53,712 research outputs found
Stochastic Database Cracking: Towards Robust Adaptive Indexing in Main-Memory Column-Stores
Modern business applications and scientific databases call for inherently
dynamic data storage environments. Such environments are characterized by two
challenging features: (a) they have little idle system time to devote on
physical design; and (b) there is little, if any, a priori workload knowledge,
while the query and data workload keeps changing dynamically. In such
environments, traditional approaches to index building and maintenance cannot
apply. Database cracking has been proposed as a solution that allows on-the-fly
physical data reorganization, as a collateral effect of query processing.
Cracking aims to continuously and automatically adapt indexes to the workload
at hand, without human intervention. Indexes are built incrementally,
adaptively, and on demand. Nevertheless, as we show, existing adaptive indexing
methods fail to deliver workload-robustness; they perform much better with
random workloads than with others. This frailty derives from the inelasticity
with which these approaches interpret each query as a hint on how data should
be stored. Current cracking schemes blindly reorganize the data within each
query's range, even if that results into successive expensive operations with
minimal indexing benefit. In this paper, we introduce stochastic cracking, a
significantly more resilient approach to adaptive indexing. Stochastic cracking
also uses each query as a hint on how to reorganize data, but not blindly so;
it gains resilience and avoids performance bottlenecks by deliberately applying
certain arbitrary choices in its decision-making. Thereby, we bring adaptive
indexing forward to a mature formulation that confers the workload-robustness
previous approaches lacked. Our extensive experimental study verifies that
stochastic cracking maintains the desired properties of original database
cracking while at the same time it performs well with diverse realistic
workloads.Comment: VLDB201
QUASII: QUery-Aware Spatial Incremental Index.
With large-scale simulations of increasingly detailed models and improvement of data acquisition technologies, massive amounts of data are easily and quickly created and collected. Traditional systems require indexes to be built before analytic queries can be executed efficiently. Such an indexing step requires substantial computing resources and introduces a considerable and growing data-to-insight gap where scientists need to wait before they can perform any analysis. Moreover, scientists often only use a small fraction of the data - the parts containing interesting phenomena - and indexing it fully does not always pay off. In this paper we develop a novel incremental index for the exploration of spatial data. Our approach, QUASII, builds a data-oriented index as a side-effect of query execution. QUASII distributes the cost of indexing across all queries, while building the index structure only for the subset of data queried. It reduces data-to-insight time and curbs the cost of incremental indexing by gradually and partially sorting the data, while producing a data-oriented hierarchical structure at the same time. As our experiments show, QUASII reduces the data-to-insight time by up to a factor of 11.4x, while its performance converges to that of the state-of-the-art static indexes
Vertical vs. Adiabatic Ionization Energies in Solution and Gas-Phase: Probing Ionization-Induced Reorganization in Conformationally-Mobile Bichromophoric Actuators Using Photoelectron Spectroscopy, Electrochemistry and Theory
Ionization-induced structural and conformational reorganization in various Ï-stacked dimers and covalently linked bichromophores is relevant to many processes in biological systems and functional materials. In this work, we examine the role of structural, conformational, and solvent reorganization in a set of conformationally mobile bichromophoric donors, using a combination of gas-phase photoelectron spectroscopy, solution-phase electrochemistry, and density functional theory (DFT) calculations. Photoelectron spectral analysis yields both adiabatic and vertical ionization energies (AIE/VIE), which are compared with measured (adiabatic) solution-phase oxidation potentials (Eox). Importantly, we find a strong correlation of Eox with AIE, but not VIE, reflecting variations in the attendant structural/conformational reorganization upon ionization. A careful comparison of the experimental data with the DFT calculations allowed us to probe the extent of charge stabilization in the gas phase and solution and to parse the reorganizational energy into its various components. This study highlights the importance of a synergistic approach of experiment and theory to study ionization-induced structural and conformational reorganization
Legal reform and aggregate small and micro business bankruptcy rates: Evidence from the 1997 Belgian Bankruptcy Code.
Bankruptcy; Business;
Basel II and Operational Risk: Implications for risk measurement and management in the financial sector
This paper proposes a methodology to analyze the implications of the Advanced Measurement Approach (AMA) for the assessment of operational risk put forward by the Basel II Accord. The methodology relies on an integrated procedure for the construction of the distribution of aggregate losses, using internal and external loss data. It is illustrated on a 2x2 matrix of two selected business lines and two event types, drawn from a database of 3000 losses obtained from a large European banking institution. For each cell, the method calibrates three truncated distributions functions for the body of internal data, the tail of internal data, and external data. When the dependence structure between aggregate losses and the non-linear adjustment of external data are explicitly taken into account, the regulatory capital computed with the AMA method proves to be substantially lower than with less sophisticated approaches allowed by the Basel II Accord, although the effect is not uniform for all business lines and event types. In a second phase, our models are used to estimate the effects of operational risk management actions on bank profitability, through a measure of RAROC adapted to operational risk. The results suggest that substantial savings can be achieved through active management techniques, although the estimated effect of a reduction of the number, frequency or severity of operational losses crucially depends on the calibration of the aggregate loss distributions.operational risk management, basel II, advanced measurement approach, copulae, external data, EVT, RAROC, cost-benefit analysis.
Acquisitions of Bankrupt and Distressed Firms
In this paper we focus on acquisitions of bankrupt firms and firms that recently emerged from Chapter 11 and compare these firms with acquired distressed firms to determine whether or not transaction timing plays a role in the outcomes of the mergers. We analyze deal premiums (or lack thereof) and evaluate post-merger operating cash flows to determine whether or not timing of the transactions impacts their effectiveness and success. We also evaluate targets and their acquirersâ stock price reactions to the announcements of acquisitions. We find that distressed targets sell their assets at a premium or at a discount smaller than bankrupt firms do, thereby benefiting from acquisitions more than bankrupt targetsâand the announcement day abnormal returns are reflective of the disparity of these purchases with bankrupt firms having significant negative abnormal returns and distressed firms having significant positive announcement day abnormal returns and acquirers of both having material announcement day abnormal returns. We also find that abnormal post-merger cash flow and cumulative abnormal return changes are more pronounced for bankrupt than distressed firms, indicating that acquisitions in Chapter 11 add greater economic value for both target and its acquirer than do acquisitions outside of bankruptcy. We also find post-merger market performance improvements for bankrupt and not distressed firms. In summary, distressed firms get a merger announcement premium and bankrupt firms give it away to their acquirers whose shareholders benefit from acquisition premiums in a year after the mergers
Evolution of Wikipedia's Category Structure
Wikipedia, as a social phenomenon of collaborative knowledge creating, has
been studied extensively from various points of views. The category system of
Wikipedia, introduced in 2004, has attracted relatively little attention. In
this study, we focus on the documentation of knowledge, and the transformation
of this documentation with time. We take Wikipedia as a proxy for knowledge in
general and its category system as an aspect of the structure of this
knowledge. We investigate the evolution of the category structure of the
English Wikipedia from its birth in 2004 to 2008. We treat the category system
as if it is a hierarchical Knowledge Organization System, capturing the changes
in the distributions of the top categories. We investigate how the clustering
of articles, defined by the category system, matches the direct link network
between the articles and show how it changes over time. We find the Wikipedia
category network mostly stable, but with occasional reorganization. We show
that the clustering matches the link structure quite well, except short periods
preceding the reorganizations.Comment: Preprint of an article submitted for consideration in Advances in
Complex Systems (2012) http://www.worldscinet.com/acs/, 19 pages, 7 figure
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