Group Title: Department of Computer and Information Science and Engineering Technical Reports
Title: A Formal approach to the vertical partitioning problem in distributed database design
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Title: A Formal approach to the vertical partitioning problem in distributed database design
Alternate Title: Department of Computer and Information Science and Engineering Technical Report
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Language: English
Creator: Muthuraj, J.
Chakravarthy, Sharma
Varadarajan, R.
Navathe, S. B.
Affiliation: University of Florida
University of Florida
University of Florida
Georgia Institute of Technology
Publisher: Department of Computer and Information Sciences, University of Florida
Place of Publication: Gainesville, Fla.
Copyright Date: 1992
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A Formal Approach to the Vertical Partitioning Problem
in Distributed Database Design


J. Muthuraj1
S. Chakravarthy2
R. Varadarajan

Computer and Information Sciences Department
University of Florida, Gainesville, FL 32611, USA
Email: sharma@snapper.cis.ufl.edu

S. B. Navathe1

College of Computing
Georgia Institute of Technology
Atlanta, GA .;i.:.:;-


Abstract

The design of distributed databases is an optimization problem requiring solutions to
several interrelated problems: data fragmentation, allocation, and local optimization. Each
problem can be solved with several different approaches thereby making the distributed
database design a very difficult task.
Although there is a large body of work on the design of data fragmentation, most of
them are either ad hoc solutions or formal solutions for special cases (e. g., binary vertical
partitioning). In this paper, we address the problem of n-ary vertical partitioning problem
and derive an objective function that generalizes and subsumes earlier work. The objective
function derived in this paper is being used for developing heuristic algorithms that can
be shown to satisfy the objective function. The objective function is also being used for
comparing previously proposed algorithms for vertical partitioning. We first derive an
objective function that is suited to distributed transaction processing and then show how
it can be extended to include additional information, such as transaction types, different
local and remote accessing costs and replication. Finally, we indicate the current status of
implementation.


1 Introduction




The design of distributed databases is an optimization problem requiring solutions to several
interrelated problems: data fragmentation, allocation, and local optimization. Each problem
phase can be solved with several different approaches thereby making the distributed database
design a very difficult task. Traditionally database design has been heuristic in nature. Al-
though the metric being optimized is not stated quantitatively, it is implicitly assumed to
be the processing cost for a given set of important transactions that constitute the bulk of
transaction load for the given database.

1This work is supported by the National Science Foundation Research Initiation Grant IRI-8716798
This work is supported by the National Science Foundation Research Initiation Grant IRI-9011216










































SDistribution of the enterprise schema into local logical schemas
PHYSICAL DATABASE DESIGN FOR EACH LOCAL DATABASE
SImplementation Schema
OPERATIONAL DATABASE


Figure 1: Distributed Database Design Methodology


Figure 1 gives an outline of the overall distributed database design methodology [2]. Dis-
tributed database design deviates from conventional non-distributed database design only in
the distribution aspect which is highlighted by the box titled distribution design in figure 1.
The distribution design involves data acquisition, partitioning of the database, allocation and
replication of the partitions and local optimization. Partitioning of the database is done in
several ways: vertical, horizontal, and hybrid (also called mixed). Our long-term objective
is to develop a distributed database design testbed in which different algorithms for various
components of distribution design can be mixed and matched. This work is a first step in that
direction and addresses the partitioning (or fragmentation) problem.

In this paper, we delimit our discussion to one of the data fragmentation problems, namely
the vertical partitioning problem. More information on this vertical partitioning problem, can
be found in Muthuraj's thesis [14]. Vertical Partitioning (also called attribute partitioning)
is a technique that is used during the design of a database to improve the performance of










transactions [15]. In vertical partitioning, attributes of a relation R3 are clustered into non-
overlapping4 groups and the relation R is projected into fragment relations according to these
attribute groups. In distributed database systems, these fragments are allocated among the
different sites. Thus the objective of vertical partitioning is to create vertical fragments of
a relation so as to minimize the cost of accessing data items during transaction processing.
If the fragments closely match the requirements of the set of transactions provided, then
the transaction processing cost could be minimized. Vertical partitioning also has its use in
partitioning individual files in centralized databases, and dividing data among different levels
of memory hierarchies etc. [15, 18]. In the case of distributed database design, transaction
processing cost is minimized by increasing the local processing of transactions (at a site) as
well as by reducing the amount of accesses to data items that are not local. The aim of vertical
partitioning technique (and in general data partitioning techniques) is to find a partitioning
scheme which would satisfy the above objective.


It should be noted that the problem of partitioning can be addressed at various levels of detail
by taking additional information into consideration. Figure 1 clearly distinguishes various
levels and a feedback path is provided to refine the outcome of the earlier levels if it does not
suit the objectives of the next level. In this paper, we are taking only transaction information
as input to keep the problem manageable. In essence, the global optimization problem (which
includes a large number of parameters and a very complex metric) is partitioned into several
smaller optimization problems to reduce the search space and the complexity of each problem.
Other detailed information need to be considered on the outcome of this stage in order to
obtain a design at the physical level.


Several vertical partitioning algorithms have been proposed in the literature. Hoffer and
Severance [7] measure the affinity between pairs of attributes and try to cluster attributes
according to their pairwise affinity by using the bond r. ,,, ././,"i, i, (BEA) [12]. Hammer
and Niamir [6] use a file design cost estimator and a heuristic to arrive at a "bottom up"
partitioning scheme. Navathe, et al [15] extend the BEA approach and propose a two phase
approach for vertical partitioning. Cornell and Yu [4] apply the work of Navathe [15] to
physical design of relational databases. Ceri, Pernici and Wiederhold [3] extend the work of
Navathe [15] by considering it as a 'divide' tool and by adding a 'conquer' tool. Navathe
and Ra [16] construct a graph-based algorithm to the vertical partitioning problem where the
heuristics used includes an intuitive objective function which is not explicitly quantified. In
addition to these vertical partitioning algorithms, there are many data clustering techniques
[9], traditionally used in pattern recognition and statistics, some of which can be adapted to
partitioning of a database. These data clustering algorithms include Square-error clustering
[9], Zahn's clustering [22], Nearest-neighbor clustering [11] and Fuzzy [9]clustering.


The partitioning algorithms mentioned above use some heuristics to create fragments of a
relation. The input to most of these algorithms is an Attribute Usage Matrix (A UM). AUM is
3For most of our discussion it does not matter whether R is a Universal relation or a relation that is in some
normal form as long as key attributes are identified. In the section on extensions, we discuss how we can take
that information into account.
4Overlapping partitions (of non-primary key attributes) may also be considered when availability is an
important criterion. This is discussed in the section on extensions.










a matrix which has attributes as columns, and transactions as rows and the access frequency
of the transactions as values in the matrix. Most of the earlier data fragmentation algorithms
use an Attribute Affinity Matrix (AAM) derived from the AUM provided as input. An AAM
is a matrix in which for each pair of attributes, the sum total of frequencies of transactions
accessing that pair of attributes together is stored. The results of the different algorithms are
sometimes different even for the same attribute affinity matrix indicating that the objective
functions used by these algorithms are different. Most of the proposed vertical partitioning
algorithms do not have an objective function to evaluate the ..... I ,. of partitions that they
produce. Also, there is no common criterion or objective function to compare and evaluate the
results of these vertical partitioning algorithms.


1.1 Contributions

This paper makes several contributions to the problem of data fragmentation in general and
the design of vertical partitioning in particular [14]. Specifically:


1. We have, perhaps for the first time, studied the applicability of some data clustering
algorithms for distributed database design5 proposed in areas such as pattern classifica-
tion, statistics etc., [9], [22], [11], to data fragmentation problem. In fact, we start from
one such objective function proposed for data clustering and modify and extend it to the
specific problem at hand.

2. We have formulated an objective function for n-ary partitions, with two components that
provide the desirable behavior for minimizing transaction processing cost.

3. Finally, we are using the approach of formulating an objective function (termed Parti-
tion Evaluator in this paper) before developing (heuristic) algorithms for the partitioning
problem. This approach enables us to study the properties of algorithms with respect to
an agreed upon objective function, and also to compare different algorithms for g..... -
ness" using the same criteria. The objective function formulated in this paper is a step
in this direction. Moreover, the objective function derived in this paper can be easily
extended to include additional information (e. g., query types -retrieval/update, al-
location information about the partitions, remote processing cost, and the transaction
usage pattern at any particular site). Some of these extensions are discussed at end of
the paper.


Our long-term objective is to either extend this objective function or to develop new objective
functions to take into account additional information pertaining to replication, storage, and
transmission costs that are critical to a distributed environment. However, we view this work
as filling a void that currently exists even at the conceptual level.


The organization of the paper is as follows. Section 2 discusses previous related work on data
clustering. Section 3 describes prior work in the area of vertical partitioning and summarizes
the need for the development of our Partition Evaluator. In section 4, we derive the Partition

'Schkolnik [18] uses data clustering techniques for partitioning a hierarchical structure for an IMS database
using detailed cost information which is different from the problem addressed in this paper.










Evaluator (PE) appropriate for distributed database design at the conceptual level. In section
5, we illustrate the use of our Partition Evaluator with an example. We show the actual
behavior of the PE and compare it with the expected behavior. Section 6 describes how the PE
is amenable to extensions with illustrations and includes the current status of implementation.
We show the actual behavior of the PE and compare it with the expected behavior. Section
includes summary and future work.


2 Previous Work

In this section we briefly summarize the work done in the area of data clustering and the work
done in data fragmentation and bring out the similarities and differences between them.

A number of data clustering algorithms have been developed in application areas such as
statistics and pattern classification and analysis which address the problem of grouping or
clustering data using various criteria. The most commonly used partitioning clustering strategy
is based on the square-error criterion [9]. The general objective is to obtain that partition
which, for a fixed number of clusters, minimizes the square-error. Minimizing square-error, or
within-cluster variation, has been shown to be equivalent to maximizing the between-cluster
variation. Clusters can also be viewed as regions of the attribute pattern space in which the
patterns are dense, separated by regions of low attribute pattern density. In the mode-seeking
partitioning algorithm due to Torn [21], clusters are identified by searching for regions of high
density, called modes, in the pattern space. Each mode is associated with a cluster center and
each pattern is assigned to the cluster with the closest center.

Zahn [22] has demonstrated how the minimum spanning tree (:.IST) can be used to detect
clusters. His choice of MST was influenced by the Gestalt principle, which favors the grouping
of attribute patterns based on Euclidean distance measure. Shaffer et al [19] demonstrate the
similarity of the mode-seeking partitioning algorithm [10] to the graph algorithm of Zahn [22]
based on minimum spanning trees. Lu and Fu [11] used another graph-based approach called
"Nearest-Neighbor clustering algorithm" to cluster patterns during character recognition [9].

The concept of using fragmentation of data as a means of improving the performance of a
database management system has often appeared in the literature on file design and optimiza-
tion. Hoffer [8] developed a non-linear, zero-one program which minimizes a linear combination
of storage, retrieval and update costs, with capacity constraints for each file. Babad [1] formu-
lated a less restrictive vertical partitioning problem for variable length attributes as a non-linear
zero-one program. In the work of Eisner and Severance [5], a file can be partitioned into two
subfiles: a primary and secondary subfile. Two forms of cost function are used in this approach.
The first function is the sum of storage charges for subtuples in the primary subfile, and the
cost of accessing all the subtuples residing in the secondary subfile. The second function is
nonlinear, and measures the total costs of access, transfer, and storage for subtuples in both
primary and secondary subfiles. The limitation of this approach is that at most two subfiles
are allowed [17]. March and Severance [13] extended this model to incorporate block factors
for both primary and secondary memories. Hoffer and Severance [7] grouped the attributes
of a relation based on the extent to which they were used together (measured the .illiiily
between pairs of attributes"). This clustering of attributes based on their pairwise affinity was
done using the bond energy algorithm (BEA). The BEA produced matrix in which an cost
function was minimized for the entire matrix using the affinity attribute matrix. They left the










creation of partitions to the subjective evaluation of the designer. Schklonik [18] has examined
the problem of partitioning a hierarchical structure (for a hierarchical database) in order to
minimize the access time to it for a given access pattern. Segment sizes and scan information
is used to minimize the page faults.

Hammer and Niamir [6] developed two heuristics, grouping and regrouping, and used them to
perform the partitioning. The grouping heuristic starts by initially assigning each attribute to
a different partition. On each iteration, all possible grouping of these partitions is considered
and the one with maximum improvement is chosen as the candidate grouping for the next
iteration. During regrouping, attributes are moved between partitions to achieve any additional
improvements possible. Navathe et al [15] use a two step approach for vertical partitioning. In
the first step, they use the given input parameters in the form of an attribute usage matrix to
construct the attribute affinity matrix on which clustering is performed. After clustering, an
empirical objective function is used to perform iterative binary partitioning. In the second step,
estimated cost factors reflecting the physical environment of fragment storage are considered
for further refinement of the partitioning scheme. Cornell and Yu [4] propose an algorithm,
as an extension of Navathe et al [15] approach, which decreases the number of disk accesses
to obtain an optimal binary partitioning. This algorithm uses specific physical factors such
as number of attributes, their length and selectivity, cardinality of the relation etc. Navathe
and Ra [Nava 89] present a graph-based approach to the vertical partitioning problem. This
approach is based on the observation that all pairs of attributes in a fragment must have high
S11 lii, fragment affinity" but low "between fragment affinity". Reduction in complexity is
claimed as the main advantage of their approach.

There are important differences in the criteria that are used in traditional clustering problems
and data fragmentation problem. In data clustering algorithms, the number of clusters is
usually fixed. Otherwise, the extreme case of only a single cluster in the partition will minimize
the inter-cluster variation. However in the database design application, there is a need to
determine the number of clusters as well and hence the objective function used in data clustering
algorithms cannot be borrowed without any changes to vertical partitioning in databases. Most
importantly, in data base design problem, the number of clusters is an important factor that
influences the trade-off between local and remote transaction processing costs.

Our interest in the above was to see whether the criteria used in the data clustering domain
could be adapted, with some changes, to the data fragmentation problem.


3 Need for an objective function


Algorithms such as Bond Energy, Binary Vertical Partitioning, Ra's algorithm and Zahn's
algorithm etc. use affinity matrix as the input. The attribute affinity is a measure of an
imaginary bond between a pair of attributes. Because only a pair of attributes is involved, this
measure does not reflect the closeness or affinity when more than two attributes are involved.
Hence the algorithms which use attribute affinity matrix are using a measure (that is an ad
hoc extrapolation of pairwise affinity to cluster affinity) that has no bearing on the affinity
as measured with respect to the entire cluster. As a consequence, we believe, it was difficult
to show or even characterize affinity values for the resulting clusters having more than two
attributes.










As we wanted to obtain a general objective function and a criterion for describing affinity
value for clusters of different sizes, our approach does not assume an attribute affinity matrix.
The input model that we consider is a matrix which consists of attributes (columns) and the
transactions (rows) with the frequency of access to the attributes for each transaction, as the
values in the matrix. With this input model we overcome the limitations that are inherent to
approaches based on attribute affinity matrix.

