Even if you assume that the human programmer is better than the query optimizer, which I think you'll agree is assuming rather a lot, the query optimizer still

Even if you assume that the human programmer is better than the query optimizer, which I think you'll agree is assuming rather a lot, the query optimizer still has important advantages. It can react to changing conditions. The optimal query plan for a given query can change as data is added or deleted. Moreover, there can be multiple optimal query plans at the same time for "the same query" (or at least the same parameterized SQL statement). I'm not suggesting that optimizers don't produce bad plans that have to be worked around. My point is that comparing the optimizer to an expert human only makes sense as a thought experiment. Most bad query plans are caused by bad selectivity estimates/stats, not bad cost models.