You may have heard that when you write code to address relational databases you don't need to know HOW to access the data. The statement is true, but not in the way that it's often interpreted.
The optimizer will ensure that if you write the correct statement you will get the correct answer - but there are many reasons why the optimzer might not get to that answer in the most efficient way. If you learn a little bit about how the optimizer "thinks" and how it analyzes the problems you give it, you will soon learn to recognise when, and why, it's going to make mistakes and what you can do to push it into a reasonable execution path.
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