As is evident from the discussion in the previous section, there are a number of partitioning
algorithms available both in the database design area and in other application areas. Many of
these algorithms use different criteria to arrive at a partitioning scheme. The objective function
used by one algorithm is not suitable for evaluating the ..... I, -" of other algorithms. Thus
we do not have a common objective function to compare and evaluate the results of these
partitioning algorithms, or in general evaluate the .....II. -" of a particular partitioning
scheme. Hence we need a partition Evaluator to compare and evaluate different algorithms,
that use the same input in the database design process. Since attribute usage matrix is the
most commonly used input available during the initial design stage, we first design an Evaluator
which can be used to evaluate the g ....II. -" of partitions arrived at using this input. This
Partition Evaluator can be used as a basis for developing algorithms to create fragments of
a relation. With this approach, there is hope that admissibility aspects of algorithms can be
shown. In addition, this Partition Evaluator has the flexibility to incorporate other information,
such as type of queries (retrieval/updates), allocation information about the partitions, remote
processing cost (transmission cost) and the transaction usage pattern at any particular site.
In the next section we will discuss the development of the Partition Evaluator in detail.


4 Development of the Partition Evaluator


In any practical database application, a transaction does not usually require all the attributes
of the tuples of a relation being retrieved during the processing of the transaction. When a
relation is vertically divided into data fragments, the attributes stored in a data fragment that
are irrelevant (i.e., not accessed by the transaction) with respect to a transaction, add to the
retrieval and processing cost, especially when the number of tuples involved in the relation is
very large. In a centralized database system with memory hierarchy, this will lead to too many
accesses to the secondary storage. In a distributed database management system, when the
relevant attributes (i.e., attributes accessed by a transaction) are in different data fragments
and allocated to different sites, there is an additional cost due to remote access of data. Thus
one of the desirable characteristics of a distributed database management systems that we wish
to achieve through partitioning is the local accessibility at any site. In other words, each site
must be able to process the transactions locally with minimal access to data located at remote
sites.

Ideally, we would like any transaction to access only the attributes in a single data fragment
with no or minimal access of irrelevant attributes in that fragment. But this is impossible
to achieve in the general cased since transactions access different and overlapping subsets of
attributes of a relation. Moreover, transactions are run at different sites and hence some of
the data fragments that contain relevant attributes of a transaction may reside in remote
sites. The overall transaction processing cost in a distributed environment thus consists of
local transaction processing cost and the remote transaction processing cost. Though it is










possible to replicate the data to avoid remote processing cost, for the first step we assume
no data redundancy to avoid modeling overhead to ensure data integrity and consistency
and also additional storage costs. In this paper, we assume that during the database design
process, "partitioning" phase is followed by the "allocation" phase during which the non-
overlapping data fragments obtained during the partitioning phase are allocated to different
sites possibly with some replication. Hence the partition evaluator we propose will evaluate
vertical partitioning schemes wherein the data fragments are non-(,.'- ,1',,,','iin the attributes.
Here non-overlapping refers only to non-primary key attributes. The primary key is preserved
in each partition. This is necessary to obtain the original relation without any loss or addition
of spurious tuples. Discussion of our approach to relations that are not in Boyce-Codd normal
form is in a later section.

The goal of partitioning the attributes and allocating to different sites, is to arrive at a minimum
processing cost for any transaction originating at any site. Since the transaction processing
cost has two components, one due to local processing and another due to remote processing,
our Partition Evaluator that measures the .....I,. -" of a vertical partitioning scheme also
has two corresponding component terms; we refer to these terms as i,. I. -.,,i1 local attribute
access (..-i" and I. I. -..,i remote attribute access (..-i" terms respectively. For simplicity, we
assume that a single access of a data fragment corresponds to an unit cost; this assumption
can easily be relaxed if more information is available regarding the access methods, network
parameters etc. The irrelevant local attribute cost term measures the local processing cost of
transactions that is due to irrelevant attributes of data fragments assuming that all the data
fragments required by a transaction are available locally. The relevant remote attribute access
term measures the remote processing cost due to relevant attributes of data fragments that
are accessed remotely by transactions; note that the contribution to remote access cost due
to irrelevant attributes is already included in the first term. Since we do not know during
partitioning how the data fragments are allocated, we compute the second term assuming
that data fragments needed by a transaction are located at different sites. In the absence of
any information regarding the transaction execution strategies, we can compute the second
term either by determining the average of all the remote access costs obtained by running the
transaction at every site containing a fragment needed by the transaction or by assuming the
transaction to be run at one of the fragment sites. If more information is available regarding
the transaction strategies, we can incorporate it in the second term.


The ideal characteristic of a Partition Evaluator is summarized in Figure 2 where the
behavior of the two components as a function of partition size (number of data fragments) is
illustrated. The first component should be maximum for a partition size of 1 and should be
zero for a partition size equal to number of attributes in the relation. The second component
on the other hand should be zero and maximum respectively for these two extremes. In
between the two extremes, the first component should be more responsive to smaller partition
sizes while the second component should be more responsive to larger partition sizes. This is
necessary to avoid unnecessary bias toward one component or another since we know that in a
distributed database design, it is undesirable to have the two extreme cases of the partition size.
In addition to being responsive to partition sizes, the evaluator function should discriminate
between different distributions of the attributes among the fragments for a fixed partition
size. Later, we will present experimental evidence to demonstrate that our Partition Evaluator
meets these criteria reasonably well.
















