Cleaning Denial Constraint Violations through Relaxation

Data cleaning is a time-consuming process that depends on the data analysis that users perform. Existing solutions treat data cleaning as a separate offline process that takes place before analysis begins. Applying data cleaning before analysis assumes a priori knowledge of the inconsistencies and the query workload, thereby requiring effort on understanding and cleaning the data that is unnecessary for the analysis. We propose an approach that performs probabilistic repair of denial constraint violations on-demand, driven by the exploratory analysis that users perform. We introduce Daisy, a system that seamlessly integrates data cleaning into the analysis by relaxing query results. Daisy executes analytical query-workloads over dirty data by weaving cleaning operators into the query plan. Our evaluation shows that Daisy adapts to the workload and outperforms traditional offline cleaning on both synthetic and real-world workloads.Comment: To appear in SIGMOD 2020 proceeding

Rapidash: Efficient Constraint Discovery via Rapid Verification

Denial Constraint (DC) is a well-established formalism that captures a wide range of integrity constraints commonly encountered, including candidate keys, functional dependencies, and ordering constraints, among others. Given their significance, there has been considerable research interest in achieving fast verification and discovery of exact DCs within the database community. Despite the significant advancements in the field, prior work exhibits notable limitations when confronted with large-scale datasets. The current state-of-the-art exact DC verification algorithm demonstrates a quadratic (worst-case) time complexity relative to the dataset's number of rows. In the context of DC discovery, existing methodologies rely on a two-step algorithm that commences with an expensive data structure-building phase, often requiring hours to complete even for datasets containing only a few million rows. Consequently, users are left without any insights into the DCs that hold on their dataset until this lengthy building phase concludes. In this paper, we introduce Rapidash, a comprehensive framework for DC verification and discovery. Our work makes a dual contribution. First, we establish a connection between orthogonal range search and DC verification. We introduce a novel exact DC verification algorithm that demonstrates near-linear time complexity, representing a theoretical improvement over prior work. Second, we propose an anytime DC discovery algorithm that leverages our novel verification algorithm to gradually provide DCs to users, eliminating the need for the time-intensive building phase observed in prior work. To validate the effectiveness of our algorithms, we conduct extensive evaluations on four large-scale production datasets. Our results reveal that our DC verification algorithm achieves up to 40 times faster performance compared to state-of-the-art approaches.Comment: comments and suggestions are welcome

