# Publications

2012
R. Barber, et al., “Business Analytics in (a) Blink,” IEEE Data Engineering Bulletin, vol. 35, no. 1, pp. 9-14, 2012.Abstract

2011
Too Many Links in the Horizon; What is Next? Linked Views and Linked History,” in Proceedings of the 10th International Semantic Web Conference (ISWC), Bonn, Germany, 2011.Abstract

The trend for more online linked data becomes stronger. Foreseeing a future where “everything” will be online and linked, we ask the critical question; what is next? We envision that managing, querying and storing large amounts of links and data is far from yet another query processing task. We highlight two distinct and promising research directions towards managing and making sense of linked data. We introduce linked views to help focusing on specific link and data instances and linked history to help observe how links and data change over time.

S. Idreos, I. Alagiannis, R. Johnson, and A. Ailamaki, “Here are my Data Files. Here are my Queries. Where are my Results?” in Proceedings of the 5th International Conference on Innovative Data Systems Research (CIDR), Asilomar, California, 2011, pp. 57-68.Abstract
Database management systems (DBMS) provide incredible flexibility and performance when it comes to query processing, scalability and accuracy. To fully exploit DBMS features, however, the user must define a schema, load the data, tune the system for the expected workload, and answer several questions. Should the database use a column-store, a row-store or some hybrid format? What indices should be created? All these questions make for a formidable and time-consuming hurdle, often deterring new applications or imposing high cost to existing ones. A characteristic example is that of scientific databases with huge data sets. The prohibitive initialization cost and complexity still forces scientists to rely on ancient" tools for their data management tasks, delaying scientific understanding and progress. Users and applications collect their data in flat files, which have traditionally been considered to be outside" a DBMS. A DBMS wants control: always bring all data inside", replicate it and format it in its own secret" way. The problem has been recognized and current efforts extend existing systems with abilities such as reading information from flat files and gracefully incorporating it into the processing engine. This paper proposes a new generation of systems where the only requirement from the user is a link to the raw data files. Queries can then immediately be fired without preparation steps in between. Internally and in an abstract way, the system takes care of selectively, adaptively and incrementally providing the proper environment given the queries at hand. Only part of the data is loaded at any given time and it is being stored and accessed in the format suitable for the current workload.
P. Bonnet, et al., “Repeatability and workability evaluation of SIGMOD 2011,” SIGMOD Record, vol. 40, pp. 45-48, 2011.Abstract
SIGMOD has offered, since 2008, to verify the experiments published in the papers accepted at the conference. This year, we have been in charge of reproducing the experiments provided by the authors (repeatability), and exploring changes to experiment parameters (workability). In this paper, we assess the SIGMOD repeatability process in terms of participation, review process and results. While the participation is stable in terms of number of submissions, we find this year a sharp contrast between the high participation from Asian authors and the low participation from American authors. We also find that most experiments are distributed as Linux packages accompanied by instructions on how to setup and run the experiments. We are still far from the vision of executable papers.
M. L. Kersten, S. Idreos, S. Manegold, and E. Liarou, “The Researcher's Guide to the Data Deluge: Querying a Scientific Database in Just a Few Seconds,” Proceedings of the Very Large Databases Endowment (PVLDB), vol. 4, no. 12, pp. 1474-1477, 2011.Abstract
There is a clear need nowadays for extremely large data processing. This is especially true in the area of scientific data management where soon we expect data inputs in the order of multiple Petabytes. However, current data management technology is not suitable for such data sizes. In the light of such new database applications, we can rethink some of the strict requirements database systems adopted in the past. We argue that correctness is such a critical property, responsible for performance degradation. In this paper, we propose a new paradigm towards building database kernels that may produce wrong but fast, cheap and indicative results. Fast response times is an essential component of data analysis for exploratory applications; allowing for fast queries enables the user to develop a feeling" for the data through a series of painless" queries which eventually leads to more detailed analysis in a targeted data area. We propose a research path where a database kernel autonomously and on-the-fly decides to reduce the processing requirements of a running query based on workload, hardware and environmental parameters. It requires a complete redesign of database operators and query processing strategy. For example, typical and very common scenarios were query processing performance degrades significantly are cases where a database operator has to spill data to disk, or is forced to perform random access, or has to follow long linked lists, etc. Here we ask the question: What if we simply avoid these steps, ignoring" the side-effect in the correctness of the result?
S. Idreos, S. Manegold, H. A. Kuno, and G. Graefe, “Merging What's Cracked, Cracking What's Merged: Adaptive Indexing in Main-Memory Column-Stores,” Proceedings of the Very Large Databases Endowment (PVLDB), vol. 4, pp. 585-597, 2011.Abstract
Adaptive indexing is characterized by the partial creation and refinement of the index as side effects of query execution. Dynamic or shifting workloads may benefit from preliminary index structures focused on the columns and specific key ranges actually queried --- without incurring the cost of full index construction. The costs and benefits of adaptive indexing techniques should therefore be compared in terms of initialization costs, the overhead imposed upon queries, and the rate at which the index converges to a state that is fully-refined for a particular workload component. Based on an examination of database cracking and adaptive merging, which are two techniques for adaptive indexing, we seek a hybrid technique that has a low initialization cost and also converges rapidly. We find the strengths and weaknesses of database cracking and adaptive merging complementary. One has a relatively high initialization cost but converges rapidly. The other has a low initialization cost but converges relatively slowly. We analyze the sources of their respective strengths and explore the space of hybrid techniques. We have designed and implemented a family of hybrid algorithms in the context of a column-store database system. Our experiments compare their behavior against database cracking and adaptive merging, as well as against both traditional full index lookup and scan of unordered data. We show that the new hybrids significantly improve over past methods while at least two of the hybrids come very close to the ideal performance'' in terms of both overhead per query and convergence to a final state.
R. Barber, et al., “Blink: Not Your Father's Database!” in Proceedings of the 5th International Workshop on Business Intelligence for the Real-Time Enterprises (BIRTE), 2011, pp. 1-22.Abstract