II
Irrelevant Relevant
Local Attribute Remote Attribute
Access Cost Access Cost
(I) (II)






1 n
Number of Partitions

Figure 2: Ideal characteristics of PE components





4.1 Derivation of the Partition Evaluator


4.1.1 Database and Transaction Specification

We derive a partition evaluator in this section without making any assumptions on the input.
The input assumed is a relation (consisting of a set of attributes) and an attribute usage
matrix. We really do not have to make any assumptions as to whether the input relation is a
Universal relation or whether it is in a particular normal form. The partitioning obtained is
non-overlapping except for the key attributes in the input relation. The key attributes are part
of each partition. In a later section we indicate how normalization and functional dependency
information can be easily added to our approach.

The input model that we use is an attribute usage matrix [AUM(t,j)] which consists of the
attributes(j) in a relation as columns and the transactions(t) as rows with the frequency of
access to the attributes for each transaction as the values in the matrix. Note that we are
avoiding the construction of any attribute affinity matrix which has its own limitation pointed
out earlier.

A representative attribute usage matrix6 is shown below:

6In this section, no distinction is made between update and retrieval transactions; it is discussed as one of
the extensions in a later section












Trans\Attrs Al A2 A3 A4 A5


T1 0 ql 0 ql ql
T2 q2 q2 q2 0 q2
T3 q3 0 0 q3 q3
T4 0 q4 q4 0 0
T5 q5 q5 q5 0 0


4.1.2 Definitions and Notations

A partition (scheme) is a division of attributes of a relation into vertical fragments in which for
any two fragments, the set of attributes of one is non-overlapping with the set of attributes of
another. For example, the partition {(1, 3)(2,4)(5)} defines a collection of fragments in which
attributes 1 and 3 are in one fragment, 2 and 4 are in another and 5 is in a separate fragment.
The following are used in the derivation of the Partition Evaluator.



n :Total number of attributes in a relation that is being partitioned.
T :Total number of transactions that are under consideration.
qt : frequency of transaction t for t = 1, 2,..., T.
M Total number of fragments of a partition.
n : Number of attributes in fragment i.
nkt : Total number of attributes that are in fragment k
accessed remotely with respect to fragment i by transaction t.
f : frequency of transaction t accessing attribute j in fragment i
note that ft is either 0 or qt
Aij : Attribute Vector for attribute j in fragment i.
t-th component of this vector is f;
Sit : Set of attributes contained in fragment i that the transaction t
accesses; It is empty if t does not need fragment i.
Sai : number of attributes in fragment i that the transaction t accesses.
Ritk : Set of relevant attributes in fragment k accessed remotely
with respect to fragment i by transaction t;
these are attributes not in fragment i but needed by t
Ritk\ : number of relevant attributes in fragment k accessed remotely
with respect to fragment i by transaction t



The attribute vector All (assuming A1 is in partition 1) for the example given above is as
follows.












0
q2
All q3
0
q5

Consider the partitioning scheme (1,3),(2,4),(5), for the attribute usage matrix given above.
Fragment 1 is (1,3), fragment 2 is (2,4) and fragment 3 is (5). Assume that transaction T2
(i.e., t = 2) is run at the site where fragment 1 is located. Then Sit is (1,3) and ISat is 2. n'2
is 2 and n3t is 1. IRit2 is 1 and IRit3 is 1.

Next we explain how each of the two components of our Partition Evaluator is derived.


4.1.3 Irrelevant local attribute access cost

For the first component, we use the square-error criterion as given in [Jain "] for data cluster-
ing. The objective here is to obtain a partition which will minimize the square-error for a fixed
number of fragments. This criterion assigns a penalty factor whenever irrelevant attributes are
accessed in a particular fragment.


Let us assume that n attributes have been partitioned into M fragments (Pi, P2 ......PM)
with ni attributes in each fragment. Thus Mt1 ni = n. The mean vector Vi for fragment i is
defined as follows.


= ASj 0 < j=1



This mean vector represents an average access pattern of the transactions over all attributes of
fragment i. For an attribute vector Aij, (Aj Vi) is called the "difference vector" for attribute
j in fragment i. The square-error for the fragment Pi is the sum of the squares of the lengths
of the difference vectors of all the attributes in fragment i. It is given by


n,
= (Aj V)r(Aj Vi) 0 < i < M (2)
j=1



If Aij = Vi then e2 will be zero. This will occur for the trivial case when there is a single
attribute in each fragment or for the case when all the attribute in each fragment are relevant
to all the transactions that access that fragment. It is the latter case that we are interested in
and to avoid the former case, we will use the second component.

The square-error for the entire partition scheme containing M fragments is given by









M
EM={ e2 (3)
i=l



Smaller the value of E smaller is the cost due to access of irrelevant attributes. E however
does not reflect the cost that might be incurred by accessing attributes remotely when the
fragments may be in different sites. Hence, for distributed database applications, we cannot
evaluate partitions on the basis of E 2 alone. The behavior of E2 is given in figure 3 as curve
I for a data set consisting of 10 attributes and eight transactions (more details of this example
are discussed in the next section).

