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Title: Speeding up warehouse physical design using a randomized algorithm
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Title: Speeding up warehouse physical design using a randomized algorithm
Series Title: Department of Computer and Information Science and Engineering Technical Report ; 99-012
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Creator: Lee, Minsoo
Hammer, Joachim
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Publication Date: April, 1999
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M. Lee and J. Hammer, "Speeding Up Warehouse Physical Design Using A Randomized Algorithm," University of Florida,
Gainesville, FL, Technical Report TR99-012, April 1999.

Speeding Up Warehouse Physical Design Using A

Randomized Algorithm

Technical Report TR99-012

Minsoo Lee and Joachim Hammer
Dept. of Computer and Information Science and Engineering
University of Florida
Gainesville, FL 32611-6120
{mslee, jhammer}@cise.ufl.edu

Abstract
A data warehouse stores information that is collected from multiple, heterogeneous information
sources for the purpose of complex querying and analysis. Information in the warehouse is typically
stored in the form of materialized views. One of the most important tasks when designing a warehouse
is the selection of materialized views to be maintained in the warehouse. The goal is to select a set of
views in such a way as to minimize the total query response time over all queries, given a limited
amount of storage space and time for maintaining the views (view selection problem).
The paper focuses on an efficient solution to the view selection problem using a genetic algorithm
for computing a near-optimal set of views. Specifically, we explore the view selection problem in the
context of OR view graphs. We show that our approach represents a dramatic improvement in time
complexity over existing search-based approaches using heuristics. Our analysis shows that the
algorithm consistently yields a solution that lies within 10% of the optimal query benefit while at the
same time exhibiting only a linear increase in execution time. We have implemented a prototype version
of our algorithm which is used to simulate the measurements used in the analysis of our approach.

1 Introduction
A data warehouse stores information that is collected from multiple, heterogeneous information
sources for the purpose of complex querying and analysis [9, 17]. The information in the warehouse is
typically processed and integrated before it is loaded in order to detect and resolve any inconsistencies
and discrepancies among related data items from different sources. Since the amount of information in
a data warehouse tends to be large and queries may involve hundreds of complex aggregates at a time,
the organization of the data warehouse becomes a critical factor in supporting efficient online
analytical query processing (OLAP) as well as in allowing periodic maintenance of the warehouse
contents. Data in the warehouse is often organized in summary tables, or materialized views [15],
which represent pre-computed portions of the most frequently asked queries. In this way, the
warehouse query processor avoids having to scan the large data sets for each query, a task that is even
more wasteful if the query occurs frequently. However, in order to keep these materialized views
consistent with the data at the sources, the views have to be maintained. Rather than periodically
refreshing the entire view, a process that may be time consuming and wasteful, a view can be
maintained in an incremental fashion, whereby only the portions of the view which are affected by the
changes in the relevant sources are updated [5, 18].
Besides this so-called view maintenance or update cost, each materialized view in the
warehouse also requires additional storage space which must be taken into account when deciding
which and how many views to materialize. For example, given a set of frequently asked OLAP
queries, materializing all possible views will certainly increase query response time but will also raise
the update costs for the warehouse and may exceed the available storage capacity. Thus by trading
space for time and vice versa, the warehouse administrator must carefully decide on a particular
warehouse configuration which balances the three important factors given above: query response time,






SPEEDING UP WAREHOUSE PHYSICAL DESIGN USING A RANDOMIZED ALGORITHM


maintenance cost, and storage space. The problem of selecting a set of materialized views for a
particular warehouse configuration which represents a desirable balance among the three costs is
known as the view selection problem1.
In this paper we focus on a solution to the view selection problem which minimizes query
response time given varying upper bounds on the maintenance cost, assuming a fixed amount of
available storage space. Specifically, we explore the view selection problem in the context of OR view
graphs, in which any view can be computed from any of its related views. Although this problem has
been addressed previously (e.g., see [6], [16]), existing algorithms do not perform well when
computing warehouse configurations involving more than 20-25 views or so. In those cases, the search
space becomes too large for any kind of exhaustive search method and even the best heuristics can
only compute acceptable solutions for a small set of special cases of the problem. To this end, we have
designed a solution involving randomization techniques which have proven successful in other
combinatorial problems. We show that our solution is superior to existing solutions in terms of both its
expected run-time behavior as well as the quality of the warehouse configurations found. The analysis
proves that our genetic algorithm yields a solution that lies within 90% of the optimal query benefit
while at the same time exhibiting only a linear increase in execution time. We expect our algorithm to
be useful in data warehouse design; most importantly in those scenarios where the queries which are
supported by the existing warehouse views change frequently, making it necessary to reconfigure the
warehouse efficiently and quickly. Supporting data warehouse evolution in this way may increase the
usefulness of the data warehousing concept even further.
The paper is organized as follows. In the Sec. 2 we present an overview of the related work.
Sec. 3 describes our technical approach. Specifically, we briefly introduce the idea behind genetic
algorithms (which are a special class of randomized algorithms) and how we are using the technique to
find an efficient solution to the view selection problem. In Sec. 4 we describe the implementation of
our prototype which was used to generate the simulation runs which we present and analyze in Sec. 5 .
Sect. 6 concludes the paper with a summary of our results and future plans.

