Publications

2017
N. Dayan, M. Athanassoulis, and S. Idreos, “Monkey: Optimal Navigable Key-Value Store,” in ACM SIGMOD International Conference on Management of Data, 2017.Abstract

 

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. 

 

monkeykeyvaluestore.pdf
M. S. Kester, M. Athanassoulis, and S. Idreos, “Access Path Selection in Main-Memory Optimized Data Systems: Should I Scan or Should I Probe?,” in ACM SIGMOD International Conference on Management of Data , 2017.Abstract

The advent of columnar data analytics engines fueled a series of optimizations on the scan operator. New designs include column-group storage, vectorized execution, shared scans, working directly over compressed data, and operating using SIMD and multi-core execution. Larger main memories and deeper cache hierarchies increase the efficiency of modern scans, prompting a revisit of the question of access path selection.


In this paper, we compare modern sequential scans and secondary index scans. Through detailed analytical modeling and experimentation we show that while scans have become useful in more cases than before, both access paths are still useful, and so, access path selection (APS) is still required to achieve the best performance when considering variable workloads. We show how to perform access path selection. In particular, contrary to the way traditional systems choose between scans and secondary indexes, we find that in addition to the query selectivity, the underlying hardware, and the system design, modern optimizers also need to take into account query concurrency. We further discuss the implications of integrating access path selection in a modern analytical data system. We demonstrate, both theoretically and experimentally, that using the proposed model a system can quickly perform access path selection, outperforming solutions that rely on a single access path or traditional access path models. We outline a light-weight mechanism to integrate APS into main-memory analytical systems that does not interfere with low latency queries. We also use the APS model to explain how the division between sequential scan and secondary index scan has historically changed due to hardware and workload changes, which allows for future projections based on hardware advancements.

accespathselection.pdf
S. Idreos, “The Automatic Scientist will be a Data System,” in Biennial Conference on Innovative Data Systems Research (CIDR), 2017.Abstract

For thousands of years science happens in a rather manual way. Mathematics, engineering and computer science provide the means to automate some of the laborious tasks that have to do with computation, data collection and management, and to some degree predictability. As scientific fields grow more mature, though, and scientists over-specialize a new problem appears that has to do with the core of the scientific process rather with the supporting steps. It becomes increasingly harder to be aware of all research concepts and techniques that may apply to a given problem. 

 

automaticscientistcidr2017.pdf
2016
K. Zoumpatianos, S. Idreos, and T. Palpanas, “ADS: The Adaptive Data Series Index,” The Very Large Databases Journal (VLDBJ), vol. 25, no. 6, pp. 843-866, 2016.Abstract

Numerous applications continuously produce big amounts of data series, and in several time critical scenarios analysts need to be able to query these data as soon as they become available. This, however, is not currently possible with the state-of-the-art indexing methods and for very large data series collections. In this paper, we present the first adaptive indexing mechanism, specifically tailored to solve the problem of indexing and querying very large data series collections. We present a detailed design and evaluation of our method using approximate and exact query algorithms with both synthetic and real datasets. Adaptive indexing significantly outperforms previous solutions, gracefully handling large data series collections, reducing the data to query delay: by the time state-of-the-art indexing techniques finish indexing 1 billion data series (and before answering even a single query), our method has already answered 3 ∗ 105 queries.

vldbj16-ads.pdf
S. Idreos, et al., “Past and Future Steps for Adaptive Storage Data Systems: From Shallow to Deep Adaptivity,” in International Workshop on Enabling Real-Time Business Intelligence (BIRTE 2016), 2016.Abstract

 

Datasystems with adaptive storage can autonomously change their behavior by altering how data is stored and accessed. Such systems have been studied primarily for the case of adaptive indexing to auto- matically create the right indexes at the right granularity. More recently work on adaptive loading and adaptive data layouts brought even more flexibility. We survey this work and describe the need for even deeper adaptivity that goes beyond adjusting knobs in a single architecture; instead it can adapt the fundamental architecture of a data system to drastically alter its behavior. 

