Out of curiosity, is Postgres usually able to use a b-tree index's representation of the columnar data for its (set of) columns to calculate a derived value for those column(s), then scan through that in-memory derived data? Certainly not as fast as the index lookup itself, but avoids needing to have the entire table in memory, I would hope?

The documentation under https://www.postgresql.org/docs/16/indexes-types.html implies that it can be used to access the sorted values - but does that feed through the query planner?