How does LINQ query execution differ between in-memory collections and databases?

In-memory LINQ executes delegates directly, while database LINQ translates expressions into SQL via providers like EF Core.

When working with IEnumerable, LINQ uses compiled delegates that run immediately in CLR memory. In contrast, IQueryable builds expression trees that are parsed by query providers (e.g., Entity Framework Core). These providers translate expressions into SQL or other query languages. This leads to fundamentally different execution models: CPU-bound in-memory iteration vs I/O-bound database execution. The mismatch can cause performance issues if developers unintentionally force client-side evaluation.