If I synchronize based on user_id, should I create an index for it? Locally, every document will have a user_id, so it doesn’t really help. However, I am not sure if the server is effectively querying user_id when syncing, in which case an index will improve performance.
Are queryable fields for flexible sync already indexed?
Sync does not query your Atlas data directly, but rather it keeps its own copy of your data. The data copy will have an index on each of your queryable fields, so you should not need to tweak those manually.