[davidrowley]

SELECT DISTINCT has seen quite a bit of work over the past few years. As of PG15, SELECT DISTINCT can use parallel query. I imagine that might help for big tables.

I assume the recursive CTEs comment is skip scanning using an index and looking for the first value higher than the previously seen value?

Certainly skip scans would be nice. There has been some work in this area, but not recently. As far as I recall some other infrastructure needed to go in first to make it easier for the query planner to understand when skip scanning would be useful.