Roy today answered an interesting question on one of our internal mailing lists. And this reminds me to pick up that piece of information as we see this quite often on customer sites, especially after upgrades or migrations. People set OPTIMIZER_FEATURES_ENABLE (OFE) to revert the optimizer’s behaviour to another pre-current release. That’s what a lot of people think this parameter does.
But in fact this is not true. Even though our documentation states it:
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OPTIMIZER_FEATURES_ENABLEacts as an umbrella parameter for enabling a series of optimizer features based on an Oracle release number. For example, if you upgrade your database