在系统中。Linq命名空间,我们现在可以扩展我们的IEnumerable来拥有Any()和Count()扩展方法。
最近有人告诉我,如果我想检查一个集合中是否包含1个或多个项目,我应该使用. any()扩展方法而不是. count() > 0扩展方法,因为. count()扩展方法必须遍历所有项目。
其次,一些集合具有Count或Length属性(而不是扩展方法)。使用这些,而不是.Any()或.Count()会更好吗?
-是的-不?
在系统中。Linq命名空间,我们现在可以扩展我们的IEnumerable来拥有Any()和Count()扩展方法。
最近有人告诉我,如果我想检查一个集合中是否包含1个或多个项目,我应该使用. any()扩展方法而不是. count() > 0扩展方法,因为. count()扩展方法必须遍历所有项目。
其次,一些集合具有Count或Length属性(而不是扩展方法)。使用这些,而不是.Any()或.Count()会更好吗?
-是的-不?
当前回答
如果您正在使用实体框架,并且有一个包含许多记录的巨大表,Any()将会更快。我记得有一次,我想检查一个有数百万行的表是否为空。Count() > 0完成耗时20-30秒。Any()是即时的。
Any()可以提高性能,因为它可能不必迭代集合来获得事物的数量。只要击中其中一个就行。或者,对于LINQ-to-Entities,生成的SQL将是IF EXISTS(…)而不是SELECT COUNT…甚至选择* ....
其他回答
编辑:在EF 6.1.1版本中修复。这个答案是不实际的
对于SQL Server和EF4-6, Count()的执行速度比Any()快两倍。
当你运行Table.Any()时,它会生成类似这样的东西(警告:不要伤害大脑试图理解它)
SELECT
CASE WHEN ( EXISTS (SELECT
1 AS [C1]
FROM [Table] AS [Extent1]
)) THEN cast(1 as bit) WHEN ( NOT EXISTS (SELECT
1 AS [C1]
FROM [Table] AS [Extent2]
)) THEN cast(0 as bit) END AS [C1]
FROM ( SELECT 1 AS X ) AS [SingleRowTable1]
这需要对符合条件的行进行两次扫描。
我不喜欢写Count() > 0,因为它隐藏了我的意图。我更喜欢使用自定义谓词:
public static class QueryExtensions
{
public static bool Exists<TSource>(this IQueryable<TSource> source, Expression<Func<TSource, bool>> predicate)
{
return source.Count(predicate) > 0;
}
}
这取决于数据集有多大,以及您的性能要求是什么?
如果不是很大,就用最易读的形式, 这对我来说是任何,因为它比方程更短更易读。
注意:我写这个答案时实体框架4是实际的。这个答案的重点并不是要进行琐碎的. any () vs . count()性能测试。重点是要表明EF远非完美。更新的版本更好…但是如果你有部分代码很慢,并且使用EF,请直接使用TSQL测试并比较性能,而不是依赖于假设(. any()总是比. count() > 0快)。
虽然我同意大多数赞成的答案和评论-特别是在这一点上,任何信号开发人员的意图都比Count() > 0 -我有过这样的情况,在SQL Server (EntityFramework 4)上,Count的速度要快一个数量级。
下面是使用Any查询的w超时异常(在~200.000条记录上):
con = db.Contacts.
Where(a => a.CompanyId == companyId && a.ContactStatusId <= (int) Const.ContactStatusEnum.Reactivated
&& !a.NewsletterLogs.Any(b => b.NewsletterLogTypeId == (int) Const.NewsletterLogTypeEnum.Unsubscr)
).OrderBy(a => a.ContactId).
Skip(position - 1).
Take(1).FirstOrDefault();
以毫秒为单位执行的计数版本:
con = db.Contacts.
Where(a => a.CompanyId == companyId && a.ContactStatusId <= (int) Const.ContactStatusEnum.Reactivated
&& a.NewsletterLogs.Count(b => b.NewsletterLogTypeId == (int) Const.NewsletterLogTypeEnum.Unsubscr) == 0
).OrderBy(a => a.ContactId).
Skip(position - 1).
