Approximate Tree Embedding
According to the XML Information Set W3C Recommendation , XML documents can be represented as trees. Complex queries can also be represented as pattern trees to be searched in the documents.
Finding structural approximate answers to complex queries, i.e. queries with two or more branching conditions, is known to be a hard task task, namely, finding a solution to the Unordered Tree Embedding Problem, which is proved to be NP-complete .
Consider the approximations shown in Figure 2:
Figure 2: Approximate Tree Embedding of the query tree in the document tree.
The semantic condition on artist is relaxed to match to the synonym singer, the
lastname element is disregarded thus accepting a partial match of the query, and
finally the tracklist element is skipped in the data.
We recall that, given two trees t1 and t2, an injective function f from nodes of t1 to nodes of t2 is an Embedding of t1 in t2 if it preserves labels, and ancestorships in both directions .
In order to cope with the intrinsic complexity of the problem, and make the embedding function efficiently computable, we guarantee ancestorship only in one direction, from query tree to document tree. This means that injectivity is not guaranteed by the mapping, in that sibling nodes in the query may be mapped to ancestor-descendant nodes in the data.
Finally, approximations are captured by the following definition, which formulates the (one-way) Approximate Tree Embedding function:
Given two trees t1 and t2, a function e from nodes of t1 to nodes of t2 is an Approximate Embedding of t1 in t2 if:
where sim is a similarity function between labels of nodes, possibly exploiting thesauri, ontologies, and semantic networks; parent and ancestor are the usual hierarchical relationships between nodes.
 XML Information Set. http://www.w3.org/TR/xml-infoset
 P. Kilpeläinen. Tree Matching Problems with Application to Structured Text Databases. PhD. Thesis, Dept. of Computer Science, Univ. of Helsinki, SF, 1992.