Abstract
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¬ (the fragment of Datalog¬ where all rules are connected) provides an effective syntax for Datalog¬ 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 et al. 2012], namely that the classic win-move query is in F 2 (a particular class of coordination-free queries).
Highlights
A Datalog program is called connected when the graph of every rule is connected; here, the graph of a rule views the variables of the rule as vertices and each positive body atom as a hyperedge
Ameloot et al [4] obtained that every connected stratified Datalog¬ program distributes over components, that is, the program can be evaluated by taking the union of the query results over the connected components of the database instance
We have shown that membership of positive Datalog programs in D is undecidable, connected Datalog¬ provides an effective syntax for Datalog¬ ∩ D both under the stratified as well as under the well-founded semantics
Summary
A Datalog program is called connected when the graph of every rule is connected; here, the graph of a rule views the variables of the rule as vertices and each positive body atom as a hyperedge. Ameloot et al [4] obtained that every connected stratified Datalog¬ program distributes over components, that is, the program can be evaluated by taking the union of the query results over the connected components of the database instance (cf Section 3 for a formal definition).
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