 

birte2016.pdf
N. Dayan, P. Bonnet, and S. Idreos, “GeckoFTL: Scalable Flash Translation Techniques For Very Large Flash Devices,” in ACM SIGMOD International Conference on Management of Data, 2016.Abstract

The volume of metadata needed by a flash translation layer (FTL) is proportional to the storage capacity of a flash device. Ideally, this metadata should reside in the device’s integrated RAM to enable fast access. However, as flash devices scale to terabytes, the necessary volume of metadata is exceeding the available integrated RAM. Moreover, recovery time after power failure, which is proportional to the size of the metadata, is becoming impractical. The simplest solution is to persist more metadata in flash. The problem is that updating metadata in flash increases the amount of internal IOs thereby harming performance and device lifetime.

In this paper, we identify a key component of the metadata called the Page Validity Bitmap (PVB) as the bottleneck. PVB is used by the garbage-collectors of state-of-the-art FTLs to keep track of which physical pages in the device are invalid. PVB constitutes 95% of the FTL’s RAM-resident metadata, and recovering PVB after power fails takes a significant proportion of the overall recovery time. To solve this problem, we propose a page-associative FTL called GeckoFTL, whose central innovation is replacing PVB with a new data structure called Logarithmic Gecko. Logarithmic Gecko is similar to an LSM-tree in that it first logs updates and later reorganizes them to ensure fast and scalable access time. Relative to the baseline of storing PVB in flash, Logarithmic Gecko enables cheaper updates at the cost of slightly more expensive garbage-collection queries. We show that this is a good trade-off because (1) updates are intrinsically more frequent than garbage-collection queries to page validity metadata, and (2) flash writes are more expensive than flash reads. We demonstrate analytically and empirically through simulation that GeckoFTL achieves a 95% reduction in space requirements and at least a 51% reduction in recovery time by storing page validity metadata in flash while keeping the contribution to internal IO overheads 98% lower than the baseline. 

geckoftl.pdf
W. Qin and S. Idreos, “Adaptive Data Skipping in Main-Memory Systems,” in ACM SIGMOD International Conference on Management of Data, 2016.Abstract

As modern main-memory optimized data systems increasingly rely on fast scans, lightweight indexes that allow for data skipping play a crucial role in data filtering to reduce system I/O. Scans benefit from data skipping when the data order is sorted, semi-sorted, or comprised of clustered values. However data skipping loses effectiveness over arbitrary data distributions. Applying data skipping techniques over non-sorted data can significantly decrease query performance since the extra cost of metadata reads result in no corresponding scan performance gains. We introduce adaptive data skipping as a framework for structures and techniques that respond to a vast array of data distributions and query workloads. We reveal an adaptive zonemaps design and implementation on a main-memory column store prototype to demonstrate that adaptive data skipping has potential for 1.4X speedup.

adaptiveDataSkipping.pdf
M. Athanassoulis and S. Idreos, “Design Tradeoffs of Data Access Methods,” in ACM SIGMOD International Conference on Management of Data, 2016.Abstract

Database researchers and practitioners have been building methods to store, access, and update data for more than five decades. Designing access methods has been a constant effort to adapt to the ever changing underlying hardware and workload requirements. The recent explosion in data system designs – including, in addition to traditional SQL systems, NoSQL, NewSQL, and other relational and non-relational systems – makes understanding the tradeoffs of designing access methods more important than ever. Access methods are at the core of any new data system. In this tutorial we survey recent developments in access method design and we place them in the design space where each approach focuses primarily on one or a subset of read performance, update performance, and memory utilization. We discuss how to utilize designs and lessons-learned from past research. In addition, we discuss new ideas on how to build access methods that have tunable behavior, as well as, what is the scenery of open research problems.

designaccessmethods-tutorial.pdf
Z. Liu and S. Idreos, “Main Memory Adaptive Denormalization,” in ACM SIGMOD International Conference on Management of Data, 2016.Abstract

Joins have traditionally been the most expensive database operator, but they are required to query normalized schemas. In turn, normalized schemas are necessary to minimize update costs and space usage. Joins can be avoided altogether by using a denormalized schema instead of a normalized schema; this improves analytical query processing times at the tradeoff of increased update overhead, loading cost, and storage requirements.

In our work, we show that we can achieve the best of both worlds by leveraging partial, incremental, and dynamic denormalized tables to avoid join operators, resulting in fast query performance while retaining the minimized loading, update, and storage costs of a normalized schema. We introduce adaptive denormalization for modern main memory systems. We replace the traditional join operations with efficient scans over the relevant partial universal tables without incur- ring the prohibitive costs of full denormalization.

adaptivedenormalization.pdf
M. Athanassoulis, Z. Yan, and S. Idreos, “UpBit: Scalable In-Memory Updatable Bitmap Indexing,” in ACM SIGMOD International Conference on Management of Data, 2016.Abstract

Bitmap indexes are widely used in both scientific and commercial databases. They bring fast read performance for specific types of queries, such as equality and selective range queries. A major drawback of bitmap indexes, however, is that supporting updates is particularly costly. Bitmap indexes are kept compressed to minimize storage footprint; as a result, updating a bitmap index requires the expensive step of decoding and then encoding a bitvector. Today, more and more applications need support for both reads and writes, blurring the boundaries between analytical processing and transaction processing. This requires new system designs and access methods that support general updates and, at the same time, offer competitive read performance.

In this paper, we propose scalable in-memory Updatable Bitmap indexing (UpBit), which offers efficient updates, without hurting read performance. UpBit relies on two design points. First, in addition to the main bitvector for each domain value, UpBit maintains an update bitvector, to keep track of updated values. Effectively, every update can now be directed to a highly-compressible, easy-to-update bitvector. While update bitvectors double the amount of uncompressed data, they are sparse, and as a result their compressed size is small. Second, we introduce fence pointers in all update bitvectors which allow for efficient retrieval of a value at an arbitrary position. Using both synthetic and real-life data, we demonstrate that UpBit significantly outperforms state-of-the-art bitmap indexes for workloads that contain both reads and writes. In particular, compared to update-optimized bitmap index designs UpBit is 15 − 29× faster in terms of update time and 2.7× faster in terms of read performance. In addition, compared to read-optimized bitmap index designs UpBit achieves efficient and scalable updates (51 − 115× lower update latency), while allowing for comparable read performance, having up to 8% overhead.

upbit-sigmod16.pdf
P. Karras, A. Nikitin, M. Saad, R. Bhatt, D. Antyukhov, and S. Idreos, “Adaptive Indexing over Encrypted Numeric Data,” in ACM SIGMOD International Conference on Management of Data, San Francisco, 2016.Abstract

Today, outsourcing query processing tasks to remote cloud servers becomes a viable option; such outsourcing calls for encrypting data stored at the server so as to render it secure against eavesdropping adversaries and/or an honest-but-curious server itself. At the same time, to be efficiently managed, outsourced data should be indexed, and even adaptively so, as a side-effect of query pro- cessing. Computationally heavy encryption schemes render such outsourcing unattractive; an alternative, Order-Preserving Encryption Scheme (OPES), intentionally preserves and reveals the order in the data, hence is unattractive from the security viewpoint. In this paper, we propose and analyze a scheme for lightweight and indexable encryption, based on linear-algebra operations. Our scheme provides higher security than OPES and allows for range and point queries to be efficiently evaluated over encrypted numeric data, with decryption performed at the client side. We implement a prototype that performs incremental, query-triggered adaptive indexing over encrypted numeric data based on this scheme, without leaking order information in advance, and without prohibitive overhead, as our extensive experimental study demonstrates. 

Secure Adaptive Indexing
M. Athanassoulis, et al., “Designing Access Methods: The RUM Conjecture,” in International Conference on Extending Database Technology (EDBT), Bordeaux, France, 2016.Abstract

The database research community has been building methods to store, access, and update data for more than four decades. Throughout the evolution of the structures and techniques used to access data, access methods adapt to the ever changing hardware and workload requirements. Today, even small changes in the workload or the hardware lead to a redesign of access methods. The need for new designs has been increasing as data generation and workload diversification grow exponentially, and hardware advances introduce increased complexity. New workload requirements are introduced by the emergence of new applications, and data is managed by large systems composed of more and more complex and heterogeneous hardware. As a result, it is increasingly important to develop application-aware and hardware-aware access methods.

The fundamental challenges that every researcher, systems architect, or designer faces when designing a new access method are how to minimize, i) read times (R), ii) update cost (U), and iii) memory (or storage) overhead (M). In this paper, we conjecture that when optimizing the read-update-memory overheads, optimizing in any two areas negatively impacts the third. We present a simple model of the RUM overheads, and we articulate the RUM Conjecture. We show how the RUM Conjecture manifests in state-of-the-art access methods, and we envision a trend toward RUM-aware access methods for future data systems. 

