ACM Transactions on

Database Systems (TODS)

Latest Articles

Dynamic Complexity under Definable Changes

In the setting of dynamic complexity, the goal of a dynamic program is to maintain the result of a fixed query for an input database that is subject to changes, possibly using additional auxiliary relations. In other words, a dynamic program updates a materialized view whenever a base relation is changed. The update of query result and auxiliary... (more)


Updates to the TODS Editorial Board

As of January 1, 2018, five Associate Editors—Walid Aref, Graham Cormode, Gautam Das, Sabrina De Capitani di Vimercati, and Dirk Van Gucht—ended their terms, each having served on the editorial board for six years. Walid, Graham, Gautam, Sabrina, and Dirk have provided very substantial, high-caliber service to the journal and the database community. Also five new Associate Editors have joined the editorial board: Angela Bonifati, Université Claude Bernard Lyon 1, Wolfgang Lehner, TU Dresden, Dan Olteanu, University of Oxford, Evaggelia Pitoura, University of Ioannina, and Bernhard Seeger, University of Marburg. All five are highly regarded scholars in database systems.

Please read more here.

Updates to the TODS Editorial Board

As of January 1, 2017, three Associate Editors, Paolo Ciaccia, Divyakant Agrawal, and Sihem Amer-Yahia, ended their terms, each having served on the editorial board for some six years. In addition, they will stay on until they complete their current loads. We are fortunate that they have donated their time and world-class expertise during these years. Also three new Associate Editors have joined the editorial board: Feifei Li, University of Utah, Kian-Lee Tan, National University of Singapore, and Jeffrey Xu Yu, Chinese University of Hong Kong. All three are highly regarded scholars in database systems. 

Please read more here.

Forthcoming Articles
Parallelizing Sequential Graph Computations

This paper presents GRAPE, a parallel GRAPh Engine for graph computations. GRAPE differs from prior systems in its ability to parallelize existing sequential graph algorithms as a whole, without the need for recasting the entire algorithms into a new model. Underlying GRAPE are a simple programming model, and a principled approach based on fixpoint computation that starts with partial evaluation and uses an incremental function as the intermediate consequence operator. We show that sequential graph algorithms can be ``plugged into'' GRAPE with minor additions, and get parallelized. Under a monotonic condition, the GRAPE parallelization guarantees to converge at correct answers as long as the sequential algorithms are correct. Moreover, we show that algorithms in MapReduce, BSP and PRAM can be optimally simulated on GRAPE. In addition to the ease of programming, we experimentally verify that GRAPE achieves comparable performance to the state-of-the-art graph systems, using real-life and synthetic graphs.

MacroBase: Prioritizing Attention in Fast Data

As data volumes continue to rise, manual inspection is becoming increasingly untenable. In response, we present MacroBase, a data analytics engine that prioritizes end-user attention in high-volume fast data streams. MacroBase enables efficient, accurate, and modular analyses that highlight and aggregate important and unusual behavior, acting as a search engine for fast data. MacroBase is able to deliver order-of-magnitude speedups over alternatives by optimizing the combination of explanation and classification tasks and by leveraging a new reservoir sampler and heavy-hitters sketch specialized for fast data streams. As a result, MacroBase delivers accurate results at speeds of up to 2M events per second per query on a single core. The system has delivered meaningful results in production, including at a telematics company monitoring hundreds of thousands of vehicles.

Distributed Joins and Data Placement for Minimal Network Traffic

Network communication is the slowest component of many operators in distributed parallel databases deployed for large-scale analytics. Whereas considerable work has focused on speeding up databases on modern hardware, communication reduction has received less attention. Existing parallel DBMSs rely on algorithms designed for disks with minor modifications for networks. A more complicated algorithm may burden the CPUs, but could avoid redundant transfers of tuples across the network. We introduce track join, a new distributed join algorithm that minimizes network traffic by generating an optimal transfer schedule for each distinct join key. Track join extends the trade-off options between CPU and network. Track join explicitly detects and exploits locality, also allowing for advanced placement of tuples beyond hash partitioning on a single attribute. We propose a novel data placement algorithm based on track join that minimizes the total network cost of multiple joins across different dimensions in an analytical workload. Our evaluation shows that track join outperforms hash join on the most expensive queries of real workloads regarding both network traffic and execution time. Finally, we show that our data placement optimization approach is both robust and effective in minimizing the total network cost of joins in analytical workloads.

Historic Moments Discovery in Sequence Data

Many emerging applications are based on finding interesting subsequences from sequence data. Finding prominent streaks, a set of longest contiguous subsequences with values all above (or below) a certain threshold, from sequence data, is one of that kind that receives much attention. Motivated from real applications, we observe that prominent streaks alone are not insightful enough but require the discovery of something we coined as historic moments as companion. In this paper, we present an algorithm to efficiently compute historic moments from sequence data. The algorithm is incremental and space-optimal, meaning that when facing new data arrival, it is able to efficiently refresh the results by keeping minimal information. Case studies show that historic moments can significantly improve the insights offered by prominent streaks alone. Furthermore, experiments show that our algorithm can outperform the baseline in both time and space.

Optimal Bloom Filters and Adaptive Merging for LSM-Trees

In this paper, we show that key-value stores backed by an LSM-tree exhibit an intrinsic trade-off between lookup cost, update cost, and main memory footprint, yet all existing designs expose a suboptimal and difficult to tune trade-off among these metrics. We pinpoint the problem to the fact that all modern key-value stores suboptimally co-tune the merge policy, the buffer size, and the Bloom filters' false positive rates in each level. We present Monkey, an LSM-based key-value store that strikes the optimal balance between the costs of updates and lookups with any given main memory budget. The insight is that worst-case lookup cost is proportional to the sum of the false positive rates of the Bloom filters across all levels of the LSM-tree. Contrary to state-of-the-art key-value stores that assign a fixed number of bits-per-element to all Bloom filters, Monkey allocates memory to filters across different levels so as to minimize this sum. We show analytically that Monkey reduces the asymptotic complexity of the worst-case lookup I/O cost, and we verify empirically using an implementation on top of LevelDB that Monkey reduces lookup latency by an increasing margin as the data volume grows 50%-80% for the data sizes we experimented with). Furthermore, we map the LSM-tree design space onto a closed-form model that enables co-tuning the merge policy, the buffer size and the filters' false positive rates to trade among lookup cost, update cost and/or main memory, depending on the workload (proportion of lookups and updates), the dataset (number and size of entries), and the underlying hardware (main memory available, disk vs. flash). We show how to use this model to answer what-if design questions about how changes in environmental parameters impact performance and how to adapt the various LSM-tree design elements accordingly.

Learning From Query-Answers: A Scalable Approach to Belief Updating and Parameter Learning

Tuple-independent and disjoint-independent probabilistic databases (TI-, and DI-PDBs) represent uncertain data in a factorized form as a product of independent probabilistic variables that represent tuples or sets of tuples, respectively. When the user submits a query, the database derives the marginal probabilities of each output-tuple, under this assumption of independence on the inputs. While query processing in TI- and DI-PDBs has been studied extensively, limited research has been dedicated to the problems of updating or deriving the parameters from observations of query results. Addressing this problem is the main contribution of this paper. We rst re-introduce Beta Probabilistic Databases (B-PDBs), a generalization of TI-PDBs designed to support both (i) belief updating and (ii) parameter learning in a principled and scalable way. The key idea of B-PDBs is to treat each parameter as a latent, Beta-distributed random variable. We show how this simple expedient enables both belief updating and parameter learning in a principled way, without imposing any burden on regular query processing. Building on B-PDBs, we introduce our new contribution, Dirichlet Probabilistic Databases (D-PDBs), a generalization of DI-PDBs with similar properties. We use both models to provide the following key contributions: (i) we show how to scalably compute the posterior densities of the parameters given new evidence; (ii) we study the complexity of performing Bayesian belief updates, devising e cient algorithms for tractable classes of queries; (iii) we propose a soft-EM algorithm for computing maximum-likelihood estimates of the parameters; (iv) we show how to embed the proposed algorithms into a standard relational engine; (v) we support our conclusions with extensive experimental results.

