The Gems of PODS event features topics and results in PODS that have been highly influential in the PODS community and beyond. The first edition of the Gems of PODS took place at SIGMOD/PODS2016.

**Optimal Score Aggregation Algorithms [PDF]**

Ronald Fagin (IBM Research - Almaden)

#### Abstract

Assume that there is a set of "voters" and a set of "candidates", where each voter assigns a numerical score to each candidate. There is a scoring function (such as the mean or the median), and a consensus ranking is obtained by applying the scoring function to each candidate's scores. The problem is to find the top k candidates, while minimizing the number of database accesses. The speaker will present an algorithm that is optimal in an extremely strong sense: not just in the worst case or the average case, but in every case! Even though the algorithm is only 10 lines long (!), the paper containing the algorithm won the 2014 GĂ¶del Prize, the top prize for a paper in theoretical computer science.

#### Hypertree Decompositions: Questions and Answers [PDF]

Georg Gottlob (University of Oxford)

#### Abstract

In the database context, the hypertree decomposition method is used for query optimization, whereby conjunctive queries having a low hypertree width (i.e., a low degree of cyclicity) can be recognized and decomposed automatically, and efficiently evaluated. Queries having bounded hypertree width are also highly parallelizable. Hypertree decompositions were introduced at ACM PODS 1999. This talk reviews - in form of questions and answers - the main relevant concepts and algorithms and surveys selected related work including applications and test results. The talk is accompanied by a paper of the same title authored by Georg Gottlob, Gianluigi Greco, Nicola Leone, and Francesco Scarcello.