Question

哪个查询具有更好的性能来返回具有特定属性的最大值的对象？

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

或：

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

Answer 1

在SQL中，两个查询被翻译成（至少，如果你在生成的IEnumerables中执行.FirstOrDefault（）来选择想要的对象）：

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]
    ))

我不能说基于任何事实会更快哪些，但我会选择第一个订购解决方案。

- *编辑以解决查询数据库* -

如果要查询内存集合，差异应该非常小。

然而，这是一个完全不科学的测试结果，通过查询每个10次的两种不同方式查询100.000个半随机数：

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

以下是整个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; }
}

Answer 2

这两个都返回了项目的序列。我会用：

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，以及真正在数据库端的含义。

哪个查询具有更好的性能来返回具有特定属性的最大值的对象？

2 个答案: