Please refer to the following link which has very food fundamental information on how tableau works.It really helped me to know the intricacies..hope will help u in the same way.
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Tableau caches the results of all SQL queries that fit in memory and reuses them if possible. If two sheets are the same, then it will hit the cache (as long as it wasn't too big to be cached in the first place). If they are similar but not the same, Tableau will see if it can figure out the answer to one from the other by applying additional filtering or aggregation.