由于派生表,实体框架很慢

Question

我使用MySQL Connector/Net 6.5.4与LINQ实体,我经常得到糟糕的查询性能,因为实体框架生成使用派生表的查询.

这是我多次遇到的简化示例.在C#中,我写了一个这样的查询:

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

而不是像这样生成简单的查询:

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

实体框架使用派生表生成一个查询,如下所示:

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

如果我解释两个查询,我得到第一个查询:

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

......对于实体框架查询来说,这样的事情很糟糕

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

请注意第一行,其中MySQL解释说它正在扫描9000多条记录.由于派生表,MySQL正在创建临时表并加载每一行.(或者我根据这样的文章推断:派生表和视图性能)

如何防止实体框架使用派生表,或者如何说服MySQL对此类查询进行明显的优化？

为了完成,以下是该linq查询的源视图:

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

Answer 1

我认为您的查询语句缺少“选择”。您尚未确定所需的记录。您的查询：

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

//没有选择

你从表中选择什么？尝试这个

例子：

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