2 Related Research
All of the related work on view selection uses some form of greedy strategy or heuristics-based
searching technique to avoid having to exhaustively traverse the solution space in search of the
optimal solution. The problem of selecting additional structures for materialization was first studied by
Roussopoulos [14] who proposed to materialize view indices rather than the actual views themselves.
View indices are similar to views except that instead of storing the tuples in the views directly, each
tuple in the view index consists of pointers to the tuples in the base relations that derive the view tuple.
The algorithm is based on A* to find an optimal set of view indexes but uses a very simple cost model
for updating the view which does not take into account which subviews have been selected. As a
result, the maintenance cost for the selected view set is not very realistic.
More recently, Ross et al. [13] and Labio et al. [10] have examined the same problem using
exhaustive search algorithms that make use of heuristics for pruning the search space. The work by
Labio et al. is an extension of the previous work by considering indexes and also improving upon the
optimal algorithm. In addition, they are the first to provide a valuable set of rules and guidelines for
choosing a set of views and indexes when their algorithm cannot compute the optimal warehouse
configuration within a reasonable time due to the complexity of the solution. Similarly, Theodoratos et
al. [16] present an exhaustive search algorithm with pruning to find a warehouse configuration for
answering a set of queries given unlimited space for storing the views. Their work also focuses on
minimizing query evaluation and view maintenance.




1 Sometimes the problem is also referred to as the view index selection problem (VIS) when the solution includes a
recommendation on which index structures should be maintained in support of the materialized views.


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[8] present and analyze greedy algorithms for selection of views in the special case of "data
cubes" that come within 63% of the optimal configuration. However, their calculations do not figure
in the update costs for the selected views.
Our work is most closely related to that of Gupta [6] who has used both the greedy approach as
well as the A* algorithm for solving the view selection problem in the context of OR/AND view
graphs and the general case of AND-OR view graphs. His approach also balances query response time
and view maintenance cost while assuming a fixed amount of storage space.

3 Technical Approach
The view selection problem as stated in the introduction is considered an NP-hard problem [2,
3]. Roughly speaking, it is very difficult to find an optimal solution to problems in this class because
of the fact that the solution space grows exponentially as the problem size increases. Although some
good solutions for NP-hard problems in general and the view selection problem in specific exist, such
approaches encounter significant problems with performance when the problem size grows above a
certain limit. More recent approaches use randomized algorithms in solving NP-hard problems.
Randomized algorithms are based on statistical concepts where the large search space can be explored
randomly using an evaluation function to guide the search process closer to the desired goal.
Randomized algorithms can find a reasonable solution within a relatively short period of time by
trading executing time for quality. Although the resulting solution is only near-optimal, this reduction
is not as drastic as the reduction in execution time. Usually, the solution is within a few percentage
points of the optimal solution which makes randomized algorithm an attractive alternative to
traditional approaches such as the ones outlined in Sec. 2 for example.
Our approach uses a genetic algorithm, which is one form of a randomized algorithm. The
motivation to use genetic algorithms in solving the view selection problem was based on the
observation that data warehouses can have a large number of views and the queries that must be
supported may change very frequently. Thus, a fast solution is needed to provide new configurations
for the data warehouse: an ideal starting point for the genetic algorithm. However, genetic algorithms
do not provide a magical solution by themselves and their success (or failure) often depend on the
proper problem specification, the set-up of the algorithm, as well as the outcome of the extremely
difficult and tedious fine-tuning of the algorithm that must be performed during many test runs. After
a brief overview of the genetic algorithms, we provide details on how to apply the genetic algorithm to
the view selection problem. Specifically, we elaborate on a suitable representation of the solution as
well as the necessary evaluation functions needed by our genetic algorithm.

3.1 Genetic Algorithms
The idea behind the genetic algorithm comes from imitating how living organisms evolve into
superior populations from one generation to the next. The genetic algorithm works as follows. A pool
of genomes are initially established. Each genome represents a possible solution for the problem to be
solved. This pool ofgenomes is called a population. The population will undergo changes and create a
new population. Each population is referred to as generation t, generation t+1, generation t+2, ..., and
so on. After several generations, it is expected that the population t+k should be composed of superior
genomes (that have survived the evolution process) than the population in generation t.
The Genetic Algorithm repeatedly executes the following four steps:
I(t=t+l
@ select P(t) from P(t-1)
recombine P(t)
evaluate P(t)
In step (, a new generation t is created by increasing the generation variable t by one. In step @
superior genomes among the previous population P(t-1) are selected and used as the basis for


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composing the genomes in the new population P(t). A statistical method, for example, the roulette
wheel method, is used to select those genomes which are superior.
In step , several operations are executed on paired or individual genomes to create new
genomes in the population. These operations are called crossover and mutation operations,
respectively. Step ) evaluates the population that is created. A function called the fitness function,
which evaluates the superiority of a genome, is used in this process. The fitness of each genome can be
gathered and used as a metric to evaluate the improvement made in the new generation. This fitness
value is also used during the selection process (in step @) in the next iteration to select superior
genomes for the next population. Also, the genome with the best fitness so far is saved. We now
explain how this algorithm can be adapted to solve the view selection problem.

3.2 An Improved Solution to the View Selection Problem
In order to apply the Genetic Algorithm (GA) to the view selection problem two requirements
must be met: (1) We need to find a string representation of a candidate solution and (2), we need to be
able to define the fitness function, the crossover operator, and the mutation operator as outlined above.
In [11], the authors discuss several solutions to popular problems using genetic algorithms,
including the 0/1 knapsack problem (see for example [1]). The similarity of the view selection
problem to the 0/1 knapsack problem gives us a hint on how to apply the Genetic Algorithm in our
context. However, to our knowledge nobody has yet to apply genetic algorithm techniques to solving
view selection and the solutions presented here represent our own approach

3.2.1 Problem Specification
The problem to be solved can be stated as follows [6]: Given an OR view graph G and a
quantity representing the available space, select a set of views that minimizes the sum of total query
response time and total maintenance cost. An OR view graph is composed of a set of views where
each view in the graph has zero or more ways to be constructed from other source views, but each way
involves only one other view. In other words, only one view among the source views is needed to
compute a view. An example of an OR view graph is the data cube [4] where each view can be
constructed in many different ways but each derivation only involves one view. A sample OR view
graph is shown in Figure 1. For example, in this figure, view a can be computed from any of the views
b, c, d (the same is true for the views b, c, d).




OR
views


OR OR OR


base tables g h 1 k




Figure 1: Sample OR view graph for four views.
Future versions of this paper will address the cases of AND view graphs as well as AND-OR
view graphs. We are also deferring the problem of index selection for later. However, in Sec. 6 we
briefly mention how index selection can be added into the problem domain in a straightforward
manner.


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3.2.2 Problem Solution
REPRESENTATION OF THE SOLUTION.
A genome represents a candidate solution of the problem to be solved. The solution of the
problem should be represented appropriately in a string format. The string could be a binary string
composed of Os and Is, or a string of alphanumeric characters. The content of the string is very
flexible, but the representation of the solution should be carefully designed so that it is possible to
properly represent all possible solutions. The views in the OR view graph may be enumerated as vi,
V2, ..., Vm where m is the total number of views. We can represent a selection of these views as a
binary string ofm bits. If the bit in position i (starting from the leftmost bit as position 1) is 1, the view
vi is selected. Otherwise, the view vi is not selected. As an example, if a bit string is 001101001,
which encodes the fact that only views v3, v4, v6 and v9 are selected.
INITIALIZATION OF THE POPULATION.
The initial population will be a pool of randomly generated bit strings of size m. In future
implementations, however, is it straightforward to start with an initial population which represents a
favorable configuration based on external knowledge about the problem and its solution. It will be
interesting to see if and how this affects the quality as well as the run-time of our algorithm. For the
experiments described in this paper, we have chosen a population size of 30.
SELECTION, CROSSOVER, MUTATION, TERMINATION
The selection process will use the popular roulette wheel method [11]. The crossover and
mutation operators are assigned probabilities pc and pm, respectively. The specific values used in our
simulation are 0.001 and 0.9. The crossover operation is applied to two genes by exchanging
information between the two, thereby creating two new genes. Crossover works as follows:


Each genome is selected with a probability of pc.
Pair genomes
For each pair, do the following :
// Assuming two genomes gl = (bl b2 ... bpos bpos,+ ... bi) and
// g2 = (ci c2 .. Cpos Cpos+ . cm)
Randomly decide a crossover point pos
Exchange information among genomes, and replace gl, g2 with gl', g2'
// (ex) gl' = (bl b ... bpos Cpos+ ... cm) and
// g2' = ( C2 . cpos bpos+l ... bm)


The mutation operator works as follows.

For all genomes,
For each bit in genome
mutate (flip) bit with probability of pm


The termination condition is 400 generations. These values are based on empirical values used
in other examples, and although reference [4] mentioned that no improvement was observed after 500
generations, we have reduced this value to 400 in our experiments as the convergence was observed to
be more rapid in our algorithm.
EVALUATION PROCESS


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The fitness function measures how good a solution (i.e., a genome) is by providing a fitness
value as follows: If the fitness is high, the solution satisfies the goal; if the fitness is low, the genome
should be discarded. There are many possible fitness functions and finding the best possible one (i.e.,
one that can truthfully evaluate the quality of a particular warehouse configuration) requires a lot of
fine-tuning.
For our problem, the fitness function has to evaluate a genome (i.e., a set of selected views to
materialize) with respect to the query benefit as well as with respect to the maintenance constraint.
This is similar to the goal of the 0/1 knapsack problem, where the goal is to maximize the profit of the
packed load while satisfying a specific capacity constraint of the knapsack. The difference is that in
the view selection problem, when a view is selected, the benefit will not only depend on the view itself
but also on other views that are selected. A good way to model such a complex problem is by
introducing a penalty value as part of the fitness function. This penalty function will reduce the fitness
if the maintenance constraint is not satisfied. When the maintenance constraint is satisfied, the penalty
function will have no effect and only the query benefit should be evaluated. Thus, the fitness function
Eval can be defined as follows:
Subtract mode (S): Eval(x) = B(G,M) -Pen(x) (if B(G,M)-Pen(x) 0 ) ......... (
= 0 (if B(G,M)-Pen(x) < 0 )
for all x[i] = 1, vi EM
for all x[i] = 0, vi M
where B is the query benefit function, Pen is the penalty function, x is a genome, G is the OR view
graph, and M is the set of selected views given by x. Note that we can avoid a negative fitness value
by setting the penalty to zero whenever the assigned penalty exceeds the query benefit.
There are several other ways to apply the penalty function as illustrated by the following two
examples:
Divide (D): Eval(x) = B(G,M) /Pen(x) ( if Pen(x) > 1) .................
= B(G,M) ( if Pen(x) x 1)
Subtract&Divide (SD): Eval(x) = B(G,M) Pen(x) ( if B(G,M) > Pen(x)) ..................
= B(G,M) /Pen(x) ( if Pen(x) 2 B(G,M) and Pen(x) > 1 )
= B(G,M) ( if Pen(x) 2 B(G,M) and Pen(x) 1 )
The penalty function can either be applied by dividing the query benefit as shown in @ (divide
mode), or by combining the subtract and divide modes together as shown in .
The penalty function itself can also have various forms. For example, we have experimented
with linear penalty and exponential penalty functions sa shown in ,, and .
Logarithmic penalty (LG): Pen(x) = log 2 (1 + p (UM) S)) ...........................
Linear penalty (LN): Pen(x) = (1 + p (U(M) S)) .................................
Exponential penalty (EX): Pen(x) ( p (U(M) S))2 .................................
where p is defined as a constant calculated from the given OR-view graph G, UM) is the total
maintenance cost of the set of materialized views M, and S is the maintenance cost constraint.
We combined the three types of penalty modes (i.e., S, D, SD) and the three types of penalty
functions (i.e., LG, LN, EX) in our prototype to evaluate and determine the best possible strategy for
solving the view selection problem. Please note, the details as well as the formulas for the query
benefit function B(G,M), the total maintenance cost U(M), and p are provided in the appendix.

4 Prototype Implementation
We used version 2.4.3 of the genetic algorithm toolkit from MIT called GAlib [12] to develop a
prototype of the algorithm described above. The toolkit supports various types of genetic algorithms,
mutation and crossover operators, built-in genome types such as 1-dimensional or 2-dimensional


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strings, and a statistics gathering tool that can provide summarized information about each generation
during a single run of the genetic algorithm. The prototype was written entirely in C++ using
Microsoft Visual C++ as our development platform.
The toolkit did not provide any libraries to encode a fitness function using the evaluation
strategies discussed above. The fitness function we developed can calculate the fitness in nine different
ways by pairing each type of penalty mode with each type of penalty function; in our implementation,
way the fitness function calculates and applies the penalty is controlled through a variable. The fitness
function needs to evaluate each genome using the cost values given by the OR-view graph and the
maintenance cost limit (e.g., given by the warehouse administrator). For this purpose, additional cost
functions which, when given a genome can calculate the total query cost and the total maintenance
cost of the selected views represented by the genome, must be encoded. The OR-view graph has the
related costs shown in Table 1. Each node (=view) in the graph, has associated with it a read cost
(RC), a query frequency (QF) and an update frequency (UF). Each edge of the graph, which denotes
the relationship among the views, is associated with a query cost (QC) and a maintenance cost (MC).
Parameter Description
RC Read Cost of the view, also used to represent the size of the view.
Node QF Query Frequency, represents the number of queries on the view during a
(View) given time interval.
UF Update Frequency, represents the number of updates on the view during a
given time interval.
QC Query Cost, represents the cost for calculating a view from one of its
Edge source views.
MC Maintenance Cost represents the cost for updating a view using one of its
source views.

Table 1: Cost parameters for OR view graphs.
The total query cost of the selected views represented by a genome is calculated by summing
over all calculated minimum cost paths from each selected view to another selected view or a base
table. Each minimum cost path is composed of all of the QC values of the edges on the path and the
RC values of the final selected view or base table. This calculation is implemented by using a depth-
first traversal of the OR view graph.
The total maintenance cost is calculated similarly, but the cost of each minimum cost path is
composed of only the UC values of the edges. The detailed formulae and examples are given in the
appendix. An OR-view graph generator, which can randomly generate OR-views based on the density
and using the parameter ranges given for each parameter of the graph, was also developed for
experimental purpose. In addition, we implemented an exhaustive search algorithm to find the optimal
solution in order to be able to compare the quality of our GA solution to the optimal one for each test
case.

5 Evaluation of the Algorithm
Our genetic algorithm was developed and evaluated using a Pentium II 450 MHz PC running
Windows NT 4.0. We performed two kinds of evaluations. First, the nine strategies for the fitness
functions (see Sec. 3.2.2) were compared in terms of the quality of the generated solutions with
respect to the optimal solutions. Second, we compared the run-time behavior of the genetic algorithm
to the exhaustive search algorithm in order to gain insight into the efficiency of our approach.
The OR-view graphs that were used in the experiments were as follows. The number of base
tables was fixed to 10 tables. The number of views varied from 5 to 20 views. The edge density of the
graph varied from 15% to 30% to 50% to 75%. The ranges for the values of all important parameters
of the OR-view graphs are shown in Table 2. The maintenance cost constraint for the problem was set


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to 50, 100, 300, and 500. Please note that one may interpret these values as time limits on how long
the warehouse may be down for maintenance or amount of data that must be read etc.
Nodes (Views) Edges
RC QF UF QC MC
100-10,000 for base tables (RC for views are 0.1-0.9 0.1-0.9 10 80 % of RC 10 150%
calculated from source views) of source view of QC