Discovery and application of data dependencies

Orientador: Prof. Dr. Eduardo Cunha de AlmeidaTese (doutorado) - Universidade Federal do Paraná, Setor de Ciências Exatas, Programa de Pós-Graduação em Informática. Defesa : Curitiba, 08/09/2020Inclui referências: p. 126-140Área de concentração: Ciência da ComputaçãoResumo: D ependências de dados (ou, simplesmente, dependências) têm um papel fundamental em muitos aspectos do gerenciam ento de dados. Em consequência, pesquisas recentes têm desenvolvido contribuições para im portante problem as relacionados à dependências. Esta tese traz contribuições que abrangem dois desses problemas. O prim eiro problem a diz respeito à descoberta de dependências com alto poder de expressividade. O objetivo é substituir o projeto m anual de dependências, o qual é sujeito a erros, por um algoritmo capaz de descobrir dependências a partir de dados apenas. N esta tese, estudamos a descoberta de restrições de negação, um tipo de dependência que contorna muitos problemas relacionados ao poder de expressividade de depêndencias. As restrições de negação têm poder de expressividade suficiente para generalizar outros tipos importantes de dependências, e expressar com plexas regras de negócios. No entanto, sua descoberta é com putacionalm ente difícil, pois possui um espaço de busca m aior do que o espaço de busca visto na descoberta de dependências mais simples. Esta tese apresenta novas técnicas na forma de um algoritmo para a descoberta de restrições de negação. Avaliamos o projeto de nosso algoritmo em uma variedade de cenários: conjuntos de dados reais e sintéticos; e núm eros variáveis de registros e colunas. N ossa avaliação m ostra que, em com paração com soluções do estado da arte, nosso algoritmo m elhora significativamente a eficiência da descoberta de restrição de negação em term os de tempo de execução. O segundo problem a diz respeito à aplicação de dependências no gerenciam ento de dados. Primeiro, estudamos a aplicação de dependências na melhoraria da consistência de dados, um aspecto crítico da qualidade dos dados. Uma m aneira comum de m odelar inconsistências é identificando violações de dependências. N esse contexto, esta tese apresenta um m étodo que estende nosso algoritm o para a descoberta de restrições de negação de form a que ele possa retornar resultados confiáveis, m esm o que o algoritm o execute sobre dados contendo alguns registros inconsistentes. M ostram os que é possível extrair evidências dos conjuntos de dados para descobrir restrições de negação que se mantêm aproximadamente. Nossa avaliação mostra que nosso método retorna dependências de negação que podem identificar, com boa precisão e recuperação, inconsistências no conjunto de dados de entrada. Esta tese traz mais um a contribuição no que diz respeito à aplicação de dependências para m elhorar a consistência de dados. Ela apresenta um sistem a para detectar violações de dependências de form a eficiente. Realizam os um a extensa avaliação de nosso sistem a usando comparações com várias abordagens; dados do mundo real e sintéticos; e vários tipos de restrições de negação. Mostramos que os sistemas de gerenciamento de banco de dados comerciais testados com eçam a apresentar baixo desem penho para conjuntos de dados relativam ente pequenos e alguns tipos de restrições de negação. Nosso sistema, por sua vez, apresenta execuções até três ordens de magnitude mais rápidas do que as de outras soluções relacionadas, especialmente para conjuntos de dados maiores e um grande número de violações identificadas. N ossa contribuição final diz respeito à aplicação de dependências na otim ização de consultas. Em particular, esta tese apresenta um sistema para a descoberta automática e seleção de dependências funcionais que potencialmente melhoram a execução de consultas. Nosso sistema com bina representações das dependências funcionais descobertas em um conjunto de dados com representações extraídas de cargas de trabalho de consulta. Essa com binação direciona a seleção de dependências funcionais que podem produzir reescritas de consulta para as consultas de entrada. N ossa avaliação experim ental m ostra que nosso sistem a seleciona dependências funcionais relevantes que podem ajudar na redução do tempo de resposta geral de consultas. Palavras-chave: Perfilamento de dados. Qualidade de dados. Limpeza de dados. Depenência de dados. Execução de consulta.Abstract: Data dependencies (or dependencies, for short) have a fundamental role in many facets of data management. As a result, recent research has been continually driving contributions to central problem s in connection w ith dependencies. This thesis makes contributions that reach two of these problems. The first problem regards the discovery of dependencies of high expressive power. The goal is to replace the error-prone process of m anual design of dependencies with an algorithm capable of discovering dependencies using only data. In this thesis, we study the discovery of denial constraints, a type of dependency that circumvents many expressiveness drawbacks. Denial constraints have enough expressive pow er to generalize other im portant types of dependencies and to express com plex business rules. However, their discovery is com putationally hard since it regards a search space that is bigger than the search space seen in the discovery of sim pler dependencies. This thesis introduces novel algorithm ic techniques in the form of an algorithm for the discovery of denial constraints. We evaluate the design of our algorithm in a variety of scenarios: real and synthetic datasets; and a varying num ber of records and columns. Our evaluation shows that, com pared to state-of-the-art solutions, our algorithm significantly improves the efficiency of denial constraint discovery in terms of runtime. The second problem concerns the application of dependencies in data management. We first study the application of dependencies for improving data consistency, a critical aspect of data quality. A com m on way to m odel data inconsistencies is by identifying violations of dependencies. in that context, this thesis presents a m ethod that extends our algorithm for the discovery of denial constraints such that it can return reliable results even if the algorithm runs on data containing some inconsistent records. A central insight is that it is possible to extract evidence from datasets to discover denial constraints that alm ost hold in the dataset. Our evaluation shows that our method returns denial dependencies that can identify, with good precision and recall, inconsistencies in the input dataset. This thesis makes one m ore contribution regarding the application of dependencies for im proving data consistency. it presents a system for detecting violations of dependencies efficiently. We perform an extensive evaluation of our system that includes comparisons with several different approaches; real-world and synthetic data; and various kinds of denial constraints. We show that the tested com m ercial database m anagem ent systems start underperform ing for relatively small datasets and production dependencies in the form of denial constraints. Our system, in turn, is up to three orders-of-m agnitude faster than related solutions, especially for larger datasets and massive numbers of identified violations. Our final contribution regards the application of dependencies in query optimization. In particular, this thesis presents a system for the automatic discovery and selection of functional dependencies that potentially improve query executions. Our system combines representations from the functional dependencies discovered in a dataset with representations of the query workloads that run for that dataset. This combination guides the selection of functional dependencies that can produce query rewritings for the incoming queries. Our experimental evaluation shows that our system selects relevant functional dependencies, which can help in reducing the overall query response time. Keywords: D ata profiling. D ata quality. D ata cleaning. D ata dependencies. Query execution

