使用 linq 连接两个表，并填充它们的字典

问题描述

我一直在寻找如何连接两个表(Data 和 DataValues，一对多)并填充类型为 的字典.

I've been searching how to join two tables (Data and DataValues, one to many) and fill a dictionary of type .

Data(s) 的记录可能是数千(例如 500,000 或更多)，并且每个 Data 可能有 10 到 20 个 DataValues，这使得它的查询量更大，因此性能在这里非常重要.

The records of Data(s) might be thousands (e.g. 500,000 or more) and each Data may have 10 to 20 DataValues which makes it a much heavier query, so the performance is really important here.

这是我写的代码:

// Passed via the arguments, for example, sensorIDs would contain:
int[] sensorIDs = { 0, 1, 2, 3, 4, 5, 6, 17, 18 };
Dictionary<Data, List<DataValue>> dict = new Dictionary<Data, List<DataValue>>();

foreach (Data Data in dt.Datas)
{
    var dValues = from d in dt.Datas
                        join dV in dt.DataValues on d.DataID equals dV.DataID
                        where (SensorIDs.Contains(dV.SensorID))
                        select dV;
    dict.Add(Data, dValues.ToList<DataValue>());
}

但是这种方法存在严重的性能问题并且需要很长时间才能执行.不确定是否需要使用 SQL 视图.有什么建议吗?

But this approach has a significant performance issue and takes a long time to execute. Not sure if I need to use SQL Views. any suggestions?

推荐答案

您查询的次数太多了.您可以在一个查询中完成此操作.

You're querying way too many times. You can do this in one query.

var dict = (from d in dt.Datas
            join dV in dt.DataValues on d.DataID equals dv.DataID
            where SensorIDs.Contains(dv.SensorID)
            select new { d, dV }).ToDictionary(o => o.d, o => o.dV.ToList());

在您的 foreach 循环中，您正在获取所有 Data 并且对于它们中的每一个，您都在做同样的事情.

In your foreach loop, you are fetching all Data and for each of them, you are doing the same thing.

现在还不是很清楚，但我认为您只想加入 SensorIDs 数组中的 DataValues.在这种情况下:

Now that wasn't very clear, but I think you want to join only the DataValues that are in the SensorIDs array. In this case:

var dict = (from d in dt.Datas
            let dV = (from dataValue in dt.DataValues
                      where SensorIDs.Contains(dataValue.SensorID) &&
                            dataValue.DataID = d.DataID
                      select dataValue)
            select new { d, dV }).ToDictionary(o => o.d, o => o.dV.ToList());