DDB Trees: A Basis for Deductive Database Explanations

The power of deductive systems in general is that programs express what should be done and not how it should be done Nevertheless deductive systems need debugging and explanation facilities Indeed their operational semantics is less abstract than the declarative semantics of the programs If users have to understand all the low level details of the operational semantics much of the bene ts of using a deductive system is lost Existing explanation systems for deductive databases produce proof trees to be shown to users Although useful proof trees give a fragmented view of query evaluations and users face a most of the time large forest of proof trees We propose a new data structure called the DDB tree which merges the information of a proof tree forest into one concise tree A DDB tree gives a global picture of a query evaluation in a dramatically reduced structure with no loss of information DDB trees can be shown to users or can be analyzed further by an explanation system