Paraskevas V. Lekeas

Cooperative oligopoly games with boundedly rational firms

We analyze cooperative Cournot games with boundedly rational firms. Due to cognitive constraints, the members of a coalition cannot accurately predict the coalitional structure of the non-members. Thus, they compute their value using simple heuristics. In particular, they assign various non-equilibrium probability distributions over the outsiders’ set of partitions. We construct the characteristic function of a coalition in such an environment and we analyze the core of the corresponding games. We show that the core is non-empty provided the number of firms in the market is sufficiently large. Moreover, we show that if two distributions over the set of partitions are related via first-order dominance, then the core of the game under the dominated distribution is a subset of the core under the dominant distribution.

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.

On Estimating Semantic Loss in Peer Data Management Systems

Traditional data integration techniques have focused on centralized components to allow transparent querying of multiple heterogeneous databases. This requirement on global/mediated schemas impedes the deployment and success of such systems in modern highly dynamic and scalable computing environments. Peer Data Management Systems (PDMS) offer a truly decentralized solution by utilizing compositions of local pair-wise mappings to propagate queries through the network. Current PDMS proposals have been ignorant of the fact that query reformulation could result in semantic degradation due to the inherent heterogeneity of the nodes. A critical issue to effectively address this problem is the existence of a technique that computes the query difference, which results from this reformulation. In this work, we propose an algorithm that estimates the semantic loss of the rewritten queries in a generic way, based on the notion of containment mapping. This information can then be used as the basis for extending appropriately the schema mappings and improving the quality of the retrieved answers.

Answering aggregation queries on hierarchical web sites using adaptive sampling

We study how to answer aggregation queries over hierarchical Web sites using adaptive sampling.

STRATEGIC DELEGATION IN A STACKELBERG GAME WITH MULTIPLE STAGES

We analyze strategic delegation in a Stackelberg model with an arbitrary number, n, of firms. We show that n-1 firms delegate their production decisions and only one firm (the one whose manager is the first mover) does not. The later a manager commits to a quantity, the higher his incentive rate. Letting

A Game Theoretic Approach for Resource Usage

u_i^*

Observing Spatial and Temporal Metadata in Hostnames

denote the equilibrium payoff of the firm whose manager commits in the ith stage, we show that

Adaptive-sampling algorithms for answering aggregation queries on Web sites

u_n^*>u_{n-1}^*>\cdots>u_2^*>u_1^*

Coalitional Beliefs In Cournot Oligopoly Tu Games

. We also compare the delegation outcome of our game with that of a corresponding Cournot oligopoly and show that managers who commit late (early) are given higher (lower) incentive rates than managers in the Cournot market.

Should I quit using my resource? Modeling Resource Usage through Game Theory

Existing web infrastructures support the publication of a tremendous amount of resources, and over the past few years Data Resource Usage has become an everyday task for millions of users all over the world. In this work we model Resource Usage as a Cooperative Cournot Game in which a resource user and the various resource services are engaged. We give quantified answers as to when it is of interest for the user to stop using part of a resource and to switch to a different one. Moreover, we do the same from the perspective of a resources provider. We show that providers should use their resources in a cooperative manner to prevent a users deviation. We also prove that providers can avoid losing users if they reduce user cost for the offered services, if they increase their resource size, and, finally, if they reduce the number of their unpopular services.