Book contents
- Frontmatter
- Dedication
- Contents
- Preface
- Part One GETTING STARTED
- Part Two RELATIONAL DATA EXCHANGE
- 4 The problem of relational data exchange
- 5 Existence of solutions
- 6 Good solutions
- 7 Query answering and rewriting
- 8 Alternative semantics
- 9 Endnotes to Part Two
- Part Three XML DATA EXCHANGE
- Part Four METADATA MANAGEMENT
- References
- Index
6 - Good solutions
from Part Two - RELATIONAL DATA EXCHANGE
Published online by Cambridge University Press: 05 June 2014
- Frontmatter
- Dedication
- Contents
- Preface
- Part One GETTING STARTED
- Part Two RELATIONAL DATA EXCHANGE
- 4 The problem of relational data exchange
- 5 Existence of solutions
- 6 Good solutions
- 7 Query answering and rewriting
- 8 Alternative semantics
- 9 Endnotes to Part Two
- Part Three XML DATA EXCHANGE
- Part Four METADATA MANAGEMENT
- References
- Index
Summary
As we know by now, solutions in data exchange are not unique; indeed, if a source instance has a solution under a relational mapping M, then it has infinitely many of them. But if many solutions exist, which one should we materialize? To answer this question, we must be able to distinguish good solutions from others. As we already mentioned in Section 4.2, good solutions in data exchange are usually identified with the most general solutions (see Example 4.3), whichinturn can be characterized as the universal solutions. We will see in this chapter that universal solutions admit different equivalent characterizations.
The existence of solutions does not in general imply the existence of universal solutions (in fact, checking whether universal solutions exist is an undecidable problem). However, this negative theoretical result does not pose serious problems in data exchange. Recall from the previous chapter that the class of mappings M = (Rs, Rt, Σst, Σt), such that Σt consists of a set of egds and a weakly acyclic set of tgds, is particularly well-behaved for data exchange. Indeed, for this class the existence of solutions – that is undecidable in general – can be checked in polynomial time, and, in case a solution exists, at least one solution can be efficiently computed. We will see in this chapter that for this class, the existence of solutions implies the existence of universal solutions. Furthermore, when one exists, it can be efficiently computed.
- Type
- Chapter
- Information
- Foundations of Data Exchange , pp. 56 - 74Publisher: Cambridge University PressPrint publication year: 2014