I think the article describes the solution that was used and not necessarily the best solution. If they made the “obvious” mistake in the first case they mightn

I think the article describes the solution that was used and not necessarily the best solution. If they made the “obvious” mistake in the first case they mightn’t arrive at the best solution in the second. Adding the single index and using the four tables have the same worst case complexity for reads, O(log n), with different constants and massively beat scanning at O(n). It may be that the second solution wasn’t the best, or that it was better for write performance or memory usage—this was 2002 after all. It may also be the case that the cardinal it’s of some columns was low in such a way that the four table solution was better, but maybe getting the columns in the right order for a multi column index could do that too. Either way, I don’t think it really matters that much and doesn’t affect the point of the article.