39 research outputs found
IMPROVEMENTS IN DATABASE CONCURRENCY CONTROL WITH LOCKING
Various techniques have been proposed to ensure the safe, concurrent execution of a set of database
transactions. Locking protocols are the most prominent and widely used of these techniques, with two-phase
locking and tree-locking being but two examples of these protocols. A locking protocol defines a
general set of restrictions on the placement of lock and unlock steps within transactions. In this paper we
show that it is possible to further increase the potential level of concurrency of a set of transactions,
within the context of a specific locking protocol, by further restricting the placement of lock and unlock
steps within each transaction. We also discuss a variation of the tree-locking protocol that allows
transaction to be locked with respect to a dynamically changing set of tree structures. In addition we
define and discuss the concept of a concurrency cost function for a locked transaction. This cost function
measures the potential for conflict of a transaction with other transactions.Information Systems Working Papers Serie
OBJECTS IN TIME
Two recent lines of database research, proceeding independently, have been
concerned with providing a richer, more intuitive view of information at the user
level. Historical database research has focused on ways to provide users with a
view of information anchored and evolving in the temporal dimension. Object-oriented
database research focuses on encapsulating both the structure and the
behavior of the objects that users intend to model. In this paper we explore how
these two lines of research might be brought together, providing to the user the
representation and management of objects in time.Information Systems Working Papers Serie
ON COMPLETENESS OF HISTORICAL RELATIONAL DATA MODELS
Several proposals for extending the relational data model to incorporate the
temporal dimension of data have appeared in the past several years. These
proposals have differed considerably in the way that the temporal dimension
has been incorporated both into the structure of the extended relations that
are defined as part of these extended model, and into the operations of the
extended relational algebra or calculus component of the models. Because
of these differences it has been difficult to compare the proposed models and
to make judgements as to which of them is "better" or indeed, the "best."
In this paper we propose a notion of historical relational completeness,
analogous to Codd's notion of relational completeness, and examine several
historical relational proposals in light of this standard.Information Systems Working Papers Serie
A PROBLEM-SOLVER/TMS ARCHITECTURE FOR GENERAL CONSTRAINT SATISFACTION PROBLEMS
Constraints, in various forms, are ubiquitous to design problems. In this paper, we provide a formal
characterization of a generalized constraint satisfaction problem (CSP) that can be used to model many
types of design/planning problems, and the architecture of an imlemented reasoning system for solving this
problem. The architecture includes a truth maintenance system (TMS) which is specifically designed to
reason about the relationships expressed in the constraints as a problem solution evolves. The CSP
consists of two types of data. The first type of datum corresponds to assignments that are handled by the
problem solver, and the second type corresponds to constraint terms handled by the TMS. The
dependency network, representing the relationships among constraint terms, is static and generally quite
small, depending on the number of constraint terms. Also, justifications are never manipulated (only
evaluated). This results in an architecture that makes efficient use of both space and time. The need for
efficient TMSs, even though these might deal only with certain classes of problems, is underscored by the
fact that general purpose TMSs have often been found to be highly inefficient for solving large problems.
We also show how certain instances of the generalized CSP can be formulated as an integer programming
problem, special cases of which can be solved efficiently using mathematical (integer) programming
techniques.Information Systems Working Papers Serie
THE HISTORICAL RELATIONAL DATA MODEL (HRDM) AND ALGEBRA BASED ON LIFESPANS
Critical to the design of an historical database model is the representation of the âexistenceâ
of objects across the temporal dimension -- for example, the "birth," "death," or "rebirth" of
an individual, or the establishment or dis-establishment of a relationship. The notion of the
"lifespan" of a database object is proposed as a simple framework for expressing these concepts.
An object's lifespan is simply those periods of time during which the database models the
properties of that object. In this paper we propose the historical relational data model (HRDM)
and algebra that is based upon lifespans and that views the values of all attributes as functions
from time points to simple domains. The model that we obtain is a consistent extension of the
relational data model, and provides a simple mechanism for providing both time-varying data
and time-varying schemes.Information Systems Working Papers Serie
ON CONSISTENT EXTENSIONS TO THE RELATIONAL DATABASE MODEL
Information Systems Working Papers Serie
A KNOWLEDGE REPRESENTATION FOR CONSTRAINT SATISFACTION PROBLEMS
In this paper we present a general representation for constraint satisfaction problems (CSP) and a -
framework for reasoning about their solution that unlike most constraint-based relaxation algorithms.
stresses the need for a "natural" encoding of constraint knowledge and can facilitate making inferences for
propagation, backtracking, and explanation. The representation consists of two components: a
generate-and-test problem solver which contains information about the problem variables, and a
constraint-driven reasoner that manages a set of constraints, specified as arbitrarily complex Boolean
expressions and represented in the form of a constraint network. This constraint network: incorporates
control information (reflected in the syntax of the constraints) that is used for constraint propagation:
contains dependency information that can be used for explanation and for dependency-directed
backtracking; and is incremental in the sense that if the problem specification is modified, a new solution
can be derived by modifying the existing solution.Information Systems Working Papers Serie
On Completeness of Historical Relational Query Languages
Numerous proposals for extending the relational data model to incorporate the temporal
dimension of data have appeared in the past several years. These proposals have differed
considerably in the way that the temporal dimension has been incorporated both into the
structure of the extended relations of these temporal models, and consequently into the
extended relational algebra or calculus that they define. Because of these differences it
has been difficult to compare the proposed models and to make judgments as to which of
them might in some sense be equivalent or even better. In this paper we define the notions of
temporally grouped and temporally ungrouped historical data models and propose two
notions of historical reIationa1 completeness, analogous to Codd's notion of relational
completeness, one for each type of model. We show that the temporally ungrouped models
are less expressive than the grouped models, but demonstrate a technique for extending the
ungrouped models with a grouping mechanism to capture the additional semantic power
of temporal grouping. For the ungrouped models we define three different languages, a
temporal logic, a logic with explicit reference to time, and a temporal algebra, and show
that under certain assumptions all three are equivalent in power. For the grouped models
we define a many-sorted logic with variables over ordinary values, historical values, and
times. Finally, we demonstrate the equivalence of this grouped calculus and the ungrouped
calculus extended with a grouping mechanism. We believe the classification of historical
data models into grouped and ungrouped provides a useful framework for the comparison
of models in the literature, and furthermore the exposition of equivalent languages for each
type provides reasonable standards for common, and minimal, notions of historical relational
completeness.Information Systems Working Papers Serie
ON COMPLETENESS OF HISTORICAL RELATIONAL QUERY LANGUAGES
Numerous proposals for extending the relational data model to incorporate the temporal
dimension of data have appeared in the past several years. These proposals have differed
considerably in the way that the temporal dimension has been incorporated both into the
structure of the extended relations of these temporal models, and consequently into the
extended relational algebra or calculus that they define. Because of these differences it has
been difficult to compare the proposed models and to make judgments as to which of them
might in some sense be equivalent or even better. In this paper we define the notions of
temporally grouped and temporally ungrouped historical data models and propose
two notions of historical relational completeness, analogous to Codd's notion of relational
completeness, one for each type of model. We show that the temporally ungrouped
models are less powerful than the grouped models, but demonstrate a technique for extending
the ungrouped models with a grouping mechanism to capture the additional semantic
power of temporal grouping. For the ungrouped models we define three different languages,
a temporal logic, a logic with explicit reference to time, and a temporal algebra, and show
that under certain assumptions all three are equivalent in power. For the grouped models
we define a many-sorted logic with variables over ordinary values, historical values, and
times. Finally, we demonstrate the equivalence of this grouped calculus and the ungrouped
calculus extended with the proposed grouping mechanism. We believe the classification of
historical data models into grouped and ungrouped provides a useful framework for the
comparison of models in the literature, and furthermore the exposition of equivalent languages
for each type provides reasonable standards for common, and minimal, notions of
historical relational completeness.Information Systems Working Papers Serie
DEPENDENCY DIRECTED BACKTRACKING IN GENERALIZED SATISFICING ASSIGNMENT PROBLEMS
Many authors have described search techniques for the satisficing assignment problem: the problem of
finding an interpretation for a set of discrete variables that satisfies a given set of constraints. In this paper
we present a formal specification of dependency directed backtracking as applied to this problem. We
also generalize the satisficing assignment problem to include limited resource constraints that arise in
operations research and industrial engineering. We discuss several new search heuristics that can be
applied to this generalized problem, and give some empirical results on the performance of these
heuristics.Information Systems Working Papers Serie