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Using Pattern Recognition Techniques to Derive a Formal Analysis of Why Heuristic Functions Work

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B. John Oommen

A Joint Work with Luis G. Rueda

School of Computer Science

Carleton University

Using Pattern Recognition Techniques to Derive a Formal Analysis of Why Heuristic Functions Work

- Any arbitrary optimization problem:
- Instances, drawn from a finite set, X,
- An Objective function
- Some feasibility functions
- The aim:
- Find an (hopefully the unique) instance of X,
- which leads to a maximum (or minimum)
- subject to the feasibility constraints.

- The Traveling Salesman Problem (TSP)
- Consider the cities numbered from 1 to n,
- The salesman starts from city 1,
- visits every city once, and
- Returns to city 1.
- An instance of X is a permutation of cities:
- For example, 1 4 3 2 5, if five cities considered
- The objective function:
- The sum of the inter-city distances:
- 1 4, 4 3, 3 2, 2 5, 5 1

A Heuristic algorithm is an algorithm

which attempts to find a certain instance X

that maximizes the objective function

It iteratively invokes a Heuristic function.

The heuristic function estimates (or measures) the cost of the solution.

The heuristic itself is a method that performs one or more changes to the current instance.

Consider a Heuristic algorithm that invokes any of

Two Heuristic Functions : H1 and H2

- used in estimating the solution to an
- Optimization problem
- If Estimation accuracy of H1 >
- Estimation accuracy of H2

Does it imply that

- H1 has higher probability of leading to the optimal QEP?

and

c

Pattern Recogniton Modeling

Two heuristic functions : H1 and H2

Probability of choosing a cost value of a Solution:

two independent random variables: X1 and X2

Distribution -- doubly exponential:

Our model:

Error function is doubly exponential.

Typical in reliability analysis and failure models.

How reliable is a Solution when only estimate known?

Assumptions:

Mean cost of Optimal Solution: , then

- shift the origin by E[X] = 0
- Variances:

- Estimate X1 better than Estimate of X2

Main Result (Exponential)

- H1 and H2, two heuristic functions.
- X1 and X2, two r.v. optimal solution obtained by H1 and H2
- X1’ and X2’, other two r.v. for sub-optimal solution

- Let p1 and p2 the prob. that H1 and H2 respectively make the wrong decision.

Shown that:

For a particular x, the prob. that x leads to wrong decision by H1 is given by:

X1(subopt)

X1(opt)

X2(opt)

X2(subopt)

The total probability that H1 makes the wrong decision for all values of x is:

Similarly, the prob. that H2 makes the wrong decision for all values of x is:

Solving integrals and making p1 p2, we have:

which, using ln x x - 1, implies that p1 p2 QED

- where 1=1c and 2=2c
- Also 2substituted for k1

and

Second Theorem

F(a1,k) can also be written in terms of a1 and k as:

- Suppose that a1 0and0 k 1,
- then G(a1,k) 0, and
- there are two solutions for G(a1,k) = 0

Proof:

Taking partial derivatives and solving:

R-ACM / Eq-depth

T-ACM / Eq-width

T-ACM / Eq-depth

G >>> 0, or

p1 <<< p2

R-ACM / T-ACM

Eq-width / Eq-depth

G 0, or p1p2

Minimum in a1 = 0 and 0 k 1

Graphical Analysis (Histograms)

l is estimated as where N is the # of samples

l Estimation for Histograms

Simulations performed in Query Optimization:

- 4 independent runs per simulation.
- 100 random Databases per run 400 per simulation.
- 6 Relations,
- 6 Attributes per relation,
- 100 tuples per relation.
- Four independent runs on 100 databases:

R-ACM vs. Traditional using:

11 bins, 50 values

# of times in which R-ACM yields better QEP

# of times in which Eq-width yields better QEP

# of times in which Eq-depth yields better QEP

- Applied PR Techniques to solve problem of relating Heuristic Function Accuracy and Solution Optimality
- Used a reasonable model of accuracy (doubly exponential distribution).
- Shown analytically how the high accuracy of heuristic function leads to a superior solutions.
- Numerically shown the results for normal distributions
- Shown that R-ACM yield better QEPs in a larger number of times than Equi-width and Equi-depth.
- Empirical results on randomly generated databases also shown the superiority of R-ACM.
- Graphically demonstrated the validity of our model.

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