.net核心加载大量数据需要大量时间

如何解决.net核心加载大量数据需要大量时间

我对您希望清除的概念有些困惑，我具有以下LINQ：

  var rsllt=(from s in _db.events
                      where s.m_time_stamp >= frm && s.m_time_stamp <= to && s.m_turbine_id == IPAddress.Parse(turbineId)
                      select new EventLogsViewModel
                      {
                          m_time_stamp = s.m_time_stamp,m_event_log_description = s.m_event_log_description,m_wind_speed = s.m_wind_speed,m_rpm = s.m_rpm,m_power = s.m_power

                      }).AsNoTracking().ToList();
            return rsllt;

它返回大量数据，大约需要3分钟来绑定我的网格，在这种情况下，解决方案是延迟加载？这意味着当用户滚动或转到网格的下一页时，它应该加载更多数据，是惰性加载吗？不是，在像我这样的情况下有什么解决方案？整个浏览器在加载数据时将被冻结

解决方法

解决方案是“每页”加载数据。除了最后一页，每个页面上都有固定数量的项目。

如果页面上的项目数为25，则到第3页将有项目50至74。您将Skip(50).Take(25)。

公式：

pageNumber = number of the page (first page has number zero)
pageSize = number of items on the Page
Fetch page pageNumber: Skip(pageNumber * pageSize).Take(pageSize);

为此创建一个整洁的类

假设您要显示的页面中每个页面都有PageSize行数。每行都是一个T类型的对象：

class Page<T> : IReadOnlyCollection<T>
{
    private readonly IQueryable<T> queryAll {get; set;}
    private readonly int pageNr;
    private readonly int pageSize;

    private IReadonlyList<T> fetchedData = null;

    public Page(IQueryable<T> queryall,pageNr,pageSize)
    {
        this.queryAll = queryAll;
        this.pageNr = pageNr;
        this.pageSize = pageSize;
    }

    public int PageNr => this.pageNr;
    public int PageSize => this.pageSize;
   
    public T GetItem(int index)
    {
        if (this.fetchedData == null)
        {
            this.fetchedData = this.FetchData();
        }
        return this.fetchedData[index];
    }

    public T this[int index] => this.GetItem(index);
    
    private IReadonlyList<T> FetchData()
    {
        return this.QueryAll
                   .Skip(this.PageNr * this.PageSize)
                   .Take(this.PageSize)
                   .ToList();
    }

    // implementation of IReadOnlyCollection<T> via this.fetchedData
}

请注意，直到您要求页面上的任何项目时，查询才会执行。下次您要求任何项目时，查询不会再次执行。

为简单起见，我省略了范围检查。

用法：

var dbContext = new OrderDbContext(...);
var dateLimit = DateTime.Today.AddDays(-100);
var queryOldNotPayedOrders = dbContext.Orders
    .Where(order => !order.Payed && order.OrderDate < dateLimit);

// Note: the query is not executed yet.!
// Get the first page:
int maxNumberOfRows = ...;
int pageSize = maxNumberOfRows;
Page<Order> page0 = new Page<Order>(queryOldNotPayedOrders,pageSize);

页面集合

要使其更加整洁，请执行以下操作：创建PageCollection，在其中您可以要求Page[4]

class PageCollection<T> : IReadOnlyList<Page<T>>
{
    private readonly Dictionary<int,Page<T>> fetchedPages;
    private readonly IQueryable<T> queryAll;
    private readonly int itemCount;
    private readonly int pageSize;
    
    public PageCollection(IQueryable<T> queryAll,int pageSize)
    {
        this.pages=  = new Dictionary<int,Page<T>>();
        this.queryAll = queryAll;

        // Count all items once. This is a Count(),much faster than fetching all items
        this.itemCount = this.queryAll.Count();
        this.pageSize = pageSize;
    }

    public int ItemCount => this.itemCount;
    public int NrOfPages => this.ItemCount / this.PageSize;
    public int PageSize => this.pageSize;

    public Page<T> GetPage(int pageNr)
    {
        if (!this.fetchedPages.TryGetValue(pageNr,out Page<T> fetchedPage))
        {
            // page not in the dictionary yet. Fetch it now and add it:
            fetchedPage = this.fetchPage(pageNr);
            this.fetchedPages.Add(fetchedPage);
        }
        return fetchedPage;
    }

    public Page<T> this[int index] => this.GetPage(index);

    private Page<T> FetchPage(int pageNr)
    {
        return new Page<T>(this.queryll,this.pageSize);
    }

    // implementation of IReadOnlyList via this.fetchedPages
}

用法：

var pages = new PageCollection<Order>(queryOldNotPayedOrders,pageSize);
var page4 = pages[4];
Order order10 = page4[10];

或： this.DataGridView.DataSource = new BindingList（pages）;

最终改善

如果您希望能够创建匿名类型的页面集合，则需要为IQueryable<T>创建扩展方法：

public static PageCollection<T> ToPages(this IQueryable<T> query,int pageSize)
{
    return new PageCollection<T>(query,pageSize);
}

用法：

var queryAll = dbContext.Orders
    .Where(order => !order.Payed)
    .Select(order => new
    {
         CustomerName = Order.Customer.Name,Address = Order.Customer.Address,DueData = Order.DueDate,Amount = Order.Total,});

    var pages = queryAll.ToPages(pageSize);