2010
S. Idreos, “Database Cracking: Towards Auto-tuning Database Kernels,” 2010.Abstract
G. Graefe, S. Idreos, H. A. Kuno, and S. Manegold, “Benchmarking Adaptive Indexing,” in Proceedings of the 2nd TPC Technology Conference on Performance Evaluation & Benchmarking (TPCTC), Singapore, 2010, pp. 169-184.Abstract
Ideally, realizing the best physical design for the current and all subsequent workloads would impact neither performance nor storage usage. In reality, workloads and datasets can change dramatically over time and index creation impacts the performance of concurrent user and system activity. We propose a framework that evaluates the key premise of adaptive indexing -- a new indexing paradigm where index creation and re-organization take place automatically and incrementally, as a side-effect of query execution. We focus on how the incremental costs and benefits of dynamic reorganization are distributed across the workload's lifetime. We believe measuring the costs and utility of the stages of adaptation are relevant metrics for evaluating new query processing paradigms and comparing them to traditional approaches.
S. Idreos, R. Kaushik, V. R. Narasayya, and R. Ramamurthy, “Estimating the compression fraction of an index using sampling,” in Proceedings of the 22nd IEEE International Conference in Data Engineering (ICDE), Long Beach, California, 2010, pp. 441-444.Abstract
Data compression techniques such as null suppression and dictionary compression are commonly used in today’s database systems. In order to effectively leverage compression, it is necessary to have the ability to efficiently and accurately estimate the size of an index if it were to be compressed. Such an analysis is critical if automated physical design tools are to be extended to handle compression. Several database systems today provide estimators for this problem based on random sampling. While this approach is efficient, there is no previous work that analyses its accuracy. In this paper, we analyse the problem of estimating the compressed size of an index from the point of view of worst-case guarantees. We show that the simple estimator implemented by several database systems has several “good” cases even though the estimator itself is agnostic to the internals of the specific compression algorithm.
2009
E. Liarou, R. Goncalves, and S. Idreos, “Exploiting the power of relational databases for efficient stream processing,” in Proceedings of the 19th International Conference on Extending Database Technology (EDBT), Saint-Petersburg, Russia, 2009, pp. 323-334.Abstract
S. Idreos, M. L. Kersten, and S. Manegold, “Self-organizing tuple reconstruction in column-stores,” in Proceedings of the 29th ACM SIGMOD International Conference on Management of Data, Providence, Rhode Island, 2009, pp. 297-308.Abstract
Column-stores gained popularity as a promising physical design alternative. Each attribute of a relation is physically stored as a separate column allowing queries to load only the required attributes. The overhead incurred is on-the-fly tuple reconstruction for multi-attribute queries. Each tuple reconstruction is a join of two columns based on tuple IDs, making it a significant cost component. The ultimate physical design is to have multiple presorted copies of each base table such that tuples are already appropriately organized in multiple different orders across the various columns. This requires the ability to predict the workload, idle time to prepare, and infrequent updates. In this paper, we propose a novel design, partial sideways cracking, that minimizes the tuple reconstruction cost in a self-organizing way. It achieves performance similar to using presorted data, but without requiring the heavy initial presorting step itself. Instead, it handles dynamic, unpredictable workloads with no idle time and frequent updates. Auxiliary dynamic data structures, called cracker maps, provide a direct mapping between pairs of attributes used together in queries for tuple reconstruction. A map is continuously physically reorganized as an integral part of query evaluation, providing faster and reduced data access for future queries. To enable flexible and self-organizing behavior in storage-limited environments, maps are materialized only partially as demanded by the workload. Each map is a collection of separate chunks that are individually reorganized, dropped or recreated as needed. We implemented partial sideways cracking in an open-source column-store. A detailed experimental analysis demonstrates that it brings significant performance benefits for multi-attribute queries.
2008
S. Idreos, E. Liarou, and M. Koubarakis, “Continuous multi-way joins over distributed hash tables,” in Proceedings of the 11th International Conference on Extending Database Technology (EDBT), Nantes, France, 2008, pp. 594-605.Abstract
