In general if you query a table twice, the query planner will visit that table twice. So DISTINCT ON is almost always more efficient. The main exception I’ve s

In general if you query a table twice, the query planner will visit that table twice. So DISTINCT ON is almost always more efficient. The main exception I’ve seen is when using it in a view, if you try to filter the results of the view, it usually can’t optimize a DISTINCT ON, but sometimes can for GROUP BY (though that might have improved on newer versions)