How to design a GSI for this DynamoDB table?

Question

This is a sample question for an AWS certification that I'm trying to clarify in my head. The question is asking that In order to be able to create a leaderboard where I can query the TopScores by User or by Game, I need to update this table to support this new ask:

A popular multiplayer online game is using an Amazon DynamoDB table named GameScore to track users’ scores. The table is configured with a partition key UserId and a sort key GameTitle as shown in the diagram below:

The answer is naturally a GSI since its an existing table but the answer goes to suggest creating an Index called GameTitleIndex which contains GameTitle and TopScore

I feel that this is incorrect since if I create a GSI with JUST TopScore - the primary keys are already projected (so it would already contain UserId and GameTitle).

What do folks suggest?

Answer 1

Think about your access pattern. If the score is made the partition key you have no way to express the query for top scores of a given game. Just because the attribute is projected doesn’t mean it’s suitably indexed.