列出慢性能与每一个慢性能

我正在构建程序，使用数据库与3个表(Worker, Task, TaskStep)我有一个方法，获取日期和建立报告为特定的工人

数据库结构如下:

Worker表列:

workerID(VARCHAR(45)),
name(VARCHAR(45)),
age(int),
...

Tasks表列:

TaskID(VARCHAR(45)),
description(VARCHAR(45)),
date(DATE),
...

TaskSteps表列:

TaskStepID(VARCHAR(45)),
description(VARCHAR(45)),
date(DATE),
...

没有索引

问题是它非常非常慢!(~ 20秒)

代码如下:

using WorkerDailyReport = Dictionary<task, IEnumerable<taskStep>>;
private void Buildreport(DateTime date)
{
    var report = new WorkerDailyReport();    
    // Load from DB
    var sw = new Stopwatch();
    sw.Start();
    var startOfDay  = date.Date;
    var endOfDay    = startOfDay.AddDays(1);
    var db          = new WorkEntities();
    const string    workerID   = "80900855";
    IEnumerable<task> _tasks = db.task
                    .Where(ta =>    ta.date     >= startOfDay   &&
                                    ta.date     <  endOfDay     &&
                                    ta.workerID == workerID)
                    .ToList();
    sw.Stop();
    Console.WriteLine("Load From DB time - " + sw.Elapsed + 
                      ", Count - "           + _tasks.Count());   
    // Build the report
    sw.Restart();
    foreach (var t in _tasks)
    {
        var ts = db.taskStep.Where(s => s.taskID == task.taskID);
        report.Add(t, ts);
    }
    sw.Stop();
    Console.WriteLine("Build report time - " + sw.Elapsed);
    // Do somthing with the report
    foreach (var t in report)
    {
        sw.Restart();
        foreach (var subNode in t.Value)
        {
            // Do somthing..
        }
        Console.WriteLine("Do somthing time - " + sw.Elapsed + 
                          ", Count - " + t.Value.Count());
    }
}

正如你所看到的，我把秒表在每个部分检查什么需要这么长时间结果如下:

1)

如果我运行以上代码:

Load From DB time - 00:00:00.0013774, Count - 577
Build report time - 00:00:03.6305722
Do somthing time - 00:00:07.7573754, Count - 21
Do somthing time - 00:00:08.2811928, Count - 11
Do somthing time - 00:00:07.8715531, Count - 14
Do somthing time - 00:00:08.0430597, Count - 0
Do somthing time - 00:00:07.7867790, Count - 9
Do somthing time - 00:00:07.3485209, Count - 39
.........

内部foreach每次运行大约需要7-9!!我不想再碾过去了40记录。

2)

如果我只改变了一件事，在第一个查询之后添加.ToList()当我从数据库加载工作任务时，它会发生变化everithing .

Load From DB time - 00:00:04.3568445, Count - 577
Build report time - 00:00:00.0018535
Do somthing time - 00:00:00.0191099, Count - 21
Do somthing time - 00:00:00.0144895, Count - 11
Do somthing time - 00:00:00.0150208, Count - 14
Do somthing time - 00:00:00.0179021, Count - 0
Do somthing time - 00:00:00.0151372, Count - 9
Do somthing time - 00:00:00.0155703, Count - 39
.........

现在从DataBase加载需要更多的时间，4秒以上。但是build报告时间大约是~1ms每个内foreach需要~10ms

第一种方式是不可能的(577 * ~8秒)，第二种选择它也很慢，我看不见。

你知道这里发生了什么吗?

1)为什么ToList()这么慢?

2)为什么没有ToList()，内部foreach和构建报告正在变慢?

我怎样才能使它更快?

非常感谢。