View determinacy for preserving selected information in data transformations

Abstract

When transforming data one often wants certain information in the data source to be preserved, i.e., we identify parts of the source data and require these parts to be transformed without loss of information. We characterize the preservation of selected information in terms of the notions of invertibility and query preservation, in a setting when transformations are specified as a view V (a set of queries), and source information is selected by a query Q. We investigate the problem for determining whether transformations V preserve the information selected by Q. (1) We show that the notion of invertibility coincides with view determinacy studied for query rewriting. (2) We establish the undecidability of the problem when either Q or V is in DATALOG or first-order logic, for invertibility and query preservation. (3) When Q and V are conjunctive queries (CQ), the problem is as hard as view determinacy for CQ queries and CQ views, an open problem. Nevertheless, we provide complexity bounds of the problem, either in PTIME or NP-complete, when V ranges over subclasses of CQ (i.e., SP, SC, PC), and when Q is assumed to be a minimal CQ query or not. (4) We show that CQ is complete for L-to-CQ rewriting when L is SP, SC or PC, i.e., every CQ query can be rewritten in terms of SP, SC or PC views using a query in CQ.

Determinacy and Subsumption of Single-Valued Bottom-Up Tree Transducers

“…Determinacy and subsumption (or query rewriting) have been well studied mainly for relational queries such as firstorder logic and conjunctive queries [2]- [4]. In XML context, the problems has been studied in a setting where transformations and queries are modeled by tree transducers [6].…”

“…Case (3) s 1 or s 2 contains variable x * and y 1 = z 1 . Because y 1 = z 1 , by top cancellation (Proposition 3), t 5 = t [4,5] or t (m) 2 has no variable. If t 5 = t [4,5] then t 0 t [1,3] t 5 = t 0 t [1,5] .…”

“…Because y 1 = z 1 , by top cancellation (Proposition 3), t 5 = t [4,5] or t (m) 2 has no variable. If t 5 = t [4,5] then t 0 t [1,3] t 5 = t 0 t [1,5] . If t (m) 2 has no variable then we also have t 0 t [1,3] t 5 = t 0 t [1,2] = t 0 t [1,5] .…”

“…Especially, it is desirable to guarantee that information is preserved in some sense through data transformation. As a formalization for information preservation in data transformation, the notions of determinacy and subsumption (or query rewriting) are known [2]- [4]. Let Q be a query to a database and V be a data transformation (or a view definition) of the database.…”

Determinacy and Subsumption for Single-Valued Bottom-Up Tree Transducers

“…In data transformation, it is desirable that certain information in source data be preserved through transformation. As a formalization for information preservation in data transformation, the notions of deteminacy and subsumption (or query rewriting) are known [1][2][3]. Let Q be a query to a database and V be a data transformation (or a view definition) of the database.…”