A Generic Framework for Querying and Updating Secondary XML Index Structures

1. A Generic Framework for Querying and Updating Secondary XML Index Structures Katharina Grün

2. Research Methodology

3. Motivation • Widespread use of XML • XML databases for efficient query and update processing • Require index structures on content and structure of documents • primary index structure • default index • on whole document • not optimized for specific queries • secondary index structures • created on demand • on specific document fragments • adapted to query workload • Framework for querying and updating secondary XML index structures(SCIENS) Become aware of problem

4. Running example path: projects/project[1]/@name labelpath: projects/project/@name • //resource[@date>= '2005-01-01'] • //project[@name='sciens']/milestone[@id=2]/resource[@date>='2007-01-01'] • //element(resource, Report)[author='Smith'] Become aware of problem

5. Challenges • Which secondary index structures are necessary? • each kind of query is best supported by different index structure • not possible to provide one index structure for each possible query • How to integrate them into a common framework? • each secondary index can index arbitrary properties of arbitrary fragments • query and update processing must not depend on specific indices defined • How to update them when documents change? • document updates must be propagated to affected index structures • incremental index maintenance algorithm Become aware of problem

6. Related work (1) • XML databases • limited support for secondary index structures • XML index structures • structure and/or content • mostly primary index structure • based on different models, proprietary structures • Object-oriented index structures • proprietary structures to support queries on path navigation and/or inheritance hierarchies • Multidimensional index structures • support several value dimensions • do not consider structure Become aware of problem

7. Related work (2) • Extensible indexing • object-relational databases • adapt index structures to different data types • Indexing tasks • Maintain secondary indices when documents are updated (KeyX1) • Select optimal index for specific query (XML Access Modules2) • Suggest set of indices for query workload (KeyX1) • currently no integrated approach for processing secondary index structures in an XML database 1) B.C.Hammerschmidt: KeyX: Selective Key-Oriented Indexing in Native XML Databases. Phd Thesis, University of Lübeck, 2005. 2) Arion, A., Benzaken, V. and Manolescu, I.: XML Acess Modules: Towards Physical Data Independence in XML Databases. Ximep workshop, 2005. Become aware of problem

8. SCIENS - Ideas • Structure and Content Indexing with Extensible, Nestable Structures • Which secondary index structures are necessary? • select a small set of index structures and adapt them to various properties • nest index structures to reflect hierarchical queries • How to integrate them into a common framework? • provide an index model • common index interface to query and update indices • How to update them when documents change? • index maintenance algorithm that determines updates for arbitrary indices • based on update fragments and index definitions Suggest solution

9. Index structures – one dimension (1) • Value indexing • hashtable or B+-tree on value • @date>= '2005-01-01' • Structure indexing • hashtable or B+-tree on path/labelpath/type • //resource • /project[1]//resource • /project[2]/milestone[2]/resource Construct solution

10. Index structures – one dimension (2) Construct solution

11. Index structures – multiple dimensions (1) 1) Robinson, J.: The KDB-tree: A search Structure for Large Multidimensional Dynamic Indexes. Sigmod, ACM Press, 1981. Construct solution

12. Index structures – multiple dimensions (2) Construct solution

13. Comparison • queries and indices on milestone hierarchy and date • e.g. //project[1]//resource[@date>2005-01-01] • define index that best matches query workload Evaluate solution

14. Index framework (1) • index • search function consisting of a set of index entries • provides interface to update and retrieve index entries • index entry • maps index keys (value, type, path,…) -> returned nodes • TechnicalReport, Smith -> 3.2.1, 4.3.1,... • index definition • selects nodes to be indexed • //element(resource, $V1)[author=$V2] • represented as unordered tree pattern with index variables • index structure • specific data structure (hash table, prefix B+-tree, kdb-tree) • one index can use several index structures (index nesting) Construct solution

15. Index framework (2) • index configuration • provides mapping from index to specific index structure • associates with each index variable the index structure to be used • $T1, $E2: kdb-tree • $E2: hash table, $T1: B+-tree • search configuration • used to access index • associates index key to be searched with each index variable • generated by index selection tool • $T1= Report, $E2= 'Smith' Construct solution

16. Index maintenance • propagate document updates to affected indices • steps • find embeddings of index patterns in update fragments • execute queries • generate index entries [(TechnicalReport, 'Smith')  resource] [(TechnicalReport, 'Tim')  resource] • up to 9 times faster than existing approach (KeyX) Construct / evaluate solution

17. Conclusion • select secondary index structures for XML • extensible: various properties and operations on these properties • nestable: adapt indices to hierarchical queries • integrate index structures into framework • hides indexing tasks from query and update processing tasks • provides index model (common index interface) • index maintenance algorithm • propagate updates to index structures • flexibility to define indices that match the query workload