LINQ Query Performance implementing Large Data Sets in C# - Unexpected Slowdowns
I'm writing unit tests and I've been working on this all day and I'm relatively new to this, so bear with me... I'm working on a personal project and I'm working with serious performance optimization when executing a LINQ query on a large dataset in my C# application using .NET 6... In particular, I have a `List<Order>` containing around 100,000 records, and when I try to filter and group them, the query takes an unacceptably long time to complete. Here's an example of my LINQ query: ```csharp var groupedOrders = orders .Where(o => o.OrderDate >= new DateTime(2023, 1, 1)) .GroupBy(o => o.CustomerId) .Select(g => new { CustomerId = g.Key, TotalOrders = g.Count(), TotalAmount = g.Sum(o => o.Amount) }) .ToList(); ``` The query above is supposed to group the orders by `CustomerId` and compute the total number of orders and the total amount for each customer. However, it’s taking several seconds to run, which is not acceptable for my use case. I've tried using `AsEnumerable()` to switch from deferred execution to immediate execution, hoping that might improve performance, but I saw no difference. I also profiled the database and it seems like the LINQ-to-Entities translation is not the scenario; the generated SQL seems efficient. Additionally, I’ve added indexes on `OrderDate` and `CustomerId` in the database, but the performance is still lagging. I'm concerned that using LINQ here might not be the best approach due to the size of the dataset. Is there a more efficient way to handle this, or any best practices that I might have overlooked? Any advice would be greatly appreciated! This is part of a larger API I'm building. Is there a better approach? This issue appeared after updating to Csharp stable. Any suggestions would be helpful. This is happening in both development and production on Linux. What are your experiences with this? I'm coming from a different tech stack and learning Csharp. What am I doing wrong?