RUM conjecture
2015
K. Zoumpatianos, S. Idreos, and T. Palpanas, “RINSE: Interactive Data Series Exploration with ADS+,” Proceedings of the Very Large Databases Endowment (PVLDB), vol. 8, no. 12, 2015.Abstract

Numerous applications continuously produce big amounts of data series, and in several time critical scenarios analysts need to be able to query these data as soon as they become available. An adaptive index data structure, ADS+, which is specifically tailored to solve the problem of indexing and querying very large data series collections has been recently proposed as a solution to this problem. The main idea is that instead of building the complete index over the complete data set up-front and querying only later, we interactively and adaptively build parts of the index, only for the parts of the data on which the users pose queries. The net effect is that instead of waiting for extended periods of time for the index creation, users can immediately start exploring the data series. In this work, we present a demonstration of ADS+; we introduce RINSE, a system that allows users to experience the benefits of the ADS+ adaptive index through an intuitive web interface. Users can explore large datasets and find patterns of interest, using nearest neighbor search. They can draw queries (data series) using a mouse, or touch screen, or they can select from a predefined list of data series. RINSE can scale to large data sizes, while drastically reducing the data to query delay: by the time state-of-the-art indexing techniques finish indexing 1 billion data series (and before answering even a single query), adaptive data series indexing can already answer 300K queries.

rinsevldb15.pdf
S. (L. ) Xi, O. Babarinsa, M. Athanassoulis, and S. Idreos, “Beyond the Wall: Near-Data Processing for Databases,” in Proceedings of the 11th International Workshop on Data Management on New Hardware (DaMoN), Melbourne, Australia, 2015.Abstract

The continuous growth of main memory size allows modern data systems to process entire large scale datasets in memory. The increase in memory capacity, however, is not matched by proportional decrease in memory latency, causing a mismatch for in-memory processing. As a result, data movement through the memory hierarchy is now one of the main performance bottlenecks for main memory data systems. Database systems researchers have proposed several innovative solutions to minimize data movement and to make data access patterns hardware-aware. Nevertheless, all relevant rows and columns for a given query have to be moved through the memory hierarchy; hence, movement of large data sets is on the critical path.

In this paper, we present JAFAR, a Near-Data Processing (NDP) accelerator for pushing selects down to memory in modern column-stores. JAFAR implements the select operator and allows only qualifying data to travel up the memory hierarchy. Through a detailed simulation of JAFAR hardware we show that it has the potential to provide 9x improvement for selects in column-stores. In addition, we discuss both hardware and software challenges for using NDP in database systems as well as opportunities for further NDP accelerators to boost additional relational operators. 

jafardamon2015.pdf
O. Babarinsa and S. Idreos, “JAFAR: Near-Data Processing for Databases,” in Proceedings of the ACM SIGMOD International Conference on Management of Data, Melbourne, Australia, 2015.Abstract

As main-memory sizes have grown, data systems have been able to process entire large-scale data-sets in memory. However, because memory speeds have been not been keeping pace with CPU speeds, the cost of moving data into CPU caches has begun to dominate certain operations within in-memory data systems. Recent advances in hardware architectures point to near memory computation capabilities becoming possible soon. This allows us to rethink how database systems process queries and how they split computation across the various computational units. In this paper, we present JAFAR, a near data processing accelerator for pushing selects down to memory. Through a detailed simulation of JAFAR hardware we show it has the potential to provide up to 900% improvement for select operations in modern column-stores.

jafar.pdf
S. Pantella and S. Idreos, “One Loop Does Not Fit All,” in Proceedings of the ACM SIGMOD International Conference on Management of Data, Melbourne, Australia, 2015.Abstract

Just-In-Time (JIT) compilation increasingly becomes a key technology for modern database systems. It allows the creation of code on-the-fly to perfectly match an active query. In the past, it has been argued that a query should be compiled to a single loop that performs all query actions, for example, all selects over all relevant columns. On the other hand, vectorization – a common feature in modern data systems – allows for better results by evaluating the query predicates sequentially in different tight for-loops.

