.In data applications such as information integration, there can be limited access patterns to relations, i.e., binding patterns require values to be specified for certain attributes in order to retrieve data from a relation. As a consequence, we cannot retrieve all tuples from these relations. In this article we study the problem of computing the complete answer to a query, i.e., the answer that could be computed if all the tuples could be retrieved. A query is stable if for any instance of the relations in the query, its complete answer can be computed using the access patterns permitted by the relations. We study the problem of testing stability of various classes of queries, including conjunctive queries, unions of conjunctive queries, and conjunctive queries with arithmetic comparisons. We give algorithms and complexity results for these classes of queries. We show that stability of datalog programs is undecidable, and give a sufficient condition for stability of datalog queries. Finally, we study data-dependent computability of the complete answer to a nonstable query, and propose a decision tree for guiding the process to compute the complete answer.

Computing complete answers to queries in the presence of limited access patterns

In many applications it is important to find a meaningful relationship between the schemas of a source and target database. This relationship is expressed in terms of declarative logical expressions called schema mappings. The more successful previous solutions have relied on inputs such as simple element correspondences between schemas in addition to local schema constraints such as keys and referential integrity. In this paper, we investigate the use of an alternate source of information about schemas, namely the presumed presence of semantics for each table, expressed in terms of a conceptual model (CM) associated with it. Our approach first compiles each CM into a graph and represents each table's semantics as a subtree in it. We then develop algorithms for discovering subgraphs that are plausible connections between those concepts/nodes in the CM graph that have attributes participating in element correspondences. A conceptual mapping candidate is now a pair of source and target subgraphs which are semantically similar. At the end, these are converted to expressions at the database level. We offer experimental results demonstrating that, for test cases of non-trivial mapping expressions involving schemas from a number of domains, the "semantic" approach outperforms the traditional technique in terms of recall and especially precision.

/pdf/a-semantic-approach-to-discovering-schema-mapping-7khzw8okji.pdf

A Semantic Approach to Discovering Schema Mapping Expressions

One of the problems arising upon the evolution of a database schema is that some queries and views defined on the previous schema version might no longer work properly. Thus, evolving a database schema entails the redefinition of queries and views to adapt them to the new schema. Although this problem has been mainly raised in the context of traditional information systems, solutions to it are also advocated in other database-related areas, such as Data Integration, Web Data Integration, and Data Warehouses. The problem is a critical one, since industrial organizations often need to adapt their databases and data warehouses to frequent changes in the real world. In this article, we provide a survey of existing approaches and tools to the problem of adapting queries and views upon a database schema evolution; we also propose a classification framework to enable a uniform comparison method among many heterogeneous approaches and tools.

https://dl.acm.org/doi/pdf/10.1145/2903726

Synchronization of Queries and Views Upon Schema Evolutions: A Survey

This paper describes a round-trip engineering approach for incrementally maintaining mappings between conceptual models and relational schemas. When either schema or conceptual model evolves to accommodate new information needs, the existing mapping must be maintained accordingly to continuously provide valid services. In this paper, the authors examine the mappings specifying "consistent" relationships between models. First, they define the consistency of a conceptual-relational mapping through "semantically compatible" instances. Next, the authors analyze the knowledge encoded in the standard database design process and develop round-trip algorithms for incrementally maintaining the consistency of conceptual-relational mappings under evolution. Finally, they conduct a set of comprehensive experiments. The results show that the proposed solution is efficient and provides significant benefits in comparison to the mapping reconstructing approach.

Maintaining Mappings between Conceptual Models and Relational Schemas

Previous works in data integration can be classified according to the approach used to define objects at the mediation level. One of these approaches is called global-as-view (GAV) and requires that each object is expressed as a view (a mediation query) on the data sources. One important limit of this approach is the management of the evolutions in the system. Indeed, each time a change occurs at the source schema level, all the queries defining the mediation objects have to be reconsidered and possibly redefined. In this paper, we propose an approach to cope with the evolution of mediation queries. Our claim is that if the definition of mediation queries in a GAV context follows a well-defined methodology, handling the evolution of the system becomes easier. These evolution problems are considered in the context of a methodology we have previously defined for generating mediation queries. Our solution is based on the concept of relevant relations on which propagation rules have been defined.

/pdf/managing-the-evolution-of-mediation-queries-4oe56dec6b.pdf

Managing the evolution of mediation queries

A Multi-Source Information System is composed of a set of independent data sources and a set of views defined as queries over these data sources This problem is dramatically increased in evolving information systems, such as data warehouses and Web systems, where several views are daily defined or modified by users who are not aware of the detailed metadata describing data sources and their interrelationships The purpose of the paper is to propose a design aid which helps in the definition of a user view (or query) from its schema and some integrity constraints Our approach defines a solution space which provides the set of potential queries that correspond to the user view The approach is based on the existence of metadata describing individual sources, on semantic assertions describing inter-source similarities between concepts, and on some heuristics which reduce the size of the solution space

Z. Kedad

Papers

Discovering view expressions from a multi-source information system