Currently one must chose between indexing keys or values, but what if we want both?
It would make perfect sense in some scenarios, such as a map of "spending per year": you would want to be able to use a nested predicate to look for documents where spendings were in a given range for a given year, and in order to do that you will need to index both the year and spending in the same nested document.
Another use case would be for projections, if one want to index a map for the sole purpose of being able to retrieve its value from the index. Then indexing entries would make more sense than indexing keys and values separately.
Related to HSEARCH-3688: I'm pretty sure both tickets will require an overhaul of the container extractors.