Table 2: Range of parameter values for the simulated OR-view graphs

5.1. Quality of Solutions
Initially, we used all nine different fitness functions to conduct the experiments. The quality of
the solutions was measured as a ratio of the optimal total query cost (obtained using the exhaustive
search) over the total computed query cost (obtained using the genetic algorithm). The ratio was
computed and averaged over several runs. It was initially expected that the ratio would always be less
than 100%. However, we observed that the genetic ulg" ilio sometimes relaxes the maintenance cost
constraint in order to trade off maintenance cost with a lower and better overall query cost. In those
cases, the total query cost obtained was actually lower than the optimal query cost which was
computed with a strict maintenance constraint value (i.e., the ratio exceeded 100%). This was very
interesting in the sense that although a maintenance cost constraint may be given, it may be interpreted
as a guideline (within certain limits) rather than as a strict value. Actually, in [7] the inverted-tree
greedy heuristic also does not guarantee a strict maintenance cost constraint, but satisfies a limit within
twice the constraint value. The nine different strategies are denoted LG-S, LG-D, LG-SD, LN-S, LN-
D, LN-SD, EX-S, EX-D, EX-SD, where LG, LN, S, etc. denote the different penalties and functions as
described in Sec. 3.2.2.
After an initial experiment, the logarithmic penalty functions (LG-S, LG-D, LG-SD) did not
perform well, especially LG-S and LG-SD. The reason was that the logarithmic penalty function
makes the penalty value too small to enforce a strong enough penalty on the fitness value. Thus, for
LG-S and LG-SD, it always tried to maximize the query benefit while ignoring the maintenance cost
constraint by yielding a solution that materializes all of the views. LG-D and several others such as
LN-S, EX-S did not result in such extreme solutions but tended to fluctuate wildly over the
maintenance cost limit, sometimes exceeding it by as much as 10,000%! Therefore, we disregard these
strategies in our figures and only show the results from the remaining strategies, namely LN-D, LN-
SD, EX-D, EX-SD as depicted in Figures 2 and 3 (please note, all remaining figures are collected at
the end of this paper). Figure 2 shows the results of averaging over the ratios of optimal total query
cost (based on a strict maintenance constraint) over GA total query costs. Figure 3 shows the results of
averaging over the ratios of GA total maintenance cost over the maintenance constraint. The values are
arranged in tuples in lexicographical order:
(density, number of views, maintenance constraint).
The density changes occur at the points 1, 65, 129 and 193 on the x-axis, each increasing the
densities. The numbers of views are in increasing order within a given density. The maintenance cost
is also in increasing order within each set of views.
The results in Figure 3 show that the LN-D and LN-SD still have a considerably large
fluctuation (about 380%) for the maintenance cost. These behaviors were exhibited especially for low
density OR-view graphs where the penalty values resulted in small values which were not enough to
enforce the maintenance constraint. If we discard these two mechanisms from our consideration,
Figure 2 shows that the remaining EX-D and EX-SD strategies obtain a total query cost ratio that is
guaranteed to always be over 90% which is very close to the optimal solution. Furthermore, the
maintenance cost is always within two times the value of the maintenance cost. Thus, EX-D and EX-


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SD represent good fitness functions for our genetic algorithm. Note that this result is also very close to
the one that was verified in theory in the inverted-tree greedy heuristics proposed by [7].
Figures 4 and 5 show the comparison in the execution time for our genetic algorithm and the
exhaustive search averaged over the sample OR view graphs. The exhaustive search algorithm was
developed to obtain the optimal solutions for comparing the qualities of the solutions, and by using
this algorithm we have actual proof that it is extremely time consuming to obtain an optimal solution
when the number of views exceeds 20 views. As a result, we limited our experiments to only 20
views. Although better heuristics exist (which still have polynomial time complexity), this particular
experiment is intended to provide a feel for the performance. From the figures we can see that the
execution time for the exhaustive algorithm, increases exponentially within each given density as it
goes up to 20 views. Our genetic algorithm on the other hand exhibits linear behavior. As the density
grows, the slope of the linear graph increases only slightly. The genetic algorithm took approximately
a 1/80th of the time that an exhaustive search took for an OR-view graph with density of 75% and 20
views. As the number of views goes up to 30 and beyond, this ratio is expected to be much more
impressive. And yet the quality of the solution generated by the genetic algorithm is still very close to
the optimal.

6 Conclusion

6.1 Summary
In this paper we have shown that our genetic algorithm is superior to existing solutions to the
view selection problem in the context of OR view graphs. Specifically, our genetic algorithm
consistently yields a solution that comes within 10% of the optimal solution (which we verified by
running an exhaustive search on OR view graphs with up to 20 views) while at the same time
exhibiting a linear run-time behavior. A penalty function has been included in the fitness function, and
experimental results show that the EX-D and EX-SD penalty functions produce the best results for the
maintenance cost view selection problem. We believe that this algorithm can become an invaluable
tool for warehouse evolution, especially for those data warehouses with a large number of views and
frequent changes to the queries which are supported by the given warehouse configuration.

