Proceedings of the thirtieth ACM SIGMOD-SIGACT-SIGART symposium on Principles of database systems

It is our great pleasure to welcome you to the 2011 ACM Symposium on Principles of Database Systems -- PODS'11. This year's symposium continues its tradition of being the premier international conference on the theoretical aspects of data management. PODS papers are distinguished by a rigorous approach to widely diverse problems in databases, often bringing to bear techniques from a variety of different areas, including computational logic, finite model theory, computational complexity, algorithm design and analysis, programming languages, and artificial intelligence. The first PODS conference was held in Los Angeles (CA) in 1982, with Jeffrey D. Ullman as General Chair. Since that time, virtually all new ideas, methods and techniques for data management have been investigated and presented in subsequent PODS conferences (see http://www09.sigmod.org/sigmod/pods/ for various information on the conference series). To celebrate the 30th anniversary of the Symposium, the PODS Executive Committee has organized the PODS 30th Anniversary Colloquium, a special event held in June 12, 2011, with the goal of providing a retrospective on the role of database theory, and outlining a picture of the future directions of the discipline. The Colloquium featured five invited presentations from distinguished leaders in the field, namely: Moshe Y. Vardi: "The rise, fall, and rise of dependency theory: Part 1, the rise and fall", Ronald Fagin: "The rise, fall, and rise of dependency theory: Part 2, the rise from the ashes", Jeffrey D. Ullman: "Deductive Databases", Serge Abiteboul: "Trees, semistructured data, and other strange ways to go beyond tables", Victor Vianu: "Database Theory: Back to the Future". The PODS Executive Committee is grateful to the speakers for their participation in the event, and to Frank Neven for organizing a lively discussion session ending the Colloquium. This volume contains the proceedings of the Thirtieth ACMSIGMOD-SIGACT-SIGART Symposium on Principles of Database Systems (PODS 2011), held in Athens, Greece, on June 13-15, 2011, in conjunction with the 2011 ACM SIGMOD International Conference on Management of Data. The proceedings include a paper by Daniel Deutch and Tova Milo based on the keynote address by Tova Milo and two papers, the first by Marcelo Arenas and Jorge Pérez, based on the tutorial byMarcelo Arenas, and the second based on the tutorial by S. Muthu Muthukrishnan, and 25 contributed papers that were selected by the Program Committee from 113 submissions. Most of these papers are preliminary reports on work in progress. While they have been read by program committee members, they have not been formally refereed. Many of them will probably appear in more polished and detailed form in scientific journals. The program committee selected the paper Data Exchange beyond Complete Data by Marcelo Arenas, Jorge Pérez and Juan L. Reutter for the PODS 2011 Best Paper Award. In addition, the announcement of the 2011 ACM PODS Alberto O. Mendelzon Test-of-Time Award appears in the proceedings. This year, the award is given to Optimal Aggregation Algorithms for Middleware by Ronald Fagin, Amnon Lotem, and Moni Naor. The paper originally appeared in the proceedings of PODS 2001. Warmest congratulations to the authors of these papers.