Covering indexes for branching path queries

Abstract

In this paper, we ask if the traditional relational query acceleration techniques of summary tables and covering indexes have analogs for branching path expression queries over tree-or graph-structured XML data. Our answer is yes --the forward-and-backward index already proposed in the literature can be viewed as a structure analogous to a summary table or covering index. We also show that it is the smallest such index that covers all branching path expression queries. While this index is very general, our experiments show that it can be so large in practice as to offer little performance improvement over evaluating queries directly on the data. Likening the forward-and-backward index to a covering index on all the attributes of several tables, we devise an index definition scheme to restrict the class of branching path expressions being indexed. The resulting index structures are dramatically smaller and perform better than the full forward-and-backward index for these classes of branching path expressions. This is roughly analogous to the situation in multidimensional or OLAP workloads, in which more highly aggregated summary tables can service a smaller subset of queries but can do so at increased performance. We evaluate the performance of our indexes on both relational decompositions of XML and a native storage technique. As expected, the performance benefit of an index is maximized when the query matches the index definition.

Approximate and Incremental Processing of Complex Queries against the Web of Data

“…Structure Index for Graph Structured Data. Structure indexes have been widely used for semi-structured and XML data [4,13,6]. A well-known concept is the dataguide [9], which is a structural description for rooted data graphs.…”

XIST: An XML Index Selection Tool

“…The Elem index selection strategy is interesting because it is a minimal set of indices to answer any path query. The SP follows the index selection technique recently proposed by Kaushik et al [16], which prefers to index on short paths over long paths. FP is a set of indices that requires no join when evaluating paths without value-based predicates.…”

“…Recently, Kaushik et al have proposed F&B-indexes that use the structural features of the input XML documents [16]. F&B indexes are forward-and-backward indices for answering branching path queries.…”

“…F&B indexes are forward-and-backward indices for answering branching path queries. Some heuristics in choosing indices, such as prioritizing short path indices over long path indices are proposed [16]. On the other hand, XIST takes many additional parameters, e.g., not only the path length, to assess the usefulness of the indices.…”

“…On the other hand, XIST takes many additional parameters, e.g., not only the path length, to assess the usefulness of the indices. Furthermore, XIST exploits information from a schema or a query workload while the index selection techniques [16] do not take advantage of this information.…”