Take(1).FirstOrDefault();
我需要找到一种方法来查看两个linq产生的确切SQL -但很明显,在某些情况下,Count和Any之间存在巨大的性能差异,不幸的是,似乎您不能在所有情况下都坚持使用Any。
编辑:这里是生成的sql。你可以看到的美;)
ANY:
exec sp_executesql N'SELECT TOP (1) [Project2].[ContactId] AS [ContactId], [Project2].[CompanyId] AS [CompanyId], [Project2].[ContactName] AS [ContactName], [Project2].[FullName] AS [FullName], [Project2].[ContactStatusId] AS [ContactStatusId], [Project2].[Created] AS [Created] FROM ( SELECT [Project2].[ContactId] AS [ContactId], [Project2].[CompanyId] AS [CompanyId], [Project2].[ContactName] AS [ContactName], [Project2].[FullName] AS [FullName], [Project2].[ContactStatusId] AS [ContactStatusId], [Project2].[Created] AS [Created], row_number() OVER (ORDER BY [Project2].[ContactId] ASC) AS [row_number] FROM ( SELECT [Extent1].[ContactId] AS [ContactId], [Extent1].[CompanyId] AS [CompanyId], [Extent1].[ContactName] AS [ContactName], [Extent1].[FullName] AS [FullName], [Extent1].[ContactStatusId] AS [ContactStatusId], [Extent1].[Created] AS [Created] FROM [dbo].[Contact] AS [Extent1] WHERE ([Extent1].[CompanyId] = @p__linq__0) AND ([Extent1].[ContactStatusId] <= 3) AND ( NOT EXISTS (SELECT 1 AS [C1] FROM [dbo].[NewsletterLog] AS [Extent2] WHERE ([Extent1].[ContactId] = [Extent2].[ContactId]) AND (6 = [Extent2].[NewsletterLogTypeId]) )) ) AS [Project2] ) AS [Project2] WHERE [Project2].[row_number] > 99 ORDER BY [Project2].[ContactId] ASC',N'@p__linq__0 int',@p__linq__0=4
数:
exec sp_executesql N'SELECT TOP (1) [Project2].[ContactId] AS [ContactId], [Project2].[CompanyId] AS [CompanyId], [Project2].[ContactName] AS [ContactName], [Project2].[FullName] AS [FullName], [Project2].[ContactStatusId] AS [ContactStatusId], [Project2].[Created] AS [Created] FROM ( SELECT [Project2].[ContactId] AS [ContactId], [Project2].[CompanyId] AS [CompanyId], [Project2].[ContactName] AS [ContactName], [Project2].[FullName] AS [FullName], [Project2].[ContactStatusId] AS [ContactStatusId], [Project2].[Created] AS [Created], row_number() OVER (ORDER BY [Project2].[ContactId] ASC) AS [row_number] FROM ( SELECT [Project1].[ContactId] AS [ContactId], [Project1].[CompanyId] AS [CompanyId], [Project1].[ContactName] AS [ContactName], [Project1].[FullName] AS [FullName], [Project1].[ContactStatusId] AS [ContactStatusId], [Project1].[Created] AS [Created] FROM ( SELECT [Extent1].[ContactId] AS [ContactId], [Extent1].[CompanyId] AS [CompanyId], [Extent1].[ContactName] AS [ContactName], [Extent1].[FullName] AS [FullName], [Extent1].[ContactStatusId] AS [ContactStatusId], [Extent1].[Created] AS [Created], (SELECT COUNT(1) AS [A1] FROM [dbo].[NewsletterLog] AS [Extent2] WHERE ([Extent1].[ContactId] = [Extent2].[ContactId]) AND (6 = [Extent2].[NewsletterLogTypeId])) AS [C1] FROM [dbo].[Contact] AS [Extent1] ) AS [Project1] WHERE ([Project1].[CompanyId] = @p__linq__0) AND ([Project1].[ContactStatusId] <= 3) AND (0 = [Project1].[C1]) ) AS [Project2] ) AS [Project2] WHERE [Project2].[row_number] > 99 ORDER BY [Project2].[ContactId] ASC',N'@p__linq__0 int',@p__linq__0=4
似乎纯粹的Where with EXISTS比计算Count然后用Count == 0执行Where要差得多。
如果你们发现我的发现有错误,请告诉我。不管Any vs Count的讨论如何,我们可以从所有这些中得出的结论是,任何更复杂的LINQ在重写为存储过程时都要好得多。
你可以做一个简单的测试来弄清楚:
var query = //make any query here
var timeCount = new Stopwatch();
timeCount.Start();
if (query.Count > 0)
{
}
timeCount.Stop();
var testCount = timeCount.Elapsed;
var timeAny = new Stopwatch();
timeAny.Start();
if (query.Any())
{
}
timeAny.Stop();
var testAny = timeAny.Elapsed;
检查testCount和testAny的值。
关于Count()方法,如果IEnumerable是一个ICollection,那么我们不能遍历所有的项目,因为我们可以检索ICollection的Count字段,如果IEnumerable不是一个ICollection,我们必须使用一个MoveNext while遍历所有的项目,看看。net框架代码:
public static int Count<TSource>(this IEnumerable<TSource> source)
{
if (source == null)
throw Error.ArgumentNull("source");
ICollection<TSource> collectionoft = source as ICollection<TSource>;
if (collectionoft != null)
return collectionoft.Count;
ICollection collection = source as ICollection;
if (collection != null)
return collection.Count;
int count = 0;
using (IEnumerator<TSource> e = source.GetEnumerator())
{
checked
{
while (e.MoveNext()) count++;
}
}
return count;
}
参考:可枚举的参考源