1

我正在编写适用于 Windows Phone (SDK 7.1) 的 Silverlight 应用程序，并在LongListSelector适用于 Windows Phone 的 Silverlight 工具包的控件中显示来自 CompactSQL DB 的数据。

一旦列表长到大约 150 个项目，该应用程序真的会减慢加载数据的速度，在页面之间导航和动画无法显示（我知道使用后台线程将有助于释放 UI 线程用于动画）。

我目前有三个经常使用的查询——每次更新 LongListSelector 中的数据或页面为 NavigatedTo 时。我已经转换MoviesByTitle成 a CompiledQuery，这有很大帮助，所以我希望对我的其他两个查询（groupedMovies和LongListSelector.ItemSourcetype List<Group<Movie>>）做同样的事情，但是我似乎无法弄清楚正确的语法。

关于如何使这些查询更有效的任何建议 - 通过使用CompiledQuery或其他方式？

MoviesByTitle在另一个名为Queries

public static Func<MovieDBContext, IOrderedQueryable<Movies>> MoviesByTitle = CompiledQuery.Compile((MovieDBContext db) => from m in db.Movies orderby m.Title,m.Year select m);

MainPage 中的字段

private static List<String> characters = new List<String> { "#", "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z" };
public static List<Group<Movies>> emptyGroups = new List<Group<Movies>>();

在 MainPage 的 LoadDB() 方法中 - 在更新 DB 时，在 OnNavigatedTo 和其他几个地方调用此方法。

//Populates the 'empty' Groups of Movies objects only once.
if (emptyGroups.Count < 1)
{
    characters.ForEach(x => emptyGroups.Add(new Group<Movies>(x, new List<Movies>())));
}

IEnumerable<Movies> query = Queries.MoviesByTitle(App.db);

//Groups the objects 
IEnumerable<Group<Movies>> groupedMovies = (from t in query
                                            group t by t.GroupHeader into grp
                                            orderby grp.Key
                                            select new Group<Movies>(grp.Key.ToString(), grp));

//Joins the 'empty' group and groupedMovies together for the LongListSelector control
moviesLongList.ItemsSource = (from t in groupedMovies.AsEnumerable().Union(emptyGroups)
                              orderby t.Title
                              select t).ToList();

GroupHeader是数据库中的一个属性Movies和一个实体

[Column(CanBeNull=true, UpdateCheck = UpdateCheck.Never)]
public char GroupHeader
    {
        get
        {
            char l;
            //For Alphabetized Grouping, titles do NOT start with "The ..."
            if (this.Title.ToLowerInvariant().StartsWith("the "))
            {
                l = this.Title.ToLowerInvariant()[4];
            }
            else
            {
                l = this.Title.ToLower()[0];
            }
            if (l >= 'a' && l <= 'z')
            {
                return l;
            }
            return '#';
        }
        set { }
    }

班级Group如下

public class Group<T> : IEnumerable<T>
{
    public Group(string name, IEnumerable<T> items)
    {
        this.Title = name;
        this.Items = new List<T>(items);
    }

    public string Title
    {
        get;
        set;
    }

    public IList<T> Items
    {
        get;
        set;
    }
    ...
}
4

1 回答 1

4

I assume that GroupHeader is an entity stored in the DB with 1-n relationship to the Movie entity.

First of all, I don't see 3 DB queries here. A LINQ expression is not always a DB query (e.g. there's LINQ to Objects). Sometimes determining what really is going on is quite challenging. The best friend in such cases is a DB profiler tool or the IntelliTrace - they show what queries are being executed on the DB at run time.

As far as I understand the code, you actually have 1+N queries: the first is MoviesByTitle, and then you have N queries in the expression that gets the movies grouped by their headers. It's N instead of 1 because you cast query to IEnumerable<> which makes it no longer a query but a simple CLR object, which is simply iterated in a foreach-like loop sending queries to the DB each time it needs a GroupHeader entity (it's an entity, isn't it?).

Try to combine 2 queries into one. You might even not need to use CompiledQuery. Here's an approximate code:

// I left this without changes
if (emptyGroups.Count < 1)           
{           
    characters.ForEach(x => emptyGroups.Add(new Group<Movies>(x, new List<Movies>())));           
} 

// I put var here, but it actually is an IQueryable<Movie>
var query = from m in App.db.Movies orderby m.Title, m.Year select m;

// Here var is IQueryable<anonymous type>, I just can't use anything else but var here
var groupedMoviesQuery = from t in query
                    group t by t.GroupHeader into grp 
                    orderby grp.Key 
                    select new
                    {
                       Movies = grp,
                       Header = grp.Key
                    }; 

// I use AsEnumerable to mark that what goes after AsEnumerable is to be executed on the client
IEnumerable<Group<Movie>> groupedMovies = groupedMoviesQuery.AsEnumerable()
                                            .Select(x => new Group<Movie>(x.Header, x.Movies))
                                            .ToList();

//No changes here
moviesLongList.ItemsSource = (from t in groupedMovies.AsEnumerable().Union(emptyGroups)            
                              orderby t.Title            
                              select t).ToList();

This code should work way better. What I actually did is that I turned your query from IEnumerable which is an iterate-only CLR object to an IQueryable which can be further wrapped into a more complex query. Now there's only one query which gets all movies grouped by headers. It should be fast.

I would introduce even more improvements to the code to make it work even faster:

  1. You use Union of entities read from the DB and of some default list. You order it afterwards. You can safely remove all other orderings from your Linq2Sql code ('query' and 'groupedMoviesQuery')
  2. Grouping seems to be not the most efficient here as compared to a join. Why don't you just query GroupHeaders including their related Movies? That should produce a JOIN in the db query which should be more efficient than a GROUP BY.
  3. If you still have issues with performance, you can turn the query into a compiled query.

I'll show an example of a compiled query for you original logic with optimization of the first item of the list above:

class Result
{
  public GroupHeader Header {get;set;}
  public IEnumerable<Movie> Movies {get;set;}
}

public static Func<MovieDBContext, IQueryable<Result>> GetGroupHeadersWithMovies =
  CompiledQuery.Compile((MovieDBContext x) => 
      from m in x.Movies
      group m by m.GroupHeader into grp 
      select new Result
      {
        Movies = grp,
        Header = grp.Key
      });
于 2012-04-15T12:26:58.793 回答