A two phase optimization technique for XML queries with multiple regular path expressions

As XML (eXtensible Markup Language) has emerged as a standard for information exchange on the World Wide Web, it has gained attention in database communities to extract information from XML seen as a database model. XML queries are based on regular path queries, which find objects reachable by given regular expressions. To answer many kinds of user queries, it is necessary to evaluate queries that have multiple regular path expressions. However, previous work on subjects such as query rewriting and query optimization in the frame work of semistructured data has usually dealt with a single regular path expression. For queries that have multiple regular path expressions we suggest a two phase optimizing technique: query rewriting using views by finding the mappings from the view's body to the query's body and for rewritten queries, evaluating each query conjunct and combining them. We show that our rewriting algorithm is sound and our query evaluation technique is more efficient than that of previous work on optimizing semistructured queries. (C) 2002 Elsevier Science Inc. All rights reserved.