From this graph, we can see that the minimum value for E^ is certainly achieved for n
partitions in an n attribute system (The minimum value may be reached even for less than
n partitions depending on the AUM). We would like to have a number of fragments which
is typically much less than n and still having the least E^ value. In some data clustering
techniques, the number of data clusters is minimized using an index called Davies-Bouldwin
(DB) index [9] which is a measure of the spread between centers of the clusters. For a typical
data set with small standard deviation (less than 0.1), this index reaches a global minimum
(highest spread) for a partition size that falls between the extremes. From the curve I of figure
3, we can see that the standard deviation of the data set given by the attribute usage matrix
is greater than 0.1, which does not meet the condition for using the DB index. For this reason,
we seek another quantity which will reflect the remote access cost. In the next section, we will
discuss the development of a quantity for remote attribute access cost.


4.1.4 Relevant Remote Attribute Access Cost

Now we will include the second component which would compute a penalty factor that com-
putes the function shown earlier in figure 2. Given a set of partitions, for each transaction
running on a partition compute the ratio of the number of remote attributes to be accessed to
the total number of attributes in each of the remote partitions. This is summed over all the
partitions and over all transactions giving the following equation. The second term is given by

E2 T [ I Rt itk
ER= IAlt Y2 litk (4)
t=1 k#i nik L


Here A7 is an operator that is either an average, minimum or maximum over all i. These
different choices of the operator give rise to average, optimistic and pessimistic estimates of
the remote access cost. If specific information is available regarding transaction execution
strategies, then we can determine for each transaction t, the remote fragments accessed by
the transaction and the remote access cost can be refined accordingly. In our experimental
investigation, we use the optimistic estimate for illustration.

We will show in the next section how we obtained the above form for the second term. Our
Partition Evaluator (PE) function is given by

7A is introduced to keep the formula general and applicable to a wide class of problems; also, a desired new
operator can be substituted for A without having to change the objective function.










PE = EM + ER


4.1.5 Compatibility of the Two Components in PE

As the second component has frequency in quadratic form, we need to make sure that both
the components are compatible in terms of the units they produce. Specifically, we need to see
whether the frequency appears in the same way in the first component. In order to do that,
it is instructive to look closely at the first term. In particular, we will rewrite Ek differently
so as to identify the contributions due to each transaction to the square error term of each
fragment in the partition. Thus


M n,
i=1 j=Y(
i=ij=i


Now the mean Vector Vi for fragment i, can be defined as follows.


\S,t *qt


The attribute vector Aij is,


Now,


fti






ft,.j














E M n, [ I qS l ., f
i=1 j=1


Si1 qt
ni I


L ft
The above formula can be rewritten as follows so as to allow us

components that make up the irrelevant local attribute access term.


IS21*q
ra,
n,

(7)


IStl*qt
n,
to identify the different


Et=EE E q1- (2
i=1 j=l t=l


where,


Therefore,


= 1 if the attribute j is accessed by the transaction t
= 0 if the attribute j is not accessed by the transaction t.
The first term qt2 (1 i12 represents relevant attribute accesses and
the second term represents irrelevant attribute accesses. Even if we
have a 0 in Ai we still have the mean squared quantity qt s 2






L i= t= q 1 nl i ( J (qi 2J
EM=2 ^ s^*^ "-l^t ^* -
=it~ v T~t v Tt /


where


Ei j1 6t =Si l
n' 6jt) = ni


and
-I St


Hence,


MET
E = q~= S 1(
i=1 t=1


Sit 2 2 (n
ni +t


S1 1) |Sn |


(10)


If n S = |St|, then E = 0. This implies that the transaction t accesses all attributes in
fragment i whenever it accesses the fragment i. We can still reduce the above equation as
follows.


Ek2 = [1 q Si + -t 12 2 + q S* (n 1 Si) Si
i t=1 1 i )I


S[qt S)


(11)











20000


16000



12000 Relevant
Remote Attribute
Access Cost
(II)
8000




4000


1 2 3 4 5 6 7 8 9 10
Number of Partitions

Figure 3: Behavior of PE components for an example


Simplifying the equation above we get,


Ei [q=1 ( S
i=1 t=l I


(12)


The equation above is the same as equation as 6, but in a much simpler form. We can clearly
see from this equation the contribution to EL by the irrelevant attributes; (1- ) is the
fraction of irrelevant attributes in fragment i as far as transaction t is concerned. As it was
mentioned earlier, E is the cost factor only for local transaction processing.

Now we can see that the E2 term shown earlier, is very much similar in form to the E2 term.
Also the necessity for having a squared frequency term in E2 is made clear. Hence the PE is
given by


PE = EM + E?


(13)


Curve II of Figure 3 illustrates the behavior of E2 for the same example of 10 attributes and
8 transactions.


