> > I'm still confused how the optimizer in any scenario would choose an index > with no relevant columns to complete a join. Is there ever a scenario > where such a choice makes sense? >
I used to ask such questions until about 15 years ago. I never got an entirely satisfactory answer. To add to the mix, how table stats would change in such a way to lead the > optimizer in such a direction, where for years it behaved reasonably. > Statistics that drive the optimised heuristics change over time. And every time they get updated there is a very small but not 0 chance that the random dive into the index tree will get sufficiently unlucky to get an estimate that is far enough removed from reality to misinform the optimiser into a suboptimal execution plan.
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