[pjerem]

Yes of course it generates the corresponding SQL and don’t iterate over the table.

But in the framework’s code, IQueryable implements IEnumerable, it’s just a totally different implementation but for the developer it’s 100% the same API and so any IQueryable can be used where a IEnumerable is expected.

[recursive]

This is a hazard that trips people up commonly. If you use an IQueryable where an IEnumerable is expected, it will use brute-force iteration semantics, and not do things like generating a WHERE clause. Linq provides similar extension methods for both interfaces, but you need to be sure your call resolves to the right interface, otherwise you'll end up doing things like pulling the whole table into memory.