XML is a rather verbose representation of semistructured data, which may require huge amounts of storage space. Several summarized representations of XML data have been proposed, which can both provide succinct information and be directly queried. In this chapter we focus on compact representations based on the extraction of association rules from XML datasets. In particular, we show how patterns can be exploited to (possibly partially) answer queries, either when fast (and approximate) answers are required, or when the actual dataset is not available, e.g., it is currently unreachable. We focus on (a) schema patterns, representing exact or approximate dataset constraints, (b) instance patterns, which represent actual data summaries, and their use for answering queries.
Cooperative Query Answering by Abstract Interpretation