Well, yes, I omitted anything that may or may not happen to explain the general principle. Cardinality-based optimizations can and do take place, but they depen

Well, yes, I omitted anything that may or may not happen to explain the general principle. Cardinality-based optimizations can and do take place, but they depend on the db being aware of the shape of your data (you need to analyze the tables often) and depend on internal factors and heuristics that are subject to change between releases, can't be assumed across different databases, and may or may not actually speed up the query (pathological cases certainly exist, even in the real world).