Scaling Machine Learning Data Repair Systems for Sparse Datasets

Machine learning data repair systems (e.g. HoloClean) have achieved state-of-the-art performance for the data repair problem on many datasets. However, these systems face significant challenges with sparse datasets. In this work, the challenges presented by such datasets to machine learning data repair systems are investigated. Dataset-independent methods are presented to mitigate the effects of data sparseness. Finally, experimental results are validated on a large, sparse real-world dataset: Census. Showing that the problem size can be reduced by more than 70%, saving significant computational costs, while still getting high accuracy data repairs (94.5% accuracy)

A comprehensive insight towards Pre-processing Methodologies applied on GPS data

Reliability in the utilization of the Global Positioning System (GPS) data demands a higher degree of accuracy with respect to time and positional information required by the user. However, various extrinsic and intrinsic parameters disrupt the data transmission phenomenon from GPS satellite to GPS receiver which always questions the trustworthiness of such data. Therefore, this manuscript offers a comprehensive insight into the data preprocessing methodologies evolved and adopted by present-day researchers. The discussion is carried out with respect to standard methods of data cleaning as well as diversified existing research-based approaches. The review finds that irrespective of a good number of work carried out to address the problem of data cleaning, there are critical loopholes in almost all the existing studies. The paper extracts open end research problems as well as it also offers an evidential insight using use-cases where it is found that still there is a critical need to investigate data cleaning methods

Scalable and Holistic Qualitative Data Cleaning

Data quality is one of the most important problems in data management, since dirty data often leads to inaccurate data analytics results and wrong business decisions. Poor data across businesses and the government cost the U.S. economy 3.1 trillion a year, according to a report by InsightSquared in 2012. Data scientists reportedly spend 60% of their time in cleaning and organizing the data according to a survey published in Forbes in 2016. Therefore, we need effective and efficient techniques to reduce the human efforts in data cleaning. Data cleaning activities usually consist of two phases: error detection and error repair. Error detection techniques can be generally classified as either quantitative or qualitative. Quantitative error detection techniques often involve statistical and machine learning methods to identify abnormal behaviors and errors. Quantitative error detection techniques have been mostly studied in the context of outlier detection. On the other hand, qualitative error detection techniques rely on descriptive approaches to specify patterns or constraints of a legal data instance. One common way of specifying those patterns or constraints is by using data quality rules expressed in some integrity constraint languages; and errors are captured by identifying violations of the specified rules. This dissertation focuses on tackling the challenges associated with detecting and repairing qualitative errors. To clean a dirty dataset using rule-based qualitative data cleaning techniques, we first need to design data quality rules that reflect the semantics of the data. Since obtaining data quality rules by consulting domain experts is usually a time-consuming processing, we need automatic techniques to discover them. We show how to mine data quality rules expressed in the formalism of denial constraints (DCs). We choose DCs as the formal integrity constraint language for capturing data quality rules because it is able to capture many real-life data quality rules, and at the same time, it allows for efficient discovery algorithm. Since error detection often requires a tuple pairwise comparison, a quadratic complexity that is expensive for a large dataset, we present a distribution strategy that distributes the error detection workload to a cluster of machines in a parallel shared-nothing computing environment. Our proposed distribution strategy aims at minimizing, across all machines, the maximum computation cost and the maximum communication cost, which are the two main types of cost one needs to consider in a shared-nothing environment. In repairing qualitative errors, we propose a holistic data cleaning technique, which accumulates evidences from a broad spectrum of data quality rules, and suggests possible data updates in a holistic manner. Compared with previous piece-meal data repairing approaches, the holistic approach produces data updates with higher accuracy because it realizes the interactions between different errors using one representation, and aims at generating data updates that can fix as many errors as possible

