Happily, the PG planner isn't thrown off by such things, at least in my experience. It has sample rows and uses them. (I really like the postgres planner, it's the best I've used.)
I've seen it be FAR wrong about memory, though, despite being right about column widths: When you join a few tables in a big select, the planner will look at some sample rows and guess the number of rows in each step of the possible plan, and if the sample is too small, that estimate can be wrong by a LARGE factor.
I've seen it be FAR wrong about memory, though, despite being right about column widths: When you join a few tables in a big select, the planner will look at some sample rows and guess the number of rows in each step of the possible plan, and if the sample is too small, that estimate can be wrong by a LARGE factor.