1. Distributed Query Processing using different Semijoin operations.

2. Presentation Outline:
1.Overview.
2.Semijoin Operation.
3. Different semijoin operations.
a. 2 way semijoin.
b. Hash Semijoin.

3. 1.1 What is distributed database system?
A distributed database system is characterized by the distribution of the system components of hardware ,control and data. For this research, a distributed system is a collection of independent computers interconnected via point-to-point communication lines.

4. 1.2 Node Characteristics:
Each computer , known as a node in the
network, has a processing capability, a
data storage capability, and is capable
of operating autonomously in the system.
Each node contains a version of a
distributed DBMS.

5. 1.3 What is distributed query processing?
The retrieval of data from different sites in a network is known as distributed query processing.

6. 1.4 Phases of distributed query processing with a semijoin operator.
1. Initial Local processing (Selections and Projects are processed at each site.)
2. Semijoin processing ( A semijoin program) is derived from the remaining join operations and executed to reduce the size of the relations in a cost-effective way)
3. Final processing (all relations involved are transmitted to final site and all joins are performed there. qs: query site)

7. 2.1 Semijoin:
A semijoin from Ri to Rj on attribute A can be denoted as Rj⋉ Ri .It is used to reduce the data transmission cost.
Computing steps:
1) Project Ri on attribute A (Ri[A] ) and ship this projection ( a semijoin projection) from the site of Ri to the site of Rj ;
2) Reduce Rj to Rj’ by eliminating tuples where attribute A are not matching any value in Ri[A] .

8. 2.2 Example: Example (semijoin s: R1—AR2): Site 2 Site 1 qs3 4 5 7 8 9 A C R2 B 1 2 6 R1
Site 1
Site 2 1 2 3
R1[A]
projection
Ship(3) 3 7
R2’
reduce qs
Ship(2)
Ship(6)
Benefit (s) = 6 -2 = 4
Cost (s) = 3
Cost effectiveness D(s) = B(s)-C(s) >0

9. 3.a.1 Definition of 2 way semijoin.
2-way Semijoin—an extended version of the semijoin
Definition: A 2-way semijoin (t) of Ri and Rj on attribute A can be denoted as
RiARj = {Ri—ARj, Rj—ARi }
So t reduces Ri and Rj to Ri’ and Rj’ respectively.

10. 3.a.2 Properties of 2 way semijoin.
Computing steps:
1) Send Ri [A] from site i to site j ;
2) Reduce Rj to Rj’ by eliminating tuples whose attribute A are not matching any of Ri [A] and at the same time partition Ri [A] to Ri [A]m (match one of Rj [A]) and Ri [A]nm(Ri [A]- Ri [A]m) ;
3) Send min(Ri [A]m , Ri [A]nm) back to site i ;
4) Reduce Ri to Ri ’ using Ri [A]m (or Ri [A]nm) .
Evaluation:
Benefit: B(t) = [S(Ri ) - S(Ri ’)] + [S(Rj) - S(Rj’)]
Cost: C(t) = S(Ri [A] ) + min[S(Ri [A]m ) , S( Ri [A]nm)]
If the benefit exceeds the cost (D(t) >0) then it is called a cost-effective 2-way semioin

11. 3.a.3 2-way semijoin example.1 2 3
R1[A]
projection
Ship(3) 3 4 5 7 8 9 A C R2 B 1 2 6 R1
Site 1
Site 2 3
R1[A]m 1 2
R1[A]nm
partition 7
R2’
reduce
Ship(1) 3 6
R1’
reduce qs
Ship(2)

12. 3.a.4 Semijoin Vs 2-way semijoin.
- It is an extended version of semijoin.
- It has more reduction power than semijoin.
- The propagation of reduction effects by the 2-way semijoin is further than by the semijoin.

13. 3.b.1 Hash-semijoin operator.
Main idea : use a search filter which represents the semijoin projection with a small bit array .
Definition:
The hash-semijoin of Ri and Rj is denoted Rj∝ Ri. It is computed as follow:
The Semijoin projection of Ri is represented as a bit array;
Shipping this bit array to the site of Rj ;
finally, the tuples of Rj are screened by the search filter.

14. 3.b.2 hash semijoin example.R2 R1 1 B
H(x)=X
Hij((Ri))Bij
S#(R1) 1 3 4 8
projection S#
Phone 2
222 3
333 4
444 5
555 6
666
Ship(Bij) Rj S#
Name 1
Cindy 3
Jemal 4
Sunny 8
Maggie
reduce 3
333 4
444 14

15. 3.b.3 Semijoin Vs Hash Semijoin.
Advantages:
Hash-semijoin is more cost-effective than semijoin
The search filter in the hash-semijoin achieves considerable savings in the cost of a semijoin operation
Limitation:
Only works on execution tree
Tightly related with the hash functions