7 research outputs found
Toward relevant answers to queries on incomplete databases
Incomplete and uncertain information is ubiquitous in database management applications. However, the techniques specifically developed to handle incomplete data are
not sufficient. Even the evaluation of SQL queries on databases containing NULL
values remains a challenge after 40 years. There is no consensus on what an answer
to a query on an incomplete database should be, and the existing notions often have
limited applicability.
One of the most prevalent techniques in the literature is based on finding answers
that are certainly true, independently of how missing values are interpreted. However,
this notion has yielded several conflicting formal definitions for certain answers. Based
on the fact that incomplete data can be enriched by some additional knowledge, we
designed a notion able to unify and explain the different definitions for certain answers.
Moreover, the knowledge-preserving certain answers notion is able to provide the first
well-founded definition of certain answers for the relational bag data model and value-inventing queries, addressing some key limitations of previous approaches. However,
it doesnât provide any guarantee about the relevancy of the answers it captures.
To understand what would be relevant answers to queries on incomplete databases,
we designed and conducted a survey on the everyday usage of NULL values among
database users. One of the findings from this socio-technical study is that even when
users agree on the possible interpretation of NULL values, they may not agree on
what a satisfactory query answer is. Therefore, to be relevant, query evaluation on
incomplete databases must account for usersâ tasks and preferences.
We model usersâ preferences and tasks with the notion of regret. The regret function
captures the task-dependent loss a user endures when he considers a database as
ground truth instead of another. Thanks to this notion, we designed the first framework
able to provide a score accounting for the risk associated with query answers. It allows
us to define the risk-minimizing answers to queries on incomplete databases. We
show that for some regret functions, regret-minimizing answers coincide with certain
answers. Moreover, as the notion is more agile, it can capture more nuanced answers
and more interpretations of incompleteness.
A different approach to improve the relevancy of an answer is to explain its provenance.
We propose to partition the incompleteness into sources and measure their respective contribution to the risk of answer. As a first milestone, we study several models
to predict the evolution of the risk when we clean a source of incompleteness. We
implemented the framework, and it exhibits promising results on relational databases
and queries with aggregate and grouping operations. Indeed, the model allows us
to infer the risk reduction obtained by cleaning an attribute. Finally, by considering a
game theoretical approach, the model can provide an explanation for answers based
on the contribution of each attributes to the risk
Troubles with Nulls, Views from the Users
International audienceIncomplete data, in the form of null values, has been extensively studied since the inception of the relational model in the 1970s. Anecdotally, one hears that the way in which SQL, the standard language for relational databases, handles nulls creates a myriad of problems in everyday applications of database systems. To the best of our knowledge, however, the actual shortcomings of SQL in this respect, as perceived by database practitioners, have not been systematically documented, and it is not known if existing research results can readily be used to address the practical challenges. Our goal is to collect and analyze the shortcomings of nulls and their treatment by SQL, and to re-evaluate existing research in this light. To this end, we designed and conducted a survey on the everyday usage of null values among database users. From the analysis of the results we reached two main conclusions. First, null values are ubiquitous and relevant in real-life scenarios, but SQL's features designed to deal with them cause multiple problems. The severity of these problems varies depending on the SQL features used, and they cannot be reduced to a single issue. Second, foundational research on nulls is misdirected and has been addressing problems of limited practical relevance. We urge the community to view the results of this survey as a way to broaden the spectrum of their researches and further bridge the theory-practice gap on null values
A Semantics-Based Approach to Design of Query Languages for Partial Information
Most of work on partial information in databases asks which operations of standard languages, like relational algebra, can still be performed correctly in the presence of nulls. In this paper a different point of view is advocated. We believe that the semantics of partiality must be clearly understood and it should give us new design principles for languages for databases with partial information. There are different sources of partial information, such as missing information and conflicts that occur when different databases are merged. In this paper, we develop a common semantic framework for them which can be applied in a context more general than the flat relational model. This ordered semantics, which is based on ideas used in the semantics of programming languages, cleanly intergrates all kinds of partial information and serves as a tool to establish connections between them. Analyzing properties of semantic domains of types suitable for representing partial information, we come up with operations that are naturally associated with those types, and we organize programming syntax around these operations. We show how the languages that we obtain can be used to ask typical queries about incomplete information in relational databases, and how they can express some previously proposed languages. Finally, we discuss a few related topics such as mixing traditional constraints with partial information and extending semantics and languages to accommodate bags and recursive types