Question

我有两个ViewModel：

public class ProductViewModel
{
    public int Id { get; set; }

    public string Name { get; set; }

    public List<PartViewModel> Parts { get; set; }
}

public class PartViewModel
{
    public int Id { get; set; }

    public string Name { get; set; }
}

我正在查询这样的数据库，以获取产品列表以及相关的部件：

var prods = _context.Products.Select(pr => new ProductViewModel
{
    Id = pr.Id,
    Name = pr.Name,
    Parts = pr.Parts.Select(prt => new PartViewModel
    {
        Id = prt.Id,
        Name = prt.Name
    }).ToList()
}).ToList();

Product表中有~8800条记录，Part表中只有1条记录。此查询需要将近4分钟才能运行。当我删除零件清单时：

var prods = _context.Products.Select(pr => new ProductViewModel
    {
        Id = pr.Id,
        Name = pr.Name
    }).ToList();

......大约需要4秒钟。

以下是数据库中的表定义，通过Code First EF创建（我确保显示索引，这可能是索引问题：

CREATE TABLE [dbo].[Product](
    [Id] [int] NOT NULL,
    [Name] [nvarchar](max) NOT NULL,
 CONSTRAINT [PK_Product] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

ALTER TABLE [dbo].[Product] ADD  CONSTRAINT [PK_Product] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

CREATE TABLE [dbo].[Part](
    [Id] [int] NOT NULL,
    [Name] [nvarchar](max) NOT NULL,
    [ProductId] [int] NULL,
 CONSTRAINT [PK_Part] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY] TEXTIMAGE_ON [PRIMARY]

GO

ALTER TABLE [dbo].[Part]  WITH CHECK ADD  CONSTRAINT [FK_Part_Product_ProductId] FOREIGN KEY([ProductId])
REFERENCES [dbo].[Product] ([Id])
GO

ALTER TABLE [dbo].[Part] CHECK CONSTRAINT [FK_Part_Product_ProductId]
GO

CREATE NONCLUSTERED INDEX [IX_Part_ProductId] ON [dbo].[Part]
(
    [ProductId] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

ALTER TABLE [dbo].[Part] ADD  CONSTRAINT [PK_Part] PRIMARY KEY CLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO

最后，这是两个代码第一个实体：

[Table("Product")]
public partial class Product
{
    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
    public Product()
    {
        Parts = new HashSet<Part>();
    }

    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    [Required]
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }

    [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2227:CollectionPropertiesShouldBeReadOnly")]
    public virtual ICollection<Part> Parts { get; set; }
}

[Table("Part")]
public class Part
{
    [DatabaseGenerated(DatabaseGeneratedOption.None)]
    [Required]
    public int Id { get; set; }

    [Required]
    public string Name { get; set; }    

    public virtual Product Product { get; set; }
}

如果您需要更多代码或信息，请与我们联系。你能看出我做错了什么吗？什么能以更快的方式恢复数据？

Answer 1

Parts = pr.Parts.Select(prt => new PartViewModel
{
    Id = prt.Id,
    Name = prt.Name

}).ToList();

问题是，对于Products中的每个产品，您要在Parts处实现项目列表，这意味着对表Parts进行了8800次查询。

如果您将Parts中ProductViewModel的类型更改为IEnumerable<PartViewModel>，则可以执行以下操作：

Parts = pr.Parts.Select(prt => new PartViewModel
{
    Id = prt.Id,
    Name = prt.Name

});

这将解决问题。

Answer 2

分开查询。

var prods = _context.Products.Select(pr => new ProductViewModel
    {
        Id = pr.Id,
        Name = pr.Name
    }).ToList();

    var parts = _context.Parts.Select(prt => new PartViewModel
    {
        Id = prt.Id,
    ProductId = prt.ProductId,
        Name = prt.Name
    }).ToList();


prods.ForEach( pr => pr.Parts = parts.Where(prt=> prt.ProductId == pr.Id).ToList())

Answer 3

你可以删除ToList（）调用但是你留下了IQueryable类型。最简单的处理方法是使用AutoMapper等工具，并在查询中直接将其映射到ProductViewModel。所以代码看起来像这样：

using AutoMapper.QueryableExtensions;

var parts = _context.Parts
                .Include(part => part.Whatever)
                .OrderByDescending(part => part.Whatever)
                .AsNoTracking()
                .ProjectTo<PartsListViewModel>()

现在您选择了所需的所有部件，您可以通过调用实际执行查询：

parts.ToList();

所以一般的想法是过滤你想要的所有东西，然后使用ToList（）或Count（）这样的调用来实际执行查询。

此外，如果添加AsNoTracking（）调用，您可以稍微优化查询。这将禁用更改跟踪，因此您对模型对象所做的任何更改都不会保存。如果需要更改数据库中的值，请注意不要调用它，但对于只读方案，最好包含该调用，因为它可以防止意外的数据更改并且运行得更快。

在https://github.com/AutoMapper/AutoMapper/wiki/Queryable-Extensions了解有关automapper的更多信息。

Linq to Entities为什么我的一对多查询速度如此之慢？

3 个答案: