Entity Framework is slow because of derived tables

问题

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

回答1:

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;

来源：https://stackoverflow.com/questions/13201049/entity-framework-is-slow-because-of-derived-tables

标签

mysql

entity-framework

linq-to-entities

database-performance