A Relational Framework for Classifier Engineering

In the design of analytical procedures and machine-learning solutions, a critical and time-consuming task is that of feature engineering, for which various recipes and tooling approaches have been developed. In this manuscript, we embark on the establishment of database foundations for feature engineering. We propose a formal framework for classification in the context of a relational database. The goal of this framework is to open the way to research and techniques to assist developers with the task of feature engineering by utilizing the database's modeling and understanding of data and queries, and by deploying the well studied principles of database management. As a first step, we demonstrate the usefulness of this framework by formally defining three key algorithmic challenges. The first challenge is that of separability, which is the problem of determining the existence of feature queries that agree with the training examples. The second is that of evaluating the VC dimension of the model class with respect to a given sequence of feature queries. The third challenge is identifiability, which is the task of testing for a property of independence among features that are represented as database queries. We give preliminary results on these challenges for the case where features are defined by means of conjunctive queries, and in particular, we study the implication of various traditional syntactic restrictions on the inherent computational complexity.

Expressive Languages for Querying the Semantic Web

The problem of querying RDF data is a central issue for the development of the Semantic Web. The query language SPARQL has become the standard language for querying RDF, since its W3C standardization in 2008. However, the 2008 version of this language missed some important functionalities: reasoning capabilities to deal with RDFS and OWL vocabularies, navigational capabilities to exploit the graph structure of RDF data, and a general form of recursion much needed to express some natural queries. To overcome these limitations, a new version of SPARQL, called SPARQL 1.1, was recently released, which includes entailment regimes for RDFS and OWL vocabularies, and a mechanism to express navigation patterns through regular expressions. Unfortunately, there is a number of useful navigation patterns that cannot be expressed in SPARQL 1.1, and the language lacks a general mechanism to express recursive queries. To the best of our knowledge, no efficient RDF query language that combines the above functionalities is known. It is the aim of this work to fill this gap. Towards this direction, we focus on the OWL 2 QL profile of OWL 2, and we show that every SPARQL query enriched with the above features can be naturally translated into a query expressed in a language that is based on an extension of Datalog, which allows for value invention and stratified negation. However, the query evaluation problem for this language is highly intractable, which is not surprising since it is expressive enough to encode some inherently hard queries. We identify a natural fragment of it, and we show it to be tractable and powerful enough to define SPARQL queries enhanced with the desired functionalities.

Wander Join and XDB: Online Aggregation via Random Walks

Joins are expensive, and online aggregation over joins was proposed to mitigate the cost, which offers users a nice and flexible tradeoff between query efficiency and accuracy in a continuous, online fashion. However, the state-of-the-art approach, in both internal and external memory, is based on ripple join, which is still very expensive and even needs unrealistic assumptions (e.g., tuples in a table are stored in random order). This paper proposes a new approach, the wander join algorithm, to the online aggregation problem by performing random walks over the underlying join graph. We also design an optimizer that chooses the optimal plan for conducting the random walks without having to collect any statistics a priori. Compared with ripple join, wander join is particularly efficient for equality joins involving multiple tables, but also supports ¸- joins. Selection predicates and group-by clauses can be handled as well. To demonstrate the usefulness of wander join, we have designed and implemented XDB (approXimate DB) by integrating wander join into various systems including PostgreSQL, Spark, and a stand-alone plug-in version using PL/SQL. The design and implementation of XDB has demonstrated wander joins practicality in a full-fledged database system. Extensive experiments using the TPC-H benchmark have demonstrated the superior performance of wander join over ripple join.

