Foreword to TODS SIGMOD/PODS 2008 special issue

This special issue of TODS has six articles, including four selected from the International Conference on Management of Data/Symposium on Principles of Database Systems (SIGMOD/PODS) 2008, Vancouver, Canada. We have two articles selected from SIGMOD 2008. “Serializable isolation for snapshot databases” (Michael Cahill, Uwe Roehm, Alan Fekete) presents a simple dynamic modification to the snapshot isolation concurrency control protocol to ensure serializable execution. Snapshot isolation has been deployed on most database systems. Unfortunately, the technique does not, by itself, guarantee serializability. For this reason, using the protocol without modification could cause databases to become (silently) inconsistent, leading to the loss of life and property. This article offers a practical solution to this problem while still maintaining most of the efficiency benefits of snapshot isolation. “Relational Joins on Graphics Processors” (Bingsheng He, Ke Yang, Rui Fang, Mian Lu, Naga Govindaraju, Qiong Luo, Pedro Sander), the second article from SIGMOD 2008, presents a group of massively parallel primitives on graphic processors to support join processing. The article is important not only in its demonstration of efficient join algorithms, but also in its two-layer architecture. The join layer sits on top of a group of useful primitive operations that will find use in a host of database and machine learning applications. Three articles were selected from PODS 2008 for publication in TODS. The article “Incorporating Constraints in Probabilistic XML” (Sara Cohen, Benny Kimelfeld and Yehoshua Sagiv), has already appeared in the September 2009 (34/3) issue. It proposes a model for probabilistic XML databases equipped with a rich constraint language that can be used to express dependencies in the data. The authors show that query answering in this model can be solved efficiently for a fairly expressive XML query language. Two articles appear in this issue. “The Recovery of a Schema Mapping: Bringing Exchanged Data Back” (Marcelo Arenas, Jorge Pérez and Cristian Riveros) studies the problem of recovery of a schema mapping. Once the data has been transferred from the source to the target according to a schema mapping, a natural question is whether one can undo the process and recover the initial data or at least part of it. The article presents a thorough investigation into this problem. “Static Analysis of Active XML Services” (Serge Abiteboul, Luc Segoufin and Victor Vianu) presents a comprehensive study on the verification of temporal properties of services expressed in Active XML, which is a high-level specification language tailored to data-intensive, distributed, and dynamic Web services. The first of the remaining two articles in this issue is “Casper: Query Processing for Location Services without Compromising Privacy” (Chi-Yin Chow, Mohamed Mokbel and Walid Aref) which proposes a privacy-aware query