6.2 Future Improvements
In the future, we are considering the following improvements:
Generate an initial population based on knowledge of a possible solution rather than using
random configurations.
Experiment with several other crossover or mutation operators to speed up convergence even
further.
Implement a more flexible termination condition that can interrupt the algorithm when the
solution lies with a certain threshold instead of always computing all generations.
Expanding our approach to include AND-OR view graphs as well as indexes. The first is
straightforward. The latter is more complicated as we have to modify the solution
representation. One possible approach may be as follows: Add the indexes related to a view
immediately after the bit position of the view. As an example, assuming view vl has indexes
ii, i2 and i3, and view V2 has indexes i4 and i5, the positions of these views and related indexes
could be fixed in the order of ( vii it i2 3 2 4 i5 ). But crossover operations and mutation
operations may need to be carefully redesigned. Because, only when a view is selected for
materialization can the associated indexes be selected. One possible way to modify the
crossover is to adjust the crossover point to be always before a view position in the genome.
For example, if the initial crossover point is k in the genomes and the position k is an index
position related to view v, ( where the bit position of v, is j, and j crossover point to be j-1 instead of k. This should allow the related view and index


Lee and Hammer






SPEEDING UP WAREHOUSE PHYSICAL DESIGN USING A RANDOMIZED ALGORITHM


information to not be separated and thus consistently maintained. The mutation may also be
changed to only flip the bit representing an index from 0 to 1 only when the view to which the
index is related is selected. And also a bit representing a view may only be flipped from 1 to 0
when it is assured that all of the indexes related to the view are not selected.
Lastly, genetic algorithms are well suited for exploiting parallelism. For further improvement
in the performance of the devised algorithm, a parallel version may be devised.

References

[1] A. V. Aho, J. E. Hopcroft, and J. D. Ullman, Data Structures and lg 1 ,,,li,,,. Addison-
Wesley Publishing Company, Reading, MA, 1983.
[2] S. A. Cook, "The Complexity of Theorem Proving Procedure," Annual ACM SIGACT
Symposium on Theory of Computing, 3151--158, 1971.
[3] M. R. Garey and D. S. Johnson, Computers and Intractability ---A Guide to the Theory of
NP-Completeness, San Francisco, 1979.
[4] J. Gray, S. Chaudhuri, A. Bosworth, A. Layman, D. Reichart, M. Venkatrao, F. Pellow, and H.
Pirahesh, "Data Cube: A Relational Aggregation Operator Generalizing Group-By, Cross-Tab,
and Sub-Totals," Data Mining and Knowledge Discovery, 1:1, pp. 29-53, 1997.
[5] A. Gupta and I. S. Mumick, "Maintenance of Materialized Views: Problems, Techniques, and
Applications," Data Engineering Bulletin, Special Issue on Materialized Views and Data
Warehousing, 18:2, pp. 3-18, 1995.
[6] H. Gupta, "Selection of Views to Materialize in a Data Warehouse," in Proceedings of the
International Conference on Database Theory, Athens, Greece, 1997.
[7] H. Gupta and I. Mumick, "Selection of Views to Materialize Under a Maintenance Cost
Constraint," Stanford University, Technical Report.
[8] V. Harinarayan, A. Rajaraman, and J. D. Ullman, "Implementing data cubes efficiently,"
SIGMOD Record (ACM Special Interest Group on Management ofData), 25:2, pp. 205--216,
1996.
[9] W. H. Inmon and C. Kelley, Rdb/VMS: Developing the Data Warehouse, QED Publishing
Group, Boston, London, Toronto, 1993.
[10] W. Labio, D. Quass, and B. Adelberg, "Physical Database Design for Data Warehouses," in
Proceedings of the International Conference on Database Engineering, Birmingham,
England, pp. 277-288, 1997.
[11] Z. Michalewicz, Genetic A lg 1, ,,ii, + Data Structures = Evolution Programs, Sringer-
Verlag, New York, New York, NY, 1994.
[12] MIT Technology Lab, "GAlib: A C++ Library of Genetic Algorithm Components", URL,
http://lancet.mit.edu/ga/.
[13] K. A. Ross, D. Srivastava, and S. Sudarshan, "Materialized view maintenance and integrity
constraint checking: Trading space for time," SIGMOD Record (ACM Special Interest Group
on Management ofData), 25:2, pp. 447-458, 1996.
[14] N. Roussopoulos, "View Indexing inrelational Databases," ACM Transactions on Database
Systems, 7:2, pp. 258-290, 1982.
[15] N. Roussopoulos, "Materialized Views and Data Warehouses," in Proceedings of the KRDB,
1997.
[16] D. Theodoratos and T. K. Sellis, "Data Warehouse Configuration," in Proceedings of the
Twenty-third International Conference on Very Large Databases, Athens, Greece, pp. 126-
135, 1997.


Lee and Hammer







SPEEDING UP WAREHOUSE PHYSICAL DESIGN USING A RANDOMIZED ALGORITHM


[17] J. Widom, "Research Problems in Data Warehousing," in Proceedings of the Fourth
International Conference on Information and Knowledge Management, Baltimore, Maryland,
pp. 25-30, 1995.
[18] Y. Zhuge, H. Garcia-Molina, J. Hammer, and J. Widom, "View Maintenance in a
Warehousing Environment," SIGMOD Record (ACM Special Interest Group on Management
ofData), 24:2, pp. 316-27, 1995.