16000




Irrelevant 12000
Local Attribute
Access Cost
(I)
8000




4000


20000










5 Analysis of the Partition Evaluator


The final form of the Partition Evaluator is given in equation 13. In order to analyze and
test the behavior of the PE, an exhaustive enumeration algorithm has been implemented. The
input to the program is an attribute usage matrix. The following input was used to test the
Partition Evaluator. For each fragment (from 1 to 10), the Partition Evaluator was computed.

Input: Attribute Usage matrix.


Trans.\Attrs. 1 2 3 4 5 6 7 8 9 10

T1 25 0 0 0 25 0 25 0 0 0
T2 0 50 50 0 0 0 0 50 50 0
T3 0 0 0 25 0 25 0 0 0 25
T4 0 35 0 0 0 0 35 35 0 0
T5 25 25 25 0 25 0 25 25 25 0
T6 25 0 0 0 25 0 0 0 0 0
T7 0 0 25 0 0 0 0 0 25 0
T8 0 0 15 15 0 15 0 0 15 15


An exhaustive enumeration program was written in C/C++ to produce all the possible combi-
nations of attributes with number of fragments varying from 1 to 10. The Partition Evaluator
was applied to each of these combinations. We assume that if a transaction is to be run on a
fragment, and that fragment does not contain even a single attribute accessed by that trans-
action, then that transaction is not run on that fragment. Each of the transactions is run on
each fragment and the minimum, maximum and the average value of the Partition Evaluator
is calculated. Total number of fragments evaluated was 115975. The optimal values (mini-
mum) along with the partitioning scheme for each partition size is given below. The results
are plotted in figure 4.










20000


15000


Partition
Evaluator 10000
Value

5000


0


1 2 3 4 5 6 7 8 9
Number of Partitions

Figure 4: Behavior of partition evaluator for an example


Number Of Fragments Min PE Value


15085

8457

5820

6024

6874

7724

8976

11692

14000


Partition Scheme


(12345678910)

(1456710)(''.:-1)

(157)(_'.:;-,)(4610)

(15)('_.:;-,)(4610)(7)

(15) (.:) (46)(46)(7)(10)

(15)('_..:-i)(4)(6)(7)(10)

(15) (4)(5) (6) (7) (10)
(1)(-_.:-,,)(4)(5)(6)(7)(10)

(1) (2'-) (3)(4)(5)(6)(7)(10)

(1)(28)(3)(4)(5)(6)(7)(9)(10)


10 1.:-., (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)


As we can see from the above table the optimum value (third row above) is found to be in a
partition of three fragments. The Partition Evaluator values above are the minimum of the
values where the second component of the PE is calculated using optimistic (minimum value)
estimate. For this particular example, other algorithms such as Ra's [16], Zahn's [22] and
Binary Vertical Partitioning [15] identify the above mentioned partition set (i.e., 3 fragments)
as optimum. In Zahn's approach, once the maximum spanning tree is obtained two different
conditions can be used to determine the partitions [9]. For this example, when these two










conditions are applied to Zahn's algorithm, they produce two different partitioning schemes.
One of them is the same as the optimal partitioning scheme obtained as above and the other
one is not. Hence, one may want to use another criterion (such as the PE) to guide the selection
of the most appropriate condition when faced with a number of alternatives. Since producing
any partitioning scheme using the Bond Energy algorithm is subjective in nature, this Partition
Evaluator can be used to guide the partitioning once the Bond Energy Algorithm is applied.
Ra's and Binary Vertical Partitioning algorithms were applied to another example with twenty
attributes and fifteen transactions [Nava 84]. The results of these two algorithms were different.
Ra's algorithm produces five fragments and Binary Vertical partitioning algorithm produced
four fragments. The Partition Evaluator was applied to both the results and it was found that
indeed the four fragment result is better than the five fragment result. This example highlights
the usefulness of the Partition Evaluator to evaluate the results of the different partitioning
algorithms and select the appropriate partitioning algorithm.

EXAMPLE: The following discussion explains how one of these figures is arrived at.
Number Of Fragments Min PE Value Partition Scheme


5820


(157)(2.; )(4610)


Let us call (1 5 7) as fragment I, (2 3 8 9) as fragment II and (4 6 10) as fragment III. The
first step is to calculate the square error for the given input.


Mean for each fragment is:
Fragmentl FragmentlI


FragmentlII

0
0
25
0
0
0
0
15


1) The square error


M n,
Ei=1 =
^M=E(Xi


E = 1234 + 2078


+ 0 = 3312


2) Cost due to relevant remote attribute accesses of a transaction (T4) is:


Viy(X1j Vi)










Value Minimum of the values


T4 run @ Fragment I: 352 2 2/4 = 1225

T4 run @ Fragment II: 352 1 1/3 = nIns 1118

T4 run @ Fragment III: cannot be run

Total cost of accessing relevant remote attributes by all transactions is

1118 + 1875 + 225 = _'".11


Therefore,

PE = 3312 + r'.,is = 5820.



The above example shows that utility of the Partition Evaluator developed in this paper
to evaluate the results of any partitioning scheme and especially for comparing previously
developed algorithms.


6 Extensions and Implementation


So far, we have discussed the formulation of an objective function (Partition Evaluator) as-
suming that attribute usage matrix is the only information available to the designer during the
partitioning phase of the design process. In this section we first discuss briefly how we can use
the objective function in the presence of additional information and then discuss the status of
the implementation effort.