In this paper, we study JIT compilation for modern in- memory column-stores in detail and we show that, contrary to the common belief that vectorization outweighs the benefits of having one loop, there are cases in which creating a single loop is actually the optimal solution. In fact, deciding between multiple or a single loop is not a static decision; instead, it depends on (per column) query selectivity. We perform our experiments on a modern column-store prototype that supports vectorization and we show that, depending on selectivity, a different code layout is optimal. When a select operator is implemented with a no-branch design, for low selectivity creating multiple loops performs better than a single loop. A single tight loop performs better otherwise. 

oneloopdoesnotfitall.pdf
A. Wasay, M. Athanassoulis, and S. Idreos, “Queriosity: Automated Data Exploration,” in Proceedings of the IEEE International Congress on Big Data, New York, USA, 2015.Abstract

Curiosity, a fundamental drive amongst higher living organisms, is what enables exploration, learning and creativity. In our increasingly data-driven world, data exploration, i.e., making sense of mounting haystacks of data, is akin to intelligence for science, business and individuals. However, modern data systems – designed for data retrieval rather than exploration – only let us retrieve data and ask if it is interesting. This makes knowledge discovery a game of hit-and-trial which can only be orchestrated by expert data scientists.

We present the vision toward Queriosity, an automated and personalized data exploration system. Designed on the principles of autonomy, learning and usability, Queriosity envisions a paradigm shift in data exploration and aims to become a a personalized “data robot” that provides a direct answer to what is interesting in a user’s data set, instead of just retrieving data. Queriosity autonomously and continuously navigates toward interesting findings based on trends, statistical properties and interactive user feedback.

queriosity.pdf
S. Idreos, “DASlab: The Data Systems Laboratory at Harvard SEAS,” ACM SIGMOD Record, 2015.Abstract

DASlab is a new laboratory at the Harvard School of Engineering and Applied Sciences (SEAS). The lab was formed in January 2014 when Stratos Idreos joined Harvard SEAS. DASlab currently consists of 3 PhD students, 1 postdoctoral researcher and 9 undergraduate researchers while it is set to double its graduate student population in the next one to two years. The lab is part of a growing community of systems and computer science researchers at Harvard; computer science faculty is scheduled to grow by 50\% in the next few years.

The main focus of DASlab is on designing data systems that (a) make it easy to extract knowledge out of increasingly diverse and growing data sets and (b) can stand the test of time.

daslab.pdf
I. Alagiannis, R. Borovica, M. Branco, S. Idreos, and A. Ailamaki, “NoDB: Efficient Query Execution on Raw Data Files,” Communications of the ACM, Research Highlights, 2015.Abstract

As data collections become larger and larger, users are faced with increasing bottlenecks in their data analysis. More data means more time to prepare and to load the data into the database before executing the desired queries. Many applications already avoid using database systems, e.g., scientific data analysis and social networks, due to the complexity and the increased data-to-query time, i.e., the time between getting the data and retrieving its first useful results. For many applications data collections keep growing fast, even on a daily basis, and this data deluge will only increase in the future, where it is expected to have much more data than what we can move or store, let alone analyze.

We here present the design and roadmap of a new paradigm in database systems, called NoDB, which do not require data loading while still maintaining the whole feature set of a modern database system. In particular, we show how to make raw data files a first-class citizen, fully integrated with the query engine. Through our design and lessons learned by implementing the NoDB philosophy over a modern DBMS, we discuss the fundamental limitations as well as the strong opportunities that such a research path brings. We identify performance bottlenecks specific for in situ processing, namely the repeated parsing and tokenizing overhead and the expensive data type conversion. To address these problems, we introduce an adaptive indexing mechanism that maintains positional information to provide efficient access to raw data files, together with a flexible caching structure. We conclude that NoDB systems are feasible to design and implement over modern DBMS, bringing an unprecedented positive effect in usability and performance.

nodb-cacm.pdf
S. Idreos, O. Papaemmanouil, and S. Chaudhuri, “Overview of Data Exploration Techniques,” in Proceedings of the ACM SIGMOD International Conference on Management of Data, Tutorial, Melbourne, Australia, 2015.Abstract

Data exploration is about efficiently extracting knowledge from data even if we do not know exactly what we are looking for. This has numerous side-effects on (a) how we design database systems at their core, i.e., at the storage and query processing layers and (b) how users or applications interact with systems.

In this tutorial, we survey recent developments in the emerging area of database systems tailored for data exploration. We discuss new ideas on how to store and access data as well as new ideas on how to interact with a data system to enable users and applications to quickly figure out which data parts are of interest. In addition, we discuss how to exploit lessons-learned from past research, the new challenges data exploration crafts, emerging applications and future directions.

exploration-tutorial.pdf

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