Lee and Hammer







SPEEDING UP WAREHOUSE PHYSICAL DESIGN USING A RANDOMIZED ALGORITHM



Collection of Figures



Figure 2: Average ratios of (optimal total query cost/GA total query
cost)


O



o
0

SE
rrC
5&


LN-D
LN-SD
-*- EX-D
- EX-SD


M- o LO N MI O (CO 0 T V- Mo LO cN OM CD
x- Mo LO CD o0 0 (cN Mo Lo I 0 C N Mo Lo
'- '- '- '- x x- (-N (N (N (N
(density,# of views, maint. constraint)















Figure 3: Average ratios of (GA total maintenance cost/maintenance
constraint)


LN-D
LN-SD
-- EX-D
-- EX-SD


Lee and Hammer


'- o0 LO NI 0) CD (M 0 M D I oM LO NI 0) CD
M- o LO CD o0 0 (c N M Lo I 0 C (N Mo Lo
(dn i- #- o- i- ,- i o n N t
(density,# of views, maint.constraint)







SPEEDING UP WAREHOUSE PHYSICAL DESIGN USING A RANDOMIZED ALGORITHM


Figure 4: Execution time for exhaustive search algorithm


(density, # of views, main. constraint)
(density, # of views, maint. constraint)


--- Exhaustive Search


Figure 5: Execution time for genetic algorithm


1.6
1.4
1.2
S1
0.8
E 0.6 -
0.4
0.2
0,
I- M e M iy LO I- M iew LanO sr i LO t
M- CO CDO O) M M- (N C.O IM (D O ( N

(density, # of views, maint. constraint)


LN-D
LN-SD
-- EX-D
--EX-SD


Lee and Hammer






SPEEDING UP WAREHOUSE PHYSICAL DESIGN USING A RANDOMIZED ALGORITHM


Appendix A. Cost Formulas

A. 1. Cost Formula for Query Benefit Function B(G, M)

Assume that G is a given OR-view graph, and M is the set of selected views to be materialized.
The formula is :

B(G, M) = (G, 0 )-z (G, M)

Note that this is called absolute benefit of M in [7]. Our notation of B(G,M) is equivalent to
Gupta's notation of B( M, D ) There are other formulas needed to compute B(G,M), such as :

T (G, M) : total query cost for AND-OR view graph in the presence of a set M of materialized
views. This is the quantity that we want to minimize in the view selection problem.

z (G, M) = I f* Q (v, M)
ve V(G)

where fv is the query frequency
V(G) denotes all views (materialized and not materialized)


Q ( v, M) : cost of answering query v in the presence of a set M of materialized views
This is calculated as the minimum query-l.. gill of a path from v to some u e (M U L). .
where L is the set of sinks (base tables) in G.
And the query-length is calculated as (See Figure 6 below) :
query-li. ,ig ofpath from v to u =
Ru (reading cost of u) + sum of query-costs associated with edges on the path
Assuming base tables are always available, Q(v,o) is the cost of answering q directly from base
tables.
v
Scl c5 : query-cost
cl m 4, m4 ml m5 : maintenance cost
0 Ru, Ry : reading cost
c2 m2 c5, m5
0 0
S y (materialized) Ry
c3, m3

(materialized) u Ru

Q (v, M) = Ru + c3 + c2 + cl (ifRu+c3+c2+cl < Ry+c5+c4)
UC(v, M) = m4 + m5 (if m4+m5 < ml+m2+m3)

Figure 6: Q(v,M) and UC(v,M) for OR view graph


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SPEEDING UP WAREHOUSE PHYSICAL DESIGN USING A RANDOMIZED ALGORITHM

A. 2. Cost Formula for Total Maintenance Cost Function U(M)

Assume that M is a set of views selected for materialization.

U(M) = gv *UC(v,M)
ve M

where gv is the update frequency, M denotes only materialized views



*UC (v, M) : cost of maintaining a materialized view v in presence of a set M of materialized
views.

This is calculated as the minimum maintenance-lk nll (sum of maintenance costs associated
with edges) of a path from v to some y E (M U L)-{v}. See Figure 6 above.


A. 3. Formula to Calculate p as Used in the Penalty Function

p = Max ( B(G, {vi}) / U({vil) )
vi e V(G)


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