含有大量数据的DataTable 设置了主键后，使用 Select() 查询效率显著提高

public class DataTableTest
{
private int dataCount = 10;
private DataTable dt = new DataTable();

    public DataTableTest(int initCount = 10)
    {
        dataCount = initCount;
        dt.Columns.Add("Id", typeof(int));
        dt.Columns.Add("CardNo", typeof(string));

        dt.PrimaryKey = new DataColumn[]
        {
            dt.Columns["CardNo"], //设置主键，可以显著提高Select效率
            //dt.Columns["Id"], //可以设置其他主键
        };


        for (int i = 0; i < dataCount; i++)
        {
            var dr = dt.NewRow();
            dr["id"] = i + 1;
            dr["CardNo"] = i + "A";
            dt.Rows.Add(dr);
        }
    }


    public int GetByForeach()
    {
        var count = 0;
        foreach ( DataRow item in dt.Rows)
        {
            //if ((int) item["Id"] > 10) count++;
            if (item["CardNo"].ToString() == "10000A") count++;
            
        }

        return count;
    }

    public int GetByLinq()
    {
        var list = (from item in dt.AsEnumerable()
                        //where item.Field<int>("Id") > 10
                        where item.Field<string>("CardNo") == "10000A"
                    select item).ToList();

        return list?.Count ?? 0;
    }

    public int GetBySelect()
    {
        //var ret = dt.Select("Id > 10");
        var cardNo = "10000A";

        var ret = dt.Select("CardNo ='" + cardNo + "'");
        return ret.Length;
    }

static void Main(string[] args)
{

        var ret = 0;
        Stopwatch sw = new Stopwatch();

        var dtTest = new DataTableTest(200000);

        
        sw.Start();
        ret = dtTest.GetByForeach();
        sw.Stop();
        Console.WriteLine("GetByForeach: ret=" + ret + ", " + sw.Elapsed.TotalMilliseconds);

        sw.Reset();
        sw.Start();
        ret = dtTest.GetByLinq();
        sw.Stop();
        Console.WriteLine("GetByLinq: ret=" + ret + ", " + sw.Elapsed.TotalMilliseconds);
        
        sw.Reset();
        sw.Start();
        ret = dtTest.GetBySelect();
        sw.Stop();
        Console.WriteLine("GetBySelect: ret=" + ret + ", " + sw.Elapsed.TotalMilliseconds);
        

        Console.Read();
    }