使用LINQ-To-Entities优化多重过滤

Question

我正在制作一个c＃应用程序，我使用多个组合框在数据网格视图上过滤项目，每个组合框的列表顶部都有“全部”，如果选中，它将不会在DGV项目中应用过滤器。我已经编写了代码，但是当同时应用2个过滤器时，查询速度很慢，我还没有完成其余的过滤器，直到我知道我使用的方法是否最佳。

namespace HDA_InfoBook
{
    public partial class frmGallery : Form
    {
        HDAEntities HDA = new HDAEntities();
        IQueryable<Hero> firstQuery = null;
        IQueryable<Hero> finalQuery = null;

        public frmGallery()
        {
            InitializeComponent();
        }

        public void ViewResults(IQueryable<Hero> viewResult)
        {
            var gridSource = from r in viewResult
                             select new
                             {
                                 Name   = r.Name,
                                 Rarity = r.Rarity,
                                 Speed  = r.Speed,
                                 Attack = r.Attack,
                                 Target = r.Target
                             };
            dgdResult.DataSource = gridSource.ToList();
        }

        public void InitializeQuery()
        {
            firstQuery = from hero in HDA.CharactersNames
                         orderby hero.HeroRarity
                         select new Hero
                         {
                             ID       = hero.ID,
                             Name     = hero.HeroName,
                             RarityID = hero.HeroRarity,
                             Rarity   = hero.Rarity.RarityName,
                             Speed    = hero.Initiative,
                             Attack   = hero.Attack,
                             Target   = hero.Attack2.Description
                         };
            ViewResults(firstQuery);
        }

        public void ReloadAll()
        {
            finalQuery = firstQuery;
        }

        public void FilterRarity(int rarityId)
        {
                finalQuery = from o in finalQuery
                            where o.RarityID == rarityId
                            select new Hero
                            {
                                ID       = o.ID,
                                Name     = o.Name,
                                RarityID = o.RarityID,
                                Rarity   = o.Rarity,
                                Speed    = o.Speed,
                                Attack   = o.Attack,
                                Target   = o.Target
                            };
        }

        public void FilterGroup(int groupId)
        {

                var subQuery = from hero in HDA.Hero_Group
                               where hero.GroupID == groupId
                               select hero.HeroID;

                finalQuery = from o in finalQuery
                             where subQuery.Contains(o.ID)
                             select new Hero
                             {
                                 ID       = o.ID,
                                 Name     = o.Name,
                                 RarityID = o.RarityID,
                                 Rarity   = o.Rarity,
                                 Speed    = o.Speed,
                                 Attack   = o.Attack,
                                 Target   = o.Target
                             };
        }

        public void ApplyFilter()
        {
            ReloadAll();
            if (cboRarity.SelectedIndex != 0)
                FilterRarity(cboRarity.SelectedIndex);
            if (cboGroup.SelectedIndex != 0/*-1 && cboGroup.ValueMember != ""*/)
                FilterGroup((byte)cboGroup.SelectedValue);
                //FilterGroup(cboGroup.SelectedIndex);
            ViewResults(finalQuery);
        }

        private void cboRarity_SelectedIndexChanged(object sender, EventArgs e)
        {
            ApplyFilter();
        }

        private void cboGroup_SelectedIndexChanged(object sender, EventArgs e)
        {
            ApplyFilter();
        }
    }
}

更新

令人沮丧的是，当我应用第三个过滤器时，查询时间大大减少，为什么会这样？

更新2

这工作得更快

    public void ApplyFilter2()
    {
        var subQuery = from hero in HDA.Hero_Group
                            where hero.GroupID == ((byte)cboGroup.SelectedValue)
                            select hero.HeroID;

        var q = from o in HDA.CharactersNames
                select new
                {
                    ID = o.ID,
                    Name = o.HeroName,
                    RarityID = o.HeroRarity,
                    Rarity = o.Rarity.RarityName,
                    Speed = o.Initiative,
                    Attack = o.Attack,
                    Target = o.Attack2.Description
                };
        if (cboRarity.SelectedIndex != 0)
            q = q.Where(o => o.RarityID == cboRarity.SelectedIndex);
        if (cboGroup.SelectedIndex != 0)
            q = q.Where(o => subQuery.Contains(o.ID));
        if (cboSpeed.SelectedIndex != 0)
            q = q.Where(o => o.Speed == cboSpeed.SelectedIndex);
        dgdResult.DataSource = q.ToList();
    }