If you are at all confused about which of PostgreSQL index to choose for a specific use case, there's a free course from Percona University designed to help you.
This free Percona University short course about the different kinds of indexes PostgreSQL supports. is available as a collection of videos on YouTube as well as on Google Classroom. It goes over the different types of indexes, when to use each and how to usethem optimally in order to squeeze performance out. The premise is that not all indexes are appropriate for all circumstances and using a ‘wrong’ index can have the opposite effect to that you intended.