实体框架是因为派生表的缓慢 [英] Entity Framework is slow because of derived tables

问题描述

我使用的MySQL Connector /净6.5.4使用LINQ到实体，因为实体框架生成使用派生表的查询我经常收到糟糕的查询性能。

这里是什么我遇到好几次一个简单的例子。在C＃中，我写这样的查询：

  VAR culverCustomers从CS =在db.CustomerSummaries那里cs.Street ==卡尔弗选CS; 
 //后... 
 VAR sortedCustomers = culverCustomers.OrderBy（CS => cs.Name）.ToList（）; 
  

而不是生成简单的查询是这样的：

  SELECT cust.id FROM customer_summary卡斯特WHERE cust.street =卡尔弗ORDER BY cust.name 
  

实体框架生成具有这样的派生表的查询：

 选择Project从.ID（
选择cust.id，cust.name，cust.street FROM customer_summary卡斯特
其中Project1.street =卡尔弗
）作为PROJECT1  - 这里就是EF产生一个毫无意义的派生表
 ORDER BY Project1.name 
  

如果我解释这两个查询，我得到这个对于第一个查询：

 的id，SELECT_TYPE，表，类型，possible_keys，行
 1，小学，地址， ALL，小学，9 
 1，小学，卡斯特，裁判，小学，4 
  

。 ..和为实体框架查询

 的id，SELECT_TYPE，表，类型，possible_keys，行$ b $可怕的东西像这样b 1，PRIMARY，< Derived2的>中ALL，9639 
 2，派生，地址，ALL，小学，9 
 2，派生，卡斯特，裁判，小学，4 
  

请注意第一行，在MySQL的解释，它的扫描 9000+记录即可。因为派生表，MySQL正在创建一个临时表和装载的每一行。 （我是这么基于像这样的文章我推断：的派生表和视图效果）

如何防止实体框架使用派生的表，或者我怎么能说服MySQL来？做了明显的优化这样的查询

有关竣工，这里就是这个LINQ查询源视图：

  CREATE VIEW customer_summary为
选择cust.id，cust.name，addr.street 
客户卡斯特
加入地址在addr.customer_id = cust.id 
  

解决方案

<地址
p>我觉得你的查询语句中缺少'选择'。您还没有确定你想要的结果。
你的查询：

  VAR culverCustomers从CS =在db.CustomerSummaries 
，其中cs.Street == 卡尔弗; 
  

//没有选择

是什么你从表中选择？试试这个

例如：

  VAR culverCustomers从CS在DB = .CustomerSummaries 
其中cs.Street ==卡尔弗
选择CS; 
  

I am using MySQL Connector/Net 6.5.4 with LINQ to entities, and I frequently get terrible query performance because the entity framework generates queries that use derived tables.

Here is a simplified example of what I've encountered several times. In C#, I write a query like this:

var culverCustomers = from cs in db.CustomerSummaries where cs.Street == "Culver" select cs;
// later...
var sortedCustomers = culverCustomers.OrderBy(cs => cs.Name).ToList();

Instead of generating simple a query like this:

SELECT cust.id FROM customer_summary cust WHERE cust.street = "Culver" ORDER BY cust.name

The entity framework generates a query with a derived table like this:

SELECT Project1.id FROM (
    SELECT cust.id, cust.name, cust.street FROM customer_summary cust
    WHERE Project1.street = "Culver"
) AS Project1 -- here is where the EF generates a pointless derived table
ORDER BY Project1.name

If I explain both queries I get this for the first query:

id, select_type, table, type, possible_keys, rows
1,  PRIMARY,     addr,  ALL,  PRIMARY,       9
1,  PRIMARY,     cust,  ref,  PRIMARY,       4

... and something awful like this for the entity framework query

id, select_type, table,      type, possible_keys, rows
1,  PRIMARY,     <derived2>, ALL,                 9639
2,  DERIVED,     addr,       ALL,  PRIMARY,       9
2,  DERIVED,     cust,       ref,  PRIMARY,       4

Note the first row, where MySQL explains that it's scanning 9000+ records. Because of the derived table, MySQL is creating a temp table and loading every row. (Or so I'm deducing based on articles like this one: Derived Tables and Views Performance)

How can I prevent the Entity Framework from using a derived table, or how can I convince MySQL to do the obvious optimization for queries like this?

For completion, here is the view that is the source for this linq query:

create view  customer_summary as
select cust.id, cust.name, addr.street
customers cust 
join addresses addr
on addr.customer_id = cust.id

解决方案

I think your query statement is missing 'select'. You have not identified the record(s) you want. your query:

var culverCustomers = from cs in db.CustomerSummaries 
                        where cs.Street == "Culver";

//no select

what are you selecting from the table? try this

example:

var culverCustomers =  from cs in db.CustomerSummaries 
                        where cs.Street == "Culver"
                        select cs;

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