Enhancing Usability and Explainability of Data Systems

The recent growth of data science expanded its reach to an ever-growing user base of nonexperts, increasing the need for usability, understandability, and explainability in these systems. Enhancing usability makes data systems accessible to people with different skills and backgrounds alike, leading to democratization of data systems. Furthermore, proper understanding of data and data-driven systems is necessary for the users to trust the function of the systems that learn from data. Finally, data systems should be transparent: when a data system behaves unexpectedly or malfunctions, the users deserve proper explanation of what caused the observed incident. Unfortunately, most existing data systems offer limited usability and support for explanations: these systems are usable only by experts with sound technical skills, and even expert users are hindered by the lack of transparency into the systems\u27 inner workings and functions. The aim of my thesis is to bridge the usability gap between nonexpert users and complex data systems, aid all sort of users, including the expert ones, in data and system understanding, and provide explanations that help reason about unexpected outcomes involving data systems. Specifically, my thesis has the following three goals: (1) enhancing usability of data systems for nonexperts, (2) enable data understanding that can assist users in a variety of tasks such as achieving trust in data-driven machine learning, gaining data understanding, and data cleaning, and (3) explaining causes of unexpected outcomes involving data and data systems. For enhancing usability, we focus on example-driven user intent discovery. We develop systems based on example-driven interactions in two different settings: querying relational databases and personalized document summarization. Towards data understanding, we develop a new data-profiling primitive that can characterize tuples for which a machine-learned model is likely to produce untrustworthy predictions. We also develop an explanation framework to explain causes of such untrustworthy predictions. Additionally, this new data-profiling primitive enables interactive data cleaning. Finally, we develop two explanation frameworks, tailored to provide explanations in debugging data system components, including the data itself. The explanation frameworks focus on explaining the root cause of a concurrent application\u27s intermittent failure and exposing issues in the data that cause a data-driven system to malfunction

Modeling and Querying Uncertainty in Data Cleaning

Data quality problems such as duplicate records, missing values, and violation of integrity constrains frequently appear in real world applications. Such problems cost enterprises billions of dollars annually, and might have unpredictable consequences in mission-critical tasks. The process of data cleaning refers to detecting and correcting errors in data in order to improve the data quality. Numerous efforts have been taken towards improving the effectiveness and the efficiency of the data cleaning. A major challenge in the data cleaning process is the inherent uncertainty about the cleaning decisions that should be taken by the cleaning algorithms (e.g., deciding whether two records are duplicates or not). Existing data cleaning systems deal with the uncertainty in data cleaning decisions by selecting one alternative, based on some heuristics, while discarding (i.e., destroying) all other alternatives, which results in a false sense of certainty. Furthermore, because of the complex dependencies among cleaning decisions, it is difficult to reverse the process of destroying some alternatives (e.g., when new external information becomes available). In most cases, restarting the data cleaning from scratch is inevitable whenever we need to incorporate new evidence. To address the uncertainty in the data cleaning process, we propose a new approach, called probabilistic data cleaning, that views data cleaning as a random process whose possible outcomes are possible clean instances (i.e., repairs). Our approach generates multiple possible clean instances to avoid the destructive aspect of current cleaning systems. In this dissertation, we apply this approach in the context of two prominent data cleaning problems: duplicate elimination, and repairing violations of functional dependencies (FDs). First, we propose a probabilistic cleaning approach for the problem of duplicate elimination. We define a space of possible repairs that can be efficiently generated. To achieve this goal, we concentrate on a family of duplicate detection approaches that are based on parameterized hierarchical clustering algorithms. We propose a novel probabilistic data model that compactly encodes the defined space of possible repairs. We show how to efficiently answer relational queries using the set of possible repairs. We also define new types of queries that reason about the uncertainty in the duplicate elimination process. Second, in the context of repairing violations of FDs, we propose a novel data cleaning approach that allows sampling from a space of possible repairs. Initially, we contrast the existing definitions of possible repairs, and we propose a new definition of possible repairs that can be sampled efficiently. We present an algorithm that randomly samples from this space, and we present multiple optimizations to improve the performance of the sampling algorithm. Third, we show how to apply our probabilistic data cleaning approach in scenarios where both data and FDs are unclean (e.g., due to data evolution or inaccurate understanding of the data semantics). We propose a framework that simultaneously modifies the data and the FDs while satisfying multiple objectives, such as consistency of the resulting data with respect to the resulting FDs, (approximate) minimality of changes of data and FDs, and leveraging the trade-off between trusting the data and trusting the FDs. In presence of uncertainty in the relative trust in data versus FDs, we show how to extend our cleaning algorithm to efficiently generate multiple possible repairs, each of which corresponds to a different level of relative trust

