The constraint database framework: lessons learned from CQA/CDB

We describe our experience with CQA/CDB, a prototype rational linear constraint database. First, we show that the standard semantics of constraint databases lead to an anomaly when queried in the presence of missing attributes. In CQA/CDB, this anomaly is avoided by enriching the CDB relational schema, resulting in heterogenous databases. Then, we present spatial databases as a special case of heterogenous databases and extend constraint query algebras (CQAs) with two additional spatial operators, proving that the resulting language is safe for linear constraints.