MoreLinq maxBy vs LINQ max + where

作者: 小仙女谖恓
来源: 51数据库
2022-12-08

问题描述

我将 EF5 与 MoreLinq 扩展一起使用，在生产中测试我的程序(非常大的数据库)时，我发现该行:

I am using EF5 with the MoreLinq extenstion, while testing my program in production (very big database), I found out that the line:

var x = db.TheBigTable.MaxBy(x => x.RecordTime);

需要很长时间(RecordTime 是一个非索引的datetime)

Takes very long time (RecordTime is a non-indexed datetime)

是不是因为MaxBy 总是在客户端运行(并且首先从数据库中获取所有记录)?

Is that because MaxBy always runs on the client side (and firstly gets ALL records from the database)?

推荐答案

这里是 MaxBy 扩展方法:

Here is the signature of the MaxBy extension method:

public static TSource MaxBy<TSource, TKey>(this IEnumerable<TSource> source,
    Func<TSource, TKey> selector)
{
    return source.MaxBy(selector, Comparer<TKey>.Default);
}

它返回 IEnumerable 的最大元素(基于给定的投影)，而不是 IQueryable.所以查询db.TheBigTable的结果确实都是先加载到内存中，然后迭代求最大值.

It returns the maximal element (based on the given projection) of an IEnumerable<T>, not an IQueryable<T>. So the results of the query db.TheBigTable are indeed all loaded into memory first, and then they are iterated to find the maximum.

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