This paper studies the problem of evaluating continuous multi-way joins on top of Distributed Hash Tables (DHTs). We present a novel algorithm, called recursive join (RJoin), that takes into account various parameters crucial in a distributed setting i.e., network traffic, query processing load distribution, storage load distribution etc. The key idea of RJoin is incremental evaluation: as relevant tuples arrive continuously, a given multi-way join is rewritten continuously into a join with fewer join operators, and is assigned continuously to different nodes of the network. In this way, RJoin distributes the responsibility of evaluating a continuous multi-way join to many network nodes by assigning parts of the evaluation of each binary join to a different node depending on the values of the join attributes. The actual nodes to be involved are decided by RJoin dynamically after taking into account the rate of incoming tuples with values equal to the values of the joined attributes. RJoin also supports sliding window joins which is a crucial feature, especially for long join paths, since it provides a mechanism to reduce the query processing state and thus keep the cost of handling incoming tuples stable. In addition, RJoin is able to handle message delays due to heavy network traffic. We present a detailed mathematical and experimental analysis of RJoin and study the performance tradeoffs that occur.
2007
E. Liarou, S. Idreos, and M. Koubarakis, “Continuous RDF Query Processing over DHTs,” in Proceedings of the 6th International Semantic Web Conference (ISWC), Busan, Korea, 2007, pp. 324-339.Abstract
We study the continuous evaluation of conjunctive triple pattern queries over RDF data stored in distributed hash tables. In a continuous query scenario network nodes subscribe with long-standing queries and receive answers whenever RDF triples satisfying their queries are published. We present two novel query processing algorithms for this scenario and analyze their properties formally. Our performance goal is to have algorithms that scale to large amounts of RDF data, distribute the storage and query processing load evenly and incur as little network traffic as possible. We discuss the various performance tradeoffs that occur through a detailed experimental evaluation of the proposed algorithms.
S. Idreos, M. L. Kersten, and S. Manegold, “Updating a cracked database,” in Proceedings of the 27th ACM SIGMOD International Conference on Management of Data, Beijing, China, 2007, pp. 413-424.Abstract
A cracked database is a datastore continuously reorganized based on operations being executed. For each query, the data of interest is physically reclustered to speed-up future access to the same, overlapping or even disjoint data. This way, a cracking DBMS self-organizes and adapts itself to the workload. So far, cracking has been considered for static databases only. In this paper, we introduce several novel algorithms for high-volume insertions, deletions and updates against a cracked database. We show that the nice performance properties of a cracked database can be maintained in a dynamic environment where updates interleave with queries. Our algorithms comply with the cracking philosophy, i.e., a table is informed on pending insertions and deletions, but only when the relevant data is needed for query processing just enough pending update actions are applied. We discuss details of our implementation in the context of an open-source DBMS and we show through a detailed experimental evaluation that our algorithms always manage to keep the cost of querying a cracked datastore with pending updates lower than the non-cracked case.
S. Idreos, M. L. Kersten, and S. Manegold, “Database Cracking,” in Proceedings of the 3rd International Conference on Innovative Data Systems Research (CIDR), Asilomar, California, 2007, pp. 68-78.Abstract
Database indices provide a non-discriminative navigational infrastructure to localize tuples of interest. Their maintenance cost is taken during database updates. In this paper, we study the complementary approach, addressing index maintenance as part of query processing using continuous physical reorganization, i.e., cracking the database into manageable pieces. The motivation is that by automatically organizing data the way users request it, we can achieve fast access and the much desired self-organized behavior. We present the first mature cracking architecture and report on our implementation of cracking in the context of a full fledged relational system. It led to a minor enhancement to its relational algebra kernel, such that cracking could be piggy-backed without incurring too much processing overhead. Furthermore, we illustrate the ripple effect of dynamic reorganization on the query plans derived by the SQL optimizer. The experiences and results obtained are indicative of a significant reduction in system complexity. We show that the resulting system is able to self-organize based on incoming requests with clear performance benefits. This behavior is visible even when the user focus is randomly shifting to different parts of the data.
2006
Z. Kaoudi, I. Miliaraki, M. Magiridou, E. Liarou, S. Idreos, and M. Koubarakis, “Semantic Grid Resource Discovery using DHTs in Atlas,” in Knowledge and Data Management in Grids, Springer, 2006.Abstract

