It's 2 different syntactically identical API's under an umbrella. 1: IEnumerable<T> that works lazily in-memory (and similar to the authors improvement) can be

It's 2 different syntactically identical API's under an umbrella. 1: IEnumerable<T> that works lazily in-memory (and similar to the authors improvement) can be done in any language with first class functions, see the authors linq.js or Java's streams library (it's not entirely the same as a chain of map/reduce/filter since it's lazy but that's mostly not a drawback since it improves performance by removing temporary storage objects). 2: IQueryable<T> is the really magical part though, by specifying certain wrapper types the compiler is informed that the library expects an bound syntax tree(AST) of the expression you write, the library can then translate the syntax tree to SQL queries and send the query to the server to be processed. Thus huge tables can be efficiently queried by just writing regular C# and never touch SQL. In most ORM's it's annoying or have impedance mistmatches but with EF you can write code and be fairly certain that it'll produce a good SQL query since the entire Linq syntax is fairly close to SQL.