单独的 LINQ2SQL 选择在两个索引表上的组和连接与一个组合 select 的性能

Question

我需要从按 ID 分组的表 (tab_activities) 中检索记录，只有按日期确定的每个组的最新记录。 在这些最新记录中，我只需要那些尚未设置特定属性（已完成）的记录。 最后，我必须交叉检查是否有任何活动也注册在黑名单（tab_flaggedActivities）中，然后对所有未完成且未标记的活动进行操作。

两个表都有关于 id 的索引。 显然，id 不是主键。

我不确定我正在做的是否是设置此 select 的最高效方式。 将这些单独的选择组合在一个 select 中会更好吗？ 如果是，我会怎么做？

// Retrieve only the latest activity entry of each known activity-ID
var latestActivityRecords = from activity in dc.tab_activities
                            where activity.dateRegistered > DateTime.Now.AddDays(-MaxAge)
                            group activity by activity.id
                            into groups
                            select groups.OrderByDescending(p => p.dateRegistered).First();

// Filter activities that are not finished
var unfinishedActivityRecords = from activity in latestActivityRecords
                                where !activity.finished
                                select activity;

// Check if any of those are in blacklist (different table)
var flaggedActivityRecords = from activity in unfinishedActivityRecords
                             join flaggedActivity in dc.tab_flaggedActivities 
                             on activity.id equals flaggedActivity.id
                             select activity;

if (unfinishedActivities != null)
{
    // Do something with all unfinished / unflagged activities 
    foreach (var activityRecord in unfinishedActivityRecords)
    {
        if (!flaggedActivityRecords.Any(p => p.id == activityRecord.id))
        {
            DoSomething(activityRecord);
        }
    }
}

Answer 1

如果您的意思是将if Any测试与查询结合起来，您可以反转最后一个查询的含义并循环遍历结果，在数据库服务器上执行过滤：

var unflaggedActivityRecords = from activity in unfinishedActivityRecords
                               where !dc.tab_flaggedActivities.Select(fa => fa.id).Contains(activity.id)
                               select activity;

if (unfinishedActivities != null) {
    // Do something with all unfinished / unflagged activities 
    foreach (var activityRecord in unflaggedActivityRecords) {
        DoSomething(activityRecord);
    }
}