Datalog Queries Distributing over Components

We investigate the class D of queries that distribute over components. These are the queries that can be evaluated by taking the union of the query results over the connected components of the database instance. We show that it is undecidable whether a (positive) Datalog program distributes over components. Additionally, we show that connected Datalog with Negation (the fragment of Datalog with Negation where all rules are connected) provides an effective syntax for Datalog with Negation programs that distribute over components under the stratified as well as under the well-founded semantics. As a corollary, we obtain a simple proof for one of the main results in previous work [Zinn, Green, and Ludäscher, ICDT2012], namely, that the classic win-move query is in F_2 (a particular class of coordination-free queries).