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8 lines
582 B
TeX
8 lines
582 B
TeX
For a basic \texttt{SELECT A, B FROM R} query, we scan the file and for each tuple output $A, B$.
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Obviously, that means that for $N$ tuples and ratio of included tuples of $Q$, we have \cost{$N + N \cdot Q$} I/O operations.
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($N$ to read, $N \cdot Q$ to write the filtered tuples)
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For a \texttt{SELECT DISTINCT A, B FROM R} query, we scan the file and eliminate duplicates before outputting.
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If this is also combined with a sort, we can prefer the sort-based approach and deduplicate after search,
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so we are projecting, then sorting and finally deduplicating using the sort method.
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