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For example, imagine that you want to retrieve Sales for a specific Region. You could have defined the Sales file as your primary file and the Sales Area file, which contains the Sales Area descriptions, as your secondary file. If your Sales file contains the Sales Area Code and your Sales Area file contained the Region Codes, then your query would run faster if your Sales Area file was the primary file.
You may also find that your query runs faster if you use the smaller file as your primary file.
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