Just like the MEMO structure in SQL Server which is kind of a "paper trail" of steps the optimizer takes in optimizing the query, is there anything in Oracle through which I can see what are the alternative plans the optimizer has ever considered (and pruned because they are more expensive than the plan that the optimizer chooses to execute)?

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Yes! This is called a 10053 trace in Oracle parlance. It's more formally known as "Tracing of Optimizer Computations".

Check out MOS doc 225598.1, or Jonathon Lewis' blog post here: https://jonathanlewis.wordpress.com/2014/05/23/10053-trace/

There's loads of hits for "10053 trace" so if you can't find what you need, it's probably out there in search-land.

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