Dependencies: If a universal relation or a relation which is not in the Boyce-Codd normal
form is used as input, several constraints need to be considered when a partition is
generated. The cost of maintaining functional and multi-valued dependencies is reduced
if all the attributes participating in a dependency are either in the same fragment or at
least allocated to the same node. The first case can be easily accommodated without
changing the objective function by defining a dependency checker which evaluates whether
a partition should be considered for cost computation. In other words, during the design
process any fragment that violates any of the dependencies would be rejected even though
that fragment might give the minimum value for the partition evaluator; such partitioning
schemes are deemed not admissible. For the second case, the information is passed to
the allocation phase.

Fixed partition size: In many cases, the number of sites are known a priori and the objective
function need to minimize the cost for a given number of fragments. This, again, does
not require any changes as one can choose the minimum cost of all the alternatives for
the number of fragments specified.

Update vs. retrieval transactions: Although no distinction was made between update and
retrieval transaction in the earlier discussion, it is quite straightforward to distinguish










between them. Typically, retrieve only transaction access data items only once where as
there is an additional overhead of writing back for updated data items on to secondary
storage (sometimes more if the data item has already been flushed to secondary storage).
A weight factor can be used to distinguish update type from retrieval type transactions.
We can increase the frequency (for example double it) of update type transactions to
reflect their processing cost. In our simulation, we can take into account the type of
queries by increasing the frequency for update type queries in the attribute usage matrix.

Local vs. remote processing cost : In our formulation of PE, we have assumed a unit cost
for processing local as well as remote data once the remote data reaches the local node.
We also assumed a uniform transmission cost between any two sites in the network. How-
ever, realistically, there is difference between accessing local data possibly using access
methods and remote data for which there are no access methods (unless dynamically
created at the local site). Also, the transmission costs between any two nodes is not
likely to be the same.
We can include another factor to reflect the ratio of local processing cost to the processing
of data coming from remote sites. This factor can be included giving different weights
to the two component terms in equation 13. If W1 is the factor for processing local data
and W2 is the factor for processing remote data, it can be accommodated by modifying
equation 13 as follows:

PE = EM + (W2/W1)Ek (14)

Note, however, that a better way would be to integrate the factors into the formula.
Non-uniform transmission cost between nodes can be taken into account by modifying
equation 4 to include a multiplicative factor TRik (which reflects the actual transmission
cost/unit data between sites i and k) inside the inner summation.

Replication: The Partition Evaluator can be extended with minimal changes to accommodate
replication of fragments. In this case, we assume that attributes are remotely accessed
only if they do not exist locally. Thus in computing the second term, only those attributes
that exist in a fragment remotely accessed with respect to the site of another fragment
(call it "local") but do not exist in the local fragment are taken into account. However,
the form of the Partition Evaluator remains the same. Although replication provides
availability, the cost of maintaining consistency of replicated data need to be considered.
The above suggestion takes only the cost of retrieval into account and not the cost of
update propagation.


6.1 Implementation Status

A distributed database design testbed is being developed at the Database Systems R & D
Center, Univ. of Florida, Gainesville. The testbed includes several different vertical partition-
ing algorithms and modules that compare and evaluate the results of these algorithms. The
algorithms that were developed as part of this effort are as follows:

Bond Energy Algorithm [12].




































Figure 5: Distributed Database Design testbed prototype


Binary Vertical Partitioning Algorithm [15].


Minyoung Ra's Graphical Algorithm [16].


Exhaustive Enumeration Algorithm.



The figure 5 gives an overall outline of the design testbed.

More algorithms can be easily "hooked" on to this design testbed prototype.


7 Summary and Future Work

In this paper, we have presented a general approach to the vertical partitioning problem. Our
study brought out the problems associated with the use of attribute affinity matrix currently
used in almost all of the earlier data partitioning algorithms. We started with the objective
function used in clustering methods and extended it to suit the requirements of database
design. As a result we have brought together work in two isolated areas and established a
correspondence between them. Using the objective function derived in this paper and the
approach proposed, one can evaluate any vertical partitioning algorithm that uses the same










input as our model. We wanted to identify and express an objective function quantitatively
before embarking on the development of algorithms. It will now be easier to develop algorithms
(heuristics-based or otherwise) exploiting the properties of the function being optimized. Our
Partition Evaluator satisfies the need for a common criteria or objective function and can be
used to compare and evaluate the extant vertical partitioning algorithms. Finally, the PE
developed in this paper has the flexibility to incorporate additional design information such
as type of queries (retrieval/updates), transmission cost, and replication as briefly outlined in
the previous section.

We are currently developing heuristic algorithms for the objective function derived in this
paper. We are in the process of developing integrated objective function to include other inputs
to the database design process, such as query types, constraints on allocation, transmission
cost, transaction usage pattern at any particular site, into the objective function derived in this
paper. We plan on comparing different algorithms in more detail using the partition evaluator.
We hope to integrate the the Partition Evaluator into a database design testbed which will
help designers to choose the right algorithm for database initial design as well as for redesign.










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