[Sesse__]

In most cases, the end result is a lot messier than it looks in this minimal example. Consider a case where the query is a 12-way join with a large IN list somewhere (which is essentially an OR); would it still look as attractive to duplicate that 12-way join a thousand times?

There _are_ optimizers that try to represent this kind of AND/OR expression as sets of distinct ranges in order to at least be able to consider whether a thousand different index scans are worth it or not, with rather limited success (it doesn't integrate all that well when you get more than one table, and getting cardinalities for that many index scans can be pretty expensive).