It's a bizarre side-effect of lacking query hints. This is my interpretation of the sequence of events: 1. Postgres will never implement query hints, because q

It's a bizarre side-effect of lacking query hints. This is my interpretation of the sequence of events: 1. Postgres will never implement query hints, because query hints are evil 2. In the real world people need them anyway, so they (ab)use implementation details that offer some control over the query plan (WITH is one example, see also OFFSET 0 and other hacks) 3. Everyone starts relying on these implementation details, e.g. you'll find people actually recommending this as a way to optimize queries in the mailing lists 4. Now when someone asks to fix the planner, someone else will point out this will break all the queries that have been "optimized" by depending on the old behavior Postgres is generally a fairly sane project though, so I have some confidence they will come to their senses. Eventually.