Inferring Insertion Times and Optimizing Error Penalties in Time-Decaying Bloom Filters

Time-Decaying Bloom Filters are probabilistic structures to answer queries on inserted items. The memory of older items decays over time, causing both false positives and false negatives. Users suffer penalties for wrong responses that are both application- and item-specific. Current filters, however, are typically tuned only for static penalties. They also ignore Bayesian priors and much information latent in the filter. We address these issues by introducing Inferential Filters, which integrate Bayesian priors and information latent in filters to make penalty-optimal, query-specific decisions. We also show how to properly infer insertion times in such filters. Our methods are general, but here we apply them to Inferential time-decaying filters, and show how to support novel query types and sliding window queries with varying error penalties. We present inferential versions of the existing Timing Bloom Filter and Generalized Bloom Filter. Our experiments on real and synthetic datasets show that when penalties are dynamic and prior probabilities are considered, these filters reduce penalties for incorrect responses to sliding-window queries by up to 70%.

Scalable Analytics on Fast Data

Today's streaming applications demand increasingly high event throughput rates and are often subject to strict latency constraints. To allow for more complex workloads, such as window-based aggregations, streaming systems need to support stateful event processing. This introduces new challenges for streaming engines as the state needs to be maintained in a consistent and durable manner and simultaneously accessed by complex queries for real-time analytics. Modern streaming systems, such as Apache Flink, do not allow for efficiently exposing the state to analytical queries. Thus, data engineers are forced to keep the state in external data stores, which significantly increases the latencies until events are visible to analytical queries. Proprietary solutions have been created to meet data freshness constraints. These solutions are expensive, error-prone, and difficult to maintain. Main-memory database systems, such as HyPer, achieve extremely low query response times while maintaining high update rates, which makes them well-suited for analytical streaming workloads. In this paper, we explore extensions to database systems to match the performance and usability of streaming systems.

Representations and Optimizations for Embedded Parallel Dataflow Languages

Parallel dataflow engines such as Apache Hadoop, Apache Spark, and Apache Flink are an established alternative of relational databases for modern data analysis applications. Characteristic for these systems is a scalable programming model based on distributed collections and parallel transformations expressed by means of second oder functions such as map and reduce. Notable examples are Flink's DataSet and Spark's RDD programming abstractions. These programming models are realized as eDSLs -- domain specific languages embedded in a general-purpose host language such as Java, Scala, or Python. This approach has several advantages over traditional stand-alone DSLs such as SQL or XQuery. First, linguistic constructs from the host language (e.g. anonymous functions syntax, value definitions, and fluent syntax via method chaining) can be reused in the eDSL. This eases the learning curve for developers already familiar with the host language. Second, it allows for seamless integration of library methods written in the host language via the function parameters passed to the parallel dataflow operators. This reduces the effort for developing analytics dataflows that go beyond pure SQL and require domain-specific logic. At the same time, however, state-of-the-art parallel dataflow eDSLs exhibit a number of shortcomings. First, one of the main advantages of a stand-alone DSL such as SQL -- the high-level, declarative Select-From-Where syntax -- is either lost completely or mimicked in a non-standard way. Second, execution aspects such as caching, join order, and partial aggregation have to be decided by the programmer. Optimizing them automatically is not possible due to the limited program context available in the intermediate representation of the DSL. In this paper, we argue that the limitations listed above are a side effect of the type-based embedding approach adopted by these state-of-the-art eDSLs. As a solution, we propose an alternative eDSL design based on quotations. We present a DSL embedded in Scala and discuss its compiler pipeline, intermediate representation, and some of the enabled optimizations. We promote the algebraic type of bags in union representation as a model for distributed collections, and its associated structural recursion scheme and monad for parallel collection processing. At the source code level, Scala's comprehension syntax over the bags monad can be used to encode Select-From-Where expressions in a standard way. At the intermediate representation level, maintaining comprehensions as a first-class citizen can be used to simplify the design and implementation of holistic dataflow optimizations that accommodate for nesting and control-flow. The proposed DSL design therefore reconciles the benefits of embedded parallel dataflow DSLs with the declarativity and optimization potential of stand-alone DSLs like SQL.

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