We study the problem of resource discovery in the Semantic Grid. We show how to solve this problem by utilizing Atlas, a P2P system for the distributed storage and retrieval of RDF(S) data. Atlas is currently under development in project OntoGrid funded by FP6. Atlas is built on top of the distributed hash table Bamboo and supports pull and push querying scenarios. It inherits all the nice features of Bamboo (openness, scalability, fault-tolerance, resistance to high churn rates) and extends Bamboo's protocols for storing and querying RDF(S) data. Atlas is being used currently to realize the metadata service of S-OGSA in a fully distributed and scalable way. In this paper, we concentrate on the main features of Atlas and demonstrate its use for Semantic Grid resource discovery in an OntoGrid use case scenario.

P. - A. Chirita, S. Idreos, M. Koubarakis, and W. Nejdl, “Designing Semantic Publish/Subscribe Networks Using Super-Peers,” in Semantic Web and Peer-to-Peer, 2006, pp. 159-179.Abstract
Publish/subscribe systems are an alternative to query-based systems in cases where the same information is asked for over and over, and where clients want to get updated answers for the same query over a period of time. Recent publish/subscribe systems such as P2P-DIET have introduced this paradigm in the P2P context. In this chapter we built on the experience gained with P2P-DIET and the Edutella super-peer infrastructure and present a semantic publish/subscribe system supporting metadata and a query language based on RDF. We define formally the basic concepts of our system and present detailed protocols for its operation.
S. Idreos, C. Tryfonopoulos, and M. Koubarakis, “Distributed Evaluation of Continuous Equi-join Queries over Large Structured Overlay Networks,” in Proceedings of the 22nd IEEE International Conference in Data Engineering (ICDE), Atlanta, Georgia, 2006, pp. 43.Abstract
We study the problem of continuous relational query processing in Internet-scale overlay networks realized by distributed hash tables. We concentrate on the case of continuous two-way equi-join queries. Joins are hard to evaluate in a distributed continuous query environment because data from more than one relations is needed, and this data is inserted in the network asynchronously. Each time a new tuple is inserted, the network nodes have to cooperate to check if this tuple can contribute to the satisfaction of a query when combined with previously inserted tuples. We propose a series of algorithms that initially index queries at network nodes using hashing. Then, they exploit the values of join attributes in incoming tuples to rewrite the given queries into simpler ones, and reindex them in the network where they might be satisfied by existing or future tuples. We present a detailed experimental evaluation in a simulated environment and we show that our algorithms are scalable, balance the storage and query processing load and keep the network traffic low.
E. Liarou, S. Idreos, and M. Koubarakis, “Evaluating Conjunctive Triple Pattern Queries over Large Structured Overlay Networks,” in Proceedings of the 5th International Semantic Web Conference (ISWC), Athens, Georgia, 2006, pp. 399-413.Abstract
We study the problem of evaluating conjunctive queries com- posed of triple patterns over RDF data stored in distributed hash tables. Our goal is to develop algorithms that scale to large amounts of RDF data, distribute the query processing load evenly and incur little network traﬃc. We present and evaluate two novel query processing algorithms with these possibly conﬂicting goals in mind. We discuss the various tradeoﬀs that occur in our setting through a detailed experimental evaluation of the proposed algorithms.