211,740 research outputs found
A New Framework for Join Product Skew
Different types of data skew can result in load imbalance in the context of
parallel joins under the shared nothing architecture. We study one important
type of skew, join product skew (JPS). A static approach based on frequency
classes is proposed which takes for granted the data distribution of join
attribute values. It comes from the observation that the join selectivity can
be expressed as a sum of products of frequencies of the join attribute values.
As a consequence, an appropriate assignment of join sub-tasks, that takes into
consideration the magnitude of the frequency products can alleviate the join
product skew. Motivated by the aforementioned remark, we propose an algorithm,
called Handling Join Product Skew (HJPS), to handle join product skew
Unachievable Region in Precision-Recall Space and Its Effect on Empirical Evaluation
Precision-recall (PR) curves and the areas under them are widely used to
summarize machine learning results, especially for data sets exhibiting class
skew. They are often used analogously to ROC curves and the area under ROC
curves. It is known that PR curves vary as class skew changes. What was not
recognized before this paper is that there is a region of PR space that is
completely unachievable, and the size of this region depends only on the skew.
This paper precisely characterizes the size of that region and discusses its
implications for empirical evaluation methodology in machine learning.Comment: ICML2012, fixed citations to use correct tech report numbe
Weak topologies for Carath\'eodory differential equations. Continuous dependence, exponential Dichotomy and attractors
We introduce new weak topologies and spaces of Carath\'eodory functions where
the solutions of the ordinary differential equations depend continuously on the
initial data and vector fields. The induced local skew-product flow is proved
to be continuous, and a notion of linearized skew-product flow is provided. Two
applications are shown. First, the propagation of the exponential dichotomy
over the trajectories of the linearized skew-product flow and the structure of
the dichotomy or Sacker-Sell spectrum. Second, how particular bounded absorbing
sets for the process defined by a Carath\'eodory vector field provide
bounded pullback attractors for the processes with vector fields in the
alpha-limit set, the omega-limit set or the whole hull of . Conditions for
the existence of a pullback or a global attractor for the skew-product
semiflow, as well as application examples are also given.Comment: 34 page
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