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Abstract:
In this paper we present {\em HOPI}, a new connection index for XML documents
based on the concept of the 2--hop cover of a directed graph introduced by
Cohen et al.
In contrast to most of the prior work on XML indexing we
consider not only paths with child or parent relationships between the nodes,
but also provide space-- and time--efficient reachability tests along the
ancestor, descendant, and link axes
to support path expressions with wildcards in our XXL search engine.
We improve the theoretical concept of a 2--hop cover by developing scalable
methods for index creation on very large XML data collections with long paths
and extensive cross--linkage, and for incremental index maintenance. Our
experiments show substantial savings in the query performance of the
HOPI index over previously proposed index structures, in combination with low
space requirements and efficient updates.
