Computing queries from derived relations

摘要: Assume that a set of derived relations are available in stored form. Given query (or subquery), can it be computed from the rela- tions and, if so, how? Variants this problem arise several areas processing. Relation fragments at site distributed database system, data- base snapshots, and intermediate results obtained dur- ing processing all examples stored, relations. In paper we give neces- sary sufficient conditions for when is com- putable single relation. It assumed both relation defined by PSJ-expressions, is, relational algebra expressions involving only projections, selections, joins, any combination. The solution constructive: not does tell whether computable or not, but also shows how to compute it.