从自动生成百万行的c# dll数据库中获取低库存

我有一个数据库(c#， dll)，它只包含一个表，自动生成百万行。

我一次只能得到一行。表的列是:int ItemID, string category, string model, string Brand).

如果两个项目具有相同的category, brand, model，则它们相同。我想找出所有类型的低库存商品，低库存是指该类型的商品剩余的数量小于等于两种。

这是我为此写的代码，但它花了2或3个小时来执行，它没有给我正确的结果。我怎么解决这个问题?

   public DataTable getLowStock()
   {
       DataTable dt = new DataTable();
       object[,] arr=new object[1000000,4];
       int  index=0;
       int rows;
       rows = db.NumRows;
       int[] IdArray;
       IdArray = db.GetItemIDList(0, rows - 1);
       string Category, Brand, Model, Condition, Location, Notes;
       DateTime ReceivedDate, LastUpdated;
       int Weight;
       double PurchasePrice, SellingPrice;
       for (int a = 0; a < IdArray.Length; a++)
       {
           Console.WriteLine(a);
           db.GetItemRecord(IdArray[a], out Category, out Brand, out Model,
                            out ReceivedDate, out Weight, out Condition, out Location,
                            out PurchasePrice, out SellingPrice, out Notes, out LastUpdated);
           if (a == 0)
           {
               arr[0, 0] = Category;
               arr[0, 1] = Brand;
               arr[0, 2] = Model;
               arr[0, 3] =  1;
               index++;
           }
           else
           {
               for (int i = 0; i < index; i++)
               {
                   if ( ( arr[i,2].ToString()==Model)&& (arr[i,1].ToString()==Brand) &&   (arr[i,0].ToString()==Category ) )
                   {
                       arr[i, 3] = (Int32)arr[i, 3] + 1;
                   }
               }
               arr[index,0]=Category;
               arr[index,1]=Brand;
               arr[index,2]=Model;
               arr[index, 3] = 1;
               index++;
           }
       }
        dt.Columns.Add("Category", typeof(string));    
        dt.Columns.Add("Model", typeof(string));
        dt.Columns.Add("Brand", typeof(string));
        dt.Columns.Add("count", typeof(int));   
        for (int i = 0; i < index; i++) {
           Console.WriteLine("i="+i);
       if((int)arr[i,3]<3){
           dt.Rows.Add(arr[i,0].ToString(),arr[i,2].ToString(),arr[i,1].ToString(),(int)arr[i,3]);
       }
       }
       return dt;
   }

SELECT category, model, brand, COUNT(*) 
FROM table 
GROUP BY category, model, brand
HAVING COUNT(*) >= 2