通过分组的linq结果进行学习非常缓慢，有什么提示吗？

Question

I did some profiling on a program that I'm running and the thing that takes the longest time is getting the results from the linq query:

var Results = 
    from a in Table
    group a by a.Value into b
    select new {Group = b};

foreach(var Result in Results)
{
    //Do calcs
}

Any ideas on how I can speed this up?

Answer 1

I think you are confusing the query object with the results of that query. Your first variable doesn't contain the results, it contains a query object. At this point the query hasn't been executed. The execution is delayed until you actually need the results, and in your example this is done when you iterate in the foreach loop. This is why the first statement executes quickly but the iteration is slow.

If you want to store the results of the query in results so that they are already calculated by the time you start the foreach loop then add a call to ToList().

var results = 
    (from a in Table
     group a by a.Value into b
     select new {Group = b}).ToList();

Answer 2

If you're using .NET 4, have a look at P-LINQ or Parallel ForEach loops. That should dramatically increase performance.

Can't really tell based on the info given, but it may be a SQL query taking too long?

If it's the ForEach loop that's really causing the bottleneck, then the Parallel ForEach will be your best bet.

Answer 3

I have had the same problem with only 2000 Sqlite records (indexed) used for an MSChart, apparently this is caused by the LINQ group statement querying all records.

The only solution I found was to go back to native SQL and the chart rendered instantaneously rather than 2 seconds using LINQ with SQlite.