Database design for mapping users to questions

I'm currently building a quiz app and I need to design a database schema for it. This is what I've tried so far: The biggest problem I have is with how to represent which questions have been already answered correctly by each user and which haven't. In the current design the *answered_questions* table would grow very fast (assuming a pool of 2000 questions and 1million users it would be 2 billion entries, which doesn't seem to be a scalable approach). I thought of partitioning the *answered_questions* table by *user_id* but I don't if that's the most optimal solution. Please note that I need to keep the information about which question is already answered because for each request i will have to retrieve the following information: * 5 questions that have been correctly answered * 5 questions that have not been correctly answered filtered by the *category*, *subcategory* and *user_id*. How could I redesign it to avoid having to query the *answered_questions* table which can become very large over time? Any help is very much appreciated.