Hello, In the category "doing more tricks with our existing btrees", which includes all that difficult stuff like skip scans and incremental sort, here's an easier planner-only one: if you have a unique index on (a) possibly "including" (b) and you have a pathkey (a, b), you can use an index [only] scan. That is, if the index is unique, and you want exactly one extra column in index order, then you don't need any extra sorting to get (a, b) in order. (If the index is not unique, or there is more than one extra trailing column in the pathkey, you need the incremental sort patch[1] to use this index). This was brought to my attention by a guru from a different RDBMS complaining about stupid stuff that PostgreSQL does and I was triggered to write this message as a kind of TODO note...
[1] https://commitfest.postgresql.org/24/1124/ -- Thomas Munro https://enterprisedb.com
