Distributed Database Management Systems: Lecture 15

Distributed Database Management Systems: Lecture 15. The main topics covered in this chapter include: PHF- Minimality of Pr; PHF-COM-MIN algorithm; primary horizontal partitioning algorithm; derived horizontal fragmentation (DHF);...

Distributed Database Management Systems

Lecture 15

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PHF- Minimality of Pr

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• A relevant predicate is the one if it influences how fragmentation is

performed (fragments f into f i and f j ) then there should be at least one

application that accesses f i and f j

differently

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If all predicates in a set Pr are relevant then the set is minimal

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PHF-COM-MIN Algorithm

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• Given: a relation R and a set of

simple predicates P r

• Output: a complete and minimal set of

simple predicates P r ’ for P r

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• Rule 1: a relation or fragment is

partitioned into at least two parts

which are accessed differently by at least one application.

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1-Initialization:

– Find a p i ∈ ∈ Pr such that p Pr such that p i ,

partitions R according to Rule 1 Pr’ ← p i

Pr ← Pr – p i

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2- Iteratively add predicates to Pr' until it is complete, find a pj ∈ ∈ Pr such that pj

partitions R according to Rule 1

set Pr' = Pr' U pi ;

Pr = Pr – pi ;

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if p k in Pr' is non-relevant then

Pr' = Pr' – p k

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Primary Horizontal Partitioning

Algorithm

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fragmentation

predicates Pr

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predicates M according to which relation R is to be fragmented

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• Pr‘ ← COM_MIN (R,Pr)

predicates

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among pi Pr

minterms from M

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PHF – Example II

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1 Find the name and budget of projects given their no issued at three sites

– p1 : LOC = "Lahore”

– p2 : LOC = “Rawalpindi"

– p3 : LOC = "Peshawar“

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2 Access project information according to budge one site accesses ≤ 200000 other accesses >200000

– p4 : BUDGET ≤ 200000

– p5 : BUDGET > 200000

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Pr = Pr' = {p 1 ,p 2 ,p 3 ,p 4 ,p 5 }

• Implications

• p 1 p 2 p 3

• p 4 p 5

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• From predicates p1 to p6 in Pr’, there

may be so many minterm predicates, like,

p1 ^ p2 ^ p3 ^ p4 ^ p5

Excluding the contradicting minterm

predicates

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• Implications must be based on the database semantics not on a

particular extension of the

database

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Derived Horizontal Fragmentation(DHF)

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• Defined on a member relation of a link according to a selection

operation specified on its owner

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• Two important points:

– Each link is an equi-join.

– Equijoin can be implemented by means

of semi-joins

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• So we are interested in defining the

partitions of member based on

fragmentation of its owner, but want to see attributes only from member, so

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R i = R ⋉ S i , 1≤ i ≤ w

where w is the maximum number of

fragments that will be defined on R and

Si = Fi (S), where Fi is formula for PHF on S

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DHF Example

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EMP

salaries one with salary less than or equal to

30,000/- and other more than that

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Thanks