Distributed Query Processing using different Semijoin operations.

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Distributed Query Processing using different
Semijoin operations.

• 
  
• 
  Presented By
• Jamal
  Uddin Ahamed
• 
  Friday,March12,2004

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Presentation Outline

• 1.Overview.
• 2.Semijoin Operation.
• 3. Different semijoin operations.
• a. 2 way semijoin.
• b.Hash Semijoin.
• c.Domain Specific Semijoin.
• d. Composite semijoin.
• 4. References.
• 5.Questions and Answer.

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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.

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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.

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1.3 What is distributed query processing?

• The retrieval of data from different sites in a
  network is known as distributed query processing.

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1.4 Phases of distributed query processing with a
semijoin operator.

• Initial Local processing (Selections and Projects
  are processed at each site.)
• 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)
• Final processing (all relations involved are
  transmitted to final site and all joins are
  performed there.)

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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
• Project Ri on attribute A (RiA ) and ship this
  projection ( a semijoin projection) from the site
  of Ri to the site of Rj
• Reduce Rj to Rj by eliminating tuples where
  attribute A are not matching any value in RiA .

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2.2 Example

• Example (semijoin s R1A?R2)

Benefit (s) 6 -2 4 Cost (s) 3 Cost
effectiveness D(s) B(s)-C(s) gt0
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3.a.1 Definition of 2 way semijoin.

• 2-way Semijoinan 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 RiA?Rj, RjA?Ri
• So t reduces Ri and Rj to Ri and Rj
  respectively.

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3.a.2 Properties of 2 way semijoin.

• Computing steps
• Send Ri A from site i to site j
• 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 Am (match
  one of Rj A) and Ri Anm(Ri A- Ri Am)
• Send min(Ri Am , Ri Anm) back to site i
• Reduce Ri to Ri using Ri Am (or Ri Anm) .
• Evaluation
• Benefit B(t) S(Ri ) - S(Ri ) S(Rj) -
  S(Rj)
• Cost C(t) S(Ri A ) minS(Ri Am ) ,
  S( Ri Anm)
• If the benefit exceeds the cost (D(t) gt0) then it
  is called a cost-effective 2-way semioin

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3.a.3 2-way semijoin example.
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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.

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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.

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3.b.2 hash semijoin example.
R2
R1
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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

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3.c.1 What is horizontally partitioned database

• We can call a distributed database system is
  horizontally partitioned (or fragmented) if the
  relations can be split horizontally into several
  disjoint sets of tuples, which are called
  horizontal fragments.

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3.c.2 Horizontally partitioned database
system.(Example)
EMP1 1?D-no ?10
EMP
E-no E-name D-no
101 johnson 01
103 jordan 03
105 erving 01
E-no E-name D-no
101 johnson 01
103 jordan 03
105 erving 01
109 jabbar 12
110 sampson 14
141 chang 16
?
EMP2 11?D-no ?20
E-no E-name D-no
109 jabbar 12
110 sampson 14
141 chang 16
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3.c.3 Horizontally partitioned database
system.(Properties)

• A fragmented relation Ri can be constructed by
  performing a union operation on all its fragment.
• Ri Uk Rik
• There is commutative rule between the binary
  operations join and union for fragmented
  relations a join between two fragmented relation
  R1 and R2 is equivalent to a union over the joins
  between each fragment of R1 and each fragment of
  R2.
• Mathematically
• (U R1k)AB (U R2m) U(R1kAB R2m)
• k m k.m

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3.c.4 Why cant we use regular semjoin between
two fragment to reduce the size of
fragments?(Continue)

• We consider a joint RiAB Rj between two
  fragmented relations Ri and Rj. We want to
  reduce the size of Rik, a fragment of Ri , by
  semijoin before it is sent to the final
  processing site. We cannot perform the semijoin
• Rik? AB Rjm
• between Rik and any fragment Rjm of Rj
  without considering the other fragment Rjm of Rj
  , because the join operation dictates that no
  tuple of a relation can be eliminate before it is
  compare with all tupls of the other joining
  relation which may be contribute to the join.

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Example
EMP1 1?D-no ?10
sal 101?E-no ?105
E-no E-name D-no
101 johnson 01
103 jordan 03
135 erving 01
E-no Sal D-no
101 1000 12
102 2000 03
105 3000 11
D-no
01
03
12
14
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EMP2 11?D-no ?20
sal 105?E-no ?110
E-no E-name D-no
109 jabbar 12
110 sampson 14
141 chang 16
E-no Sal D-no
107 1000 12
107 2000 03
110 3000 11
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3.c.5 Definition of Domain Specific Semijoin.

• The domain-specific semijoin operation, Rik(
  AB Rjm, where A and B are the joining
  attributes and Rik, Rjm are two fragments of the
  joining relation Ri and Rj respectively, is
  defined as follows
• Rik( AB Rjm rr? Rik r.A ? Rjm B
  U(DomRj.B-DomRjm.B)
• Where Rik is the restricted fragment and Rjm
  is the restricting fragment. We also called Ri
  the restricted relation and Rj is the restricting
  relation of the domain-specific semijoin.

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3.d.1 Definition of Composite Semijoin.

• Composite Semijoin a semijoin in which the
  projection and the transimssion involve multiple
  columns (attrs).

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3.d.2 Example of Composite Semijoin.
R2
R1
A1 A2 Non-join Attr
1 aa -
1 bb -
2 cc -
3 cc -
A1 A2 Non-join Attr
1 cc -
1 aa -
2 bb -
3 bb -
A1 A2 Non-join Attr
1 aa -
No False loop!!
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3.d.3 Semijoin Vs Composite Semijoin.

• Composite semijoins in a query processing
  algorithm is likely to result in substantial RT
  reduction.
• Composite semijoins should not always be used. If
  it results greater RT, ignore it.
• Strategy with composite semijoins is at least as
  good as that without composite semijoins.

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References

1. Using 2-way semijoin in distributed query
   processing. By Hyunchul Kang and Nick
   Roussopoulos.
2. Improving distributed query processing by
   hash-semijoins. By Judy Tseng and Arbee Chen.
3. Domain Specific SemijoinA new operation for
   distributed query processing. By Jason Chen and
   Victor Li.
4. Composite Semijoin in distributed query
   processing. By William Perrizio and Chun Chen

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Comments Questions??
Thank You!