4. Following Schema for a simple social network Consider the following schema for a simple social network. It maintains a collection of users, identified by unique handles (short names). Friend tells us that handle is a friend of handle2. (Friend is uni-directional: handle2 might not be a friend of handle 1.) And Message says that handle broadcasted a message (text) from city from_city. User(handle, name, home_city, bio) Friendhandle1, handle2) Message(handle, text, from_city) Explain the following relational algebra expression in English, that it represents TT handle3 [ handle1 ->handle3, handle2 ->handle4 ( Friend) P handle1 -> handie4, handle2 -> handle3 ( Friend)]