How does LINQ behave when multiple IQueryable providers are accidentally composed together?

LINQ cannot safely compose multiple IQueryable providers; it either fails translation or forces client-side evaluation.

Each IQueryable provider (e.g., EF Core, LINQ-to-SQL, Cosmos provider) has its own expression translation engine. When queries from different providers are combined, the expression tree becomes incompatible. Since there is no shared execution model, the framework cannot unify translation. This often results in runtime exceptions or silent fallback to IEnumerable, which moves execution to memory and breaks performance expectations.