Let r and s be two relations over the relation schemes R and S respectively,…
2001
Let r and s be two relations over the relation schemes R and S respectively, and let A be an attribute in R. then the relational algebra expression
is always equal to
- A.
A
- B.
B
- C.
C
- D.
D
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Correct answer: C
A tuple t is in the final result of σ (A=a)(r ⋈ s) if and only if it is a combination of a tuple from r and a tuple from s that match on common attributes, and the component from r satisfies A=a.
Filtering r beforehand removes exactly those tuples that would have been discarded anyway after the join.
Query Optimization Insight: In real-world databases, evaluating option (c) is vastly more efficient than evaluating the original expression. Joining two large tables first and then filtering takes a lot of memory and time. Filtering the table r first significantly reduces the number of rows that participate in the join operation.