An even bigger issue is that regular optimizer see a whole static world (the code). You can know very well the size of most things. For DBs, it would be 'trivi

An even bigger issue is that regular optimizer see a whole static world (the code). You can know very well the size of most things. For DBs, it would be 'trivial' if we could know the exact (or very very close) size of the tables and indexes. But the db is a mutating environment, that start with 1 row and 1 second later is 1 million, and 1 second later is 1 row again. And then you mutate it from thousands of different connections. Also, the users will mutate the shape, structure, runtime parameters, indexes, kind of indexes, data types, etc.