Distributed Database Management Systems: Lecture 14

Distributed Database Management Systems: Lecture 14. The main topics covered in this chapter include: minterm predicates; PHF-requirements; minterm selectivities sel(mi); access frequencies acc(qi); access frequencies minterm acc(mi); completeness example;...

Distributed Database Management Systems

Lecture 14

Minterm predicates:

given a set of simple predicates

Pr = {pr1, pr2, …, pm}for a relation R, the set of minterm predicates M is defined as

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PAY(title, sal)

p1: title = “El Eng”

p2: title = “Sys Ana”

p3: title = “Mech Eng”

p4: title = “Prog”

p5: SAL ≤ 20,000 p6: SAL > 20000

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Minterm Predicates

m1: title = “El Eng” SAL ≤ 20,000

m2: title = “Sys Ana” SAL > 20,000

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PHF-Requirements

3- minterm selectivities sel(mi):

number of tuples of the relation that would be accessed by a user query involving mi Like, sel(m1) = 0 as

per data on p108

4- Access frequencies acc(qi):

frequency with which user

applications access data If Q(q1,

q2, …, qn} is set of user queries, then acc(qi) determines the

access frequency of qi within a

given period, like acc(q2) = 24

5- Access frequencies minterm

acc(mi) can also be defined from acc(q )

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• A PHF is defined by a selection

operation on the owner

relations of the database

schema, given a relation R, its PHF are given by Ri = Fi

(R), 1 ≤ i ≤ w

– Where F j is a selection formula, which is (preferably) a minterm predicate.

PHF (Simple Example)

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Elect Eng 40000Sys Analyst 34000Mech Eng 27000Programmer 24000

PAY

PAY1 =    σ sal ≤ 30000  (PAY)

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• Defining simple predicates may be

tricky

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A set of simple predicates Pr is said to

tuples of the minterm fragments

defined on P r requires

that two tuples of the same minterm

fragment have the same probability of being accessed by the application

Completeness Example

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–Find the budgets of projects at

each location (1)

–Find projects with budgets less

than 2M (2)

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Pr = {LOC=“Lahore”,

LOC=“Rawalpindi”, LOC=“Peshawar”, BUDGET<=200000, BUDGET

>200000}

• Which is complete.

Thanks