Compression Vs Queryability - A Case Study.

Some compromise on compression is known to be necessary, if the relative positions of the information stored by semi-structured documents are to remain accessible under queries. With this in view, we compare, on an example, the ‘query-friendliness’ of XML documents, when compressed into straightline tree grammars which are either regular or context-free. The queries considered are in a limited fragment of XPath, corresponding to a type of patterns; each such query defines naturally a non-deterministic, bottom-up ‘query automaton’ that runs just as well on a tree as on its compressed dag.