GATE CS 2014 Set 2 — Question 40
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Databases → Relational Model → Joins
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Question
Consider a join (relation algebra) between relations and using the nested loop method. There are 3 buffers each of size equal to disk block size, out of which one buffer is reserved for intermediate results. Assuming , the join will have fewer number of disk block accesses if
Correct answer
(A) relation r(R) is in the outer loop.
Solution
In a Block Nested Loop Join with limited buffers (specifically 2 buffers for input relations: 1 for outer, 1 for inner), the cost in terms of disk block accesses is given by:where is the number of blocks in the outer relation and is the number of blocks in the inner relation.Let be the size of and be the size of .
If is outer:
If is outer: The term is common to both. The difference lies in the first term ( vs ).
Since , we have .
Therefore, .To minimize disk block accesses, the smaller relation should be in the outer loop.
If is outer:
If is outer: The term is common to both. The difference lies in the first term ( vs ).
Since , we have .
Therefore, .To minimize disk block accesses, the smaller relation should be in the outer loop.
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