## Joins

 Question 1
Consider a join (relation algebra) between relations r(R) and s(S) 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 size(r(R)) < size(s(S)), the join will have fewer number of disk block accesses if
 A relation r(R) is in the outer loop. B relation s(S) is in the outer loop. C join selection factor between r(R) and s(S) is more than 0.5. D join selection factor between r(R) and s(S) is less than 0.5.
Database-Management-System       Joins       Gate 2014 Set -02
Question 1 Explanation:
The join between r(R) and s(S) which is using nested loop will be as follows,
For each tuple r in R do
For each tuple s in S do
If r and s satisfy the join condition then
output the tuple
The above algorithm involves (nr*bs+br )
block transfer and (nr+br ) seeks.
where,
br → no. of blocks in R
bs → no. of blocks in S
hr → no. of tuples in R
To have less block accesses in nr should be less and in question it is given that size(r(R)) < size(s(S)).
To have fewer no. of disk block accesses the relation r(R) should be in outer loop.
There is 1 question to complete.