count, timer.Elapsed).Dump();

    timer = new System.Diagnostics.Stopwatch();
    timer.Start();

    // Find by .Max
    for(int i=0; i<100000; i++)
    {
        var itemWithLargestNumber = 
            from item in listOfItems
            where item.Number== (from l in listOfItems select item).Max(l => l.Number)
            select item;
    }
    timer.Stop();
    string.Format("By .Max ({0}): {1}", count, timer.Elapsed).Dump();

    "".Dump();
}

class Item 
{
    public long Number { get; set; }
}
于 2012-10-02T09:29:27.833 回答
0

好吧，这两个都返回一系列项目。我会使用：

var max = listOfItems.OrderByDescending(item => item.Number).FirstOrDefault();

然后检查查询的执行计划是什么。在 LINQ to Objects 中效率相对较低，但您确实需要查看生成的 SQL，以及这在数据库端的真正含义。

于 2012-10-02T09:08:46.840 回答