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This is the type of question that can have a million answers. If all we look at is the cost we would assume that Oracle has optimized the cost, but there are many factors that will affect performance. I would ask you, what type of data are you selecting? Are indexes being used? How is the data being selected? Are there filters? What is the state of the data in the database? Are you having network issues? Where is your data located? I can continue, but I am sure you get the point.
What you need to do instead is look at what is happening. It might be time for you to run Statspack or AWR and analyze what is happening to your statement. Are there waits? Is there a large disk read? So I would say that there is no silver bullet on this one. It will be necessary for you to investigate the symptoms and make a diagnosis.
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