A posting on the OTN database forum a few days ago demonstrated an important problem with hinting – especially (though it didn’t come up in the thread) in the face of upgrades. A simple query needed a couple of hints to produce the correct plan, but a slight change to the query seemed to result in Oracle ignoring the hints. The optimizer doesn’t ignore hints, of course, but there are many reasons why it might have appeared to so I created a little demonstration of the problem – starting with the following data set:
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