MoreLinq maxBy vs LINQ max + where

问题

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);

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

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

回答1:

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);
}

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.

来源：https://stackoverflow.com/questions/19484563/morelinq-maxby-vs-linq-max-where