Private Data Exploring, Sampling, and Profiling

Data analytics is being widely used not only as a business tool, which empowers organizations to drive efficiencies, glean deeper operational insights and identify new opportunities, but also for the greater good of society, as it is helping solve some of world's most pressing issues, such as developing COVID-19 vaccines, fighting poverty and climate change. Data analytics is a process involving a pipeline of tasks over the underlying datasets, such as data acquisition and cleaning, data exploration and profiling, building statistics and training machine learning models. In many cases, conducting data analytics faces two practical challenges. First, many sensitive datasets have restricted access and do not allow unfettered access; Second, data assets are often owned and stored in silos by multiple business units within an organization with different access control. Therefore, data scientists have to do analytics on private and siloed data. There is a fundamental trade-off between data privacy and the data analytics tasks. On the one hand, achieving good quality data analytics requires understanding the whole picture of the data; on the other hand, despite recent advances in designing privacy and security primitives such as differential privacy and secure computation, when naivly applied, they often significantly downgrade tasks' efficiency and accuracy, due to the expensive computations and injected noise, respectively. Moreover, those techniques are often piecemeal and they fall short in holistically integrating into end-to-end data analytics tasks. In this thesis, we approach this problem by treating privacy and utility as constraints on data analytics. First, we study each task and express its utility as data constraints; then, we select a principled data privacy and security model for each task; and finally, we develop mechanisms to combine them into end to end analytics tasks. This dissertation addresses the specific technical challenges of trading off privacy and utility in three popular analytics tasks. The first challenge is to ensure query accuracy in private data exploration. Current systems for answering queries with differential privacy place an inordinate burden on the data scientist to understand differential privacy, manage their privacy budget, and even implement new algorithms for noisy query answering. Moreover, current systems do not provide any guarantees to the data analyst on the quality they care about, namely accuracy of query answers. We propose APEx, a generic accuracy-aware privacy query engine for private data exploration. The key distinction of APEx is to allow the data scientist to explicitly specify the desired accuracy bounds to a SQL query. Using experiments with query benchmarks and a case study, we show that APEx allows high exploration quality with a reasonable privacy loss. The second challenge is to preserve the structure of the data in private data synthesis. Existing differentially private data synthesis methods aim to generate useful data based on applications, but they fail in keeping one of the most fundamental data properties of the structured data — the underlying correlations and dependencies among tuples and attributes. As a result, the synthesized data is not useful for any downstream tasks that require this structure to be preserved. We propose Kamino, a data synthesis system to ensure differential privacy and to preserve the structure and correlations present in the original dataset. We empirically show that while preserving the structure of the data, Kamino achieves comparable and even better usefulness in applications of training classification models and answering marginal queries than the state-of-the-art methods of differentially private data synthesis. The third challenge is efficient and secure private data profiling. Discovering functional dependencies (FDs) usually requires access to all data partitions to find constraints that hold on the whole dataset. Simply applying general secure multi-party computation protocols incurs high computation and communication cost. We propose SMFD to formulate the FD discovery problem in the secure multi-party scenario, and design secure and efficient cryptographic protocols to discover FDs over distributed partitions. Experimental results show that SMFD is practically efficient over non-secure distributed FD discovery, and can significantly outperform general purpose multi-party computation framework