为什么实体框架的AsEnumerable()从服务器下载所有数据? [英] Why is Entity Framework's AsEnumerable() downloading all data from the server?

问题描述

使用AsEnumerable()时EF下载所有结果行的解释是什么?

What is the explanation for EF downloading all result rows when AsEnumerable() is used?

我的意思是这段代码:

context.Logs.AsEnumerable().Where(x => x.Id % 2 == 0).Take(100).ToList();

将从表中下载所有行，然后再将任何行传递给Where()方法，并且表中可能有数百万行.

will download all the rows from the table before passing any row to the Where() method and there could be millions of rows in the table.

我想做的是仅下载足以收集满足Id % 2 == 0条件的100行(很可能只有200行).

What I would like it to do, is to download only enough to gather 100 rows that would satisfy the Id % 2 == 0 condition (most likely just around 200 rows).

EF是否可以像使用普通ADO.NET使用SqlDataReader的Read()方法那样按需加载行并节省时间和带宽?

Couldn't EF do on demand loading of rows like you can with plain ADO.NET using Read() method of SqlDataReader and save time and bandwidth?

我认为它不能像这样工作是有原因的，我想听到一个很好的论据来支持该设计决策.

I suppose that it does not work like that for a reason and I'd like to hear a good argument supporting that design decision.

注意:这是一个完全人为的示例，我知道通常您不应该以这种方式使用EF，但是我在某些现有代码中发现了这一点，只是感到惊讶，因为我的假设被证明是错误的.

NOTE: This is a completely contrived example and I know normally you should not use EF this way, but I found this in some existing code and was just surprised my assumptions turned out to be incorrect.

推荐答案

简而言之:不同行为的原因是，当直接使用IQueryable时，单个SQL查询可以为您的整个LINQ查询组成；但是使用IEnumerable时，必须加载整个数据表.

The short answer: The reason for the different behaviors is that, when you use IQueryable directly, a single SQL query can be formed for your entire LINQ query; but when you use IEnumerable, the entire table of data must be loaded.

长答案:考虑以下代码.

context.Logs.Where(x => x.Id % 2 == 0)

context.Logs的类型为IQueryable<Log>. IQueryable<Log>.Where使用Expression<Func<Log, bool>>作为谓词. Expression代表抽象语法树；也就是说，它不仅仅是您可以运行的代码.可以认为它是在运行时在内存中表示的，就像这样:

context.Logs is of type IQueryable<Log>. IQueryable<Log>.Where is taking an Expression<Func<Log, bool>> as the predicate. The Expression represents an abstract syntax tree; that is, it's more than just code you can run. Think of it as being represented in memory, at runtime, like this:

Lambda (=>)
  Parameters
    Variable: x
  Body
    Equals (==)
      Modulo (%)
        PropertyAccess (.)
          Variable: x
          Property: Id
        Constant: 2
      Constant: 0

LINQ-to-Entities引擎可以使用context.Logs.Where(x => x.Id % 2 == 0)并将其机械地转换为如下所示的SQL查询:

The LINQ-to-Entities engine can take context.Logs.Where(x => x.Id % 2 == 0) and mechanically convert it into a SQL query that looks something like this:

SELECT *
FROM "Logs"
WHERE "Logs"."Id" % 2 = 0;

如果将代码更改为context.Logs.Where(x => x.Id % 2 == 0).Take(100)，则SQL查询将变为以下内容:

If you change your code to context.Logs.Where(x => x.Id % 2 == 0).Take(100), the SQL query becomes something like this:

SELECT *
FROM "Logs"
WHERE "Logs"."Id" % 2 = 0
LIMIT 100;

这完全是因为IQueryable上的LINQ扩展方法使用Expression而不是Func.

This is entirely because the LINQ extension methods on IQueryable use Expression instead of just Func.

现在考虑context.Logs.AsEnumerable().Where(x => x.Id % 2 == 0). IEnumerable<Log>.Where扩展方法将Func<Log, bool>用作谓词.那只是可运行的代码.无法对其进行分析以确定其结构；它不能用于形成SQL查询.

Now consider context.Logs.AsEnumerable().Where(x => x.Id % 2 == 0). The IEnumerable<Log>.Where extension method is taking a Func<Log, bool> as a predicate. That is only runnable code. It cannot be analyzed to determine its structure; it cannot be used to form a SQL query.

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