是否有一种方法可以强制实体框架使用子查询的别名，而不是将其乘以OrderBy？
var results = Users.Select( u => new
{
u.idUser,
u.Name,
nbChilds = u.Children.Count
})
.OrderBy( u => u.nbChilds );
这将在SQL中转换为：
SELECT [u].[idUser] AS [idUser], [u].[name] AS [Name], (
SELECT COUNT(*)
FROM [Children] AS [c0]
) AS [nbChilds]
FROM [Users] AS [u]
ORDER BY (
SELECT COUNT(*)
FROM [Children] AS [c]
)
这意味着它将运行两次儿童计数。就我而言，这是我想要避免的严重性能下降。有没有办法告诉Linq在select中使用别名，所以输出将是：
SELECT [u].[idUser] AS [idUser], [u].[name] AS [Name], (
SELECT COUNT(*)
FROM [Children] AS [c0]
) AS [nbChilds]
FROM [Users] AS [u]
ORDER BY [nbChilds]
答案 0 :(得分：1)
EF6在生成此查询方面做得很好。 EF Core在ORDER BY中重复子查询，这对于SQL Server至少会导致更昂贵的计划。
SQL生成仍然是EF Core中一个积极的改进领域。请为此向EF Core提出问题：https://github.com/aspnet/EntityFrameworkCore/issues
在EF 6这样的查询中
var q = from b in db.Blogs
orderby b.Posts.Count
select new
{
b.Name,
NumPosts = b.Posts.Count
};
var l = q.ToList();
这样翻译的：
SELECT
[Project2].[Id] AS [Id],
[Project2].[Name] AS [Name],
[Project2].[C2] AS [C1]
FROM ( SELECT
[Project1].[Id] AS [Id],
[Project1].[Name] AS [Name],
[Project1].[C1] AS [C1],
(SELECT
COUNT(1) AS [A1]
FROM [dbo].[Posts] AS [Extent3]
WHERE [Project1].[Id] = [Extent3].[BlogId]) AS [C2]
FROM ( SELECT
[Extent1].[Id] AS [Id],
[Extent1].[Name] AS [Name],
(SELECT
COUNT(1) AS [A1]
FROM [dbo].[Posts] AS [Extent2]
WHERE [Extent1].[Id] = [Extent2].[BlogId]) AS [C1]
FROM [dbo].[Blogs] AS [Extent1]
) AS [Project1]
) AS [Project2]
ORDER BY [Project2].[C1] ASC
但是在EF Core上，子查询在ORDER BY子句中重复：
这是一个复制品
using Microsoft.EntityFrameworkCore;
using Newtonsoft.Json;
using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.ComponentModel.DataAnnotations.Schema;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Threading.Tasks;
namespace EfCoreTest
{
public class Category
{
public int CategoryId { get; set; }
public virtual ICollection<Product> Products { get; } = new HashSet<Product>();
}
public class Product
{
public int ProductId{ get; set; }
public string Name { get; set; }
public DateTime Date { get; set; }
public Category Category { get; set; }
}
public class Db : DbContext
{
public DbSet<Category> Categorys { get; set; }
public DbSet<Product> Products { get; set; }
protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
{
optionsBuilder.UseSqlServer("server=.;database=EfCoreTest;Integrated Security=true");
base.OnConfiguring(optionsBuilder);
}
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
base.OnModelCreating(modelBuilder);
}
}
class Program
{
static void Main(string[] args)
{
using (var db = new Db())
{
db.Database.EnsureDeleted();
db.Database.EnsureCreated();
for (int i = 0; i < 100; i++)
{
var t = new Category();
for (int j = 0; j < 1000; j++)
{
var product = new Product()
{
Category = t,
Date = DateTime.Now,
Name = $"Category {j}{i}"
};
db.Add(product);
}
db.Add(t);
}
db.SaveChanges();
}
using (var db = new Db())
{
var q = from c in db.Categorys
orderby c.Products.Count
select new
{
c.CategoryId,
ProductCount = c.Products.Count
};
var l = q.ToList();
Console.WriteLine("Hit any key to exit.");
Console.ReadKey();
}
}
}
}
这是它生成的SQL
SELECT [c].[CategoryId], (
SELECT COUNT(*)
FROM [Products] AS [p0]
WHERE [c].[CategoryId] = [p0].[CategoryId]
) AS [ProductCount]
FROM [Categorys] AS [c]
ORDER BY (
SELECT COUNT(*)
FROM [Products] AS [p]
WHERE [c].[CategoryId] = [p].[CategoryId]
)
以及该查询的时间和IO统计信息：
(100 rows affected)
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Categorys'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Products'. Scan count 2, logical reads 960, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 31 ms, elapsed time = 43 ms.
这是我们希望它生成的SQL：
SELECT [c].[CategoryId], (
SELECT COUNT(*)
FROM [Products] AS [p0]
WHERE [c].[CategoryId] = [p0].[CategoryId]
) AS [ProductCount]
FROM [Categorys] AS [c]
ORDER BY (
[ProductCount]
)
时间和IO统计信息：
(100 rows affected)
Table 'Worktable'. Scan count 0, logical reads 0, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Categorys'. Scan count 1, logical reads 2, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'Products'. Scan count 1, logical reads 480, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
SQL Server Execution Times:
CPU time = 15 ms, elapsed time = 20 ms.
答案 1 :(得分：0)
实际上，它是这样转换为SQL的（如果是Linq To SQL）：
SELECT[t2].[idUser], [t2].[Name], [t2].[value] AS [nbChilds]
FROM (
SELECT[t0].[idUser], [t0].[Name],
(SELECT COUNT(*)
FROM [Children] AS [t1]
WHERE [t1].[idUser] = [t0].[idUser]
) AS[value]
FROM[Customers] AS[t0]
) AS[t2]
ORDER BY[t2].[value]
但是，如果您将其转换为您显示的转换，则可以这样做：
var results = Users.Select( u => new
{
u.idUser,
u.Name,
nbChilds = u.Children.Count
})
.AsEnumerable()
.OrderBy( u => u.nbChilds );
那将被转换为：
SELECT [t0].[idUser], [t0].[Name], (
SELECT COUNT(*)
FROM [Children] AS [t1]
WHERE [t1].[idUser] = [t0].[idUser]
) AS [nbChilds]
FROM [Customers] AS [t0]