最好使用基于 PLINQ 的高性能库，如 System.Data.SqlClient 和 AggregationFramework。以下是一个使用 PLINQ 和 System.Data.SqlClient 的代码示例：

using System.Linq;
using System.Data.SqlClient;

...

var connectionString = "Your Connection String"; // Replace with your connection string
var query = "SELECT CustomerID, COUNT(*) AS TotalOrders FROM Orders GROUP BY CustomerID";

using (var conn = new SqlConnection(connectionString))
{
    conn.Open();
    var command = new SqlCommand(query, conn);

    var result = command.ExecuteReader()
        .Cast()
        .AsParallel()
        .ToLookup(
            r => r.GetString(0), // Group by CustomerID
            r => r.GetInt32(1)); // Get count of orders

    foreach (var item in result)
    {
        Console.WriteLine($"CustomerID: {item.Key}, Total Orders: {item.Sum()}");
    }
}