1. Yan Huang - CSCI5330 Database Implementation – Access Methods
Query Processing II
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

2. Yan Huang - CSCI5330 Database Implementation – Access Methods
Join Operation
Several different algorithms to implement joins
Block nested-loop join
Indexed nested-loop join
Merge-join
Hash-join
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

3. Block Nested-Loop Join
for each block Br of r do begin for each block Bs of s do begin for each tuple tr in Br do begin for each tuple ts in Bs do begin Check if (tr,ts) satisfy the join condition if they do, add tr • ts to the result. end end end end
r :outer relation
s: inner relation
cost = br  bs + br,
Assuming one buffer for each
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

4. Block Nested-Loop Joinr …
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

5. Block Nested-Loop Join
Requires no indices
Can be used with any join condition
cost = br  bs + br
Which relation shall be out relations?
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

6. Allocate More Buffers to Outer Loop
Cost? s r …
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

7. Allocate More Buffers to Inner Loop
Cost? r s
How to allocate M buffers to inner loop/out loop/result? … …
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

8. Indexed Nested-Loop Join
Index available on the inner relation’s join attribute
Cost = br + nr  c
Where c is the cost of traversing index and fetching all matching s tuples for one tuple or r
Index available on the outer relation’s join attribute
We do not use it, can we? r … s … …
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

9. Indices available on join attributes of both r and s
Which one should be the out loop? r … s … …
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

10. Yan Huang - CSCI5330 Database Implementation – Access Methods
Exercise
Compute depositor customer, with depositor as the outer relation.
Customer has a secondary B+-tree index on customer-name
Blocking factor 20 keys
#customer = 400b/10,000t #depositor =100b/5,000t
Block nested loops
400* = 40,100
Indexed nested loops
* 5 = 25,100 disk accesses.
CPU cost likely to be less than that for block nested loops join
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

11. Merge-Join
Sort both relations on their join attribute (if not already sorted on the join attributes).
Merge the sorted relations to join them
Join step is similar to the merge stage of the sort-merge algorithm.
Every pair with same value on join attribute must be matched
Cost = br + bs + the cost of sorting if relations are unsorted
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

12. ? Merge-Join But there is a problem! s r A, 1 A, 2 A, 3 B, 1 B, 2 C,1
D, 1
A, 5
A, 3
A, 1
A, 7
A, 2
C,1
C, 2
C, 3 ?
We assume records with same value are in the same block!
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

13. Yan Huang - CSCI5330 Database Implementation – Access Methods
Merge-Join
Can be used only for equi-joins and natural joins
Why not on inequality, great than, less than joins?
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

14. Yan Huang - CSCI5330 Database Implementation – Access Methods
Exercise
Compute depositor customer, with depositor as the outer relation.
Customer has a secondary B+-tree index on customer-name
Blocking factor 20 keys
#customer = 400b/10,000t #depositor =100b/5,000t
Merge join
log(400) + log(100) = 516
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

15. Yan Huang - CSCI5330 Database Implementation – Access Methods
Hybrid Merge-join
If one relation is sorted, and the other has a secondary B+-tree index on the join attribute
Merge the sorted relation with the leaf entries of the B+-tree .
Sort the result on the addresses of the unsorted relation’s tuples
Scan the unsorted relation in physical address order and merge with previous result, to replace addresses by the actual tuples
Sequential scan more efficient than random lookup
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods

16. Yan Huang - CSCI5330 Database Implementation – Access Methods
Hybrid Merge-join r … s … …
1/14/2005
Yan Huang - CSCI5330 Database Implementation – Access Methods