Efficient Keyword Search over Virtual XML Views

Efficient Keyword Search over Virtual XML Views FengShao and Lin Guo and ChavdarBotev and AnandBhaskar and MuthiahChettiar and Fan Yang Cornell University JayavelShanmugasundaram Yahoo! Research 2008. 02. 14. Summarized by Dongmin Shin, IDS Lab., Seoul National University Presented by Dongmin Shin, IDS Lab., Seoul National University

Index Introduction Background System Overview QPT GenerationModule PDT Generation Module Experiments Conclusion and Future Work

Introduction • The set of documents being searched is materialized. Fundamental assumption of traditional information retrieval systems

Introduction • Aggregator may not have resources to materialize all the data • If the view is materialized, the contents of the view may be • out-of-date or maintaining the view may be expensive • The data sources may not wish to provide the entire data The view is often virtual (unmaterialized) But

Introduction • Need Efficiently evaluating keyword search queries over virtual XML views

Background

Background TF-IDF method XML Scoring tf(e,k) : the number of distinct occurrences of the keyword k in element e and its descendants idf(k) = score(e,Q) =

System Overview (2) The parser redirects the query to the Query Pattern Tree(QPT) GenerationModule (3) QPT issent to the Pruned Document Tree(PDT) Generation Module (4) Generate PDTs using only the path indices and inverted list indices (5) Rewritten query and PDTs are sent to Evaluator (6) Produce the view that contains all view elements with pruned content (7) Elements are scored, only those with highest scores are fully materialized using document storage (1) Keyword queries over virtual views

System Overview • XML Storage • Dewey IDs • Popular id format • Hierarchical numbering scheme • ID of an element contains the ID of its parent

System Overview • XML Indexing • Path indices • Evaluate XML path and twig(i.e., branching path) • Store XML paths with values in a relational table • Use indices such as B+-tree • One row for each unique (Path, Value) pair • IDList : the list of ids of all elements on the path • B+-tree index is built on the (Path, Value) pair

System Overview • Inverted list indices • Store the list of XML elements that directly contain the keyword for each keyword in the document collection

QPT GenerationModule

PDT Generation Module • Output • Only contains elements that correspond to nodes in the QPT • Onlycontains element values that are required during query evaluation • Advantage • Query evaluation is likely to be more efficient and scalable • Allows us to use the regular(unmodified) query evaluator

PDT Generation Module • Key Idea • An element e in the document corresponding to a node n in the QPT is selected for inclusion only if it satisfies three types of constraints • Ancestor constraint – an ancestor element of e that corresponds to the parent of n in the QPT should also be selected • Descendant constraint – for each mandatory edge from n to a child of n in the QPT, at least one child/descendant element of e corresponding to that child of n should also be selected • Predicate Constraint – if e is a leaf node, it satisfies all predicates associated with n

PDT Generation Module • PrepareList (1) Issues a lookup on path indices for each QPT node that has no mandatory child edges (2) Identifies nodes that have a ‘v’ annotation to obtain values and ids (3) Looks up inverted lists indices and retrieves the list of Dewey IDs containing the keywords along with tf values

PDT Generation Module Candidate Tree(CT)

PDT Generation Module • Step 1 : adding new IDs • Adds the current minimum IDs in pathLists

PDT Generation Module • Step 2 : creating PDT nodes • Create PDT nodes using CT nodes • Top-down • Check DM value of each CT node • if it is “1”, create it in pdt cache • If not, check children of that node • If DM value of that children node is “1”, create is in pdt cache of parent node

PDT Generation Module • Step 3 : removing CT nodes • Bottom-up • Check if each node satisfies ancestor constraints • If not, remove • If so, propagate to the pdt cache of the ancestor • If some node has no children and does not satisfy descendant constraints, remove

PDT Generation Module • When we remove the root node “books”, all IDs in its pdt cache will be propagated to the result PDT

PDT Generation Module

Experiments • 500MB INEX dataset • Varying parameters • Size of data, # keywords, selectivity of keywords • # of joins, join selectivity, level of nestings • # of results, Avg. size of view element • Four alternative approaches • Baseline • GTP : general solution to integrate structure and keyword search queries • Efficient : proposed architecture • Proj : techniques of projecting XML documents

Experiments • The cost of generating PDTs scales gracefully • Overhead of post-processing(scoring and materializing) is negligible • The cost of the query evaluator dominates the entire cost EFFICIENT is a scalable and efficient soultion

Experiments • Run time for EFFICIENT increases • Because the cost of the query evaluation increases • Run time for EFFICIENT increases slightly • Because it accesses more inverted lists to retrieve tf values

Conclusion and Future Work • Conclusion • A general technique for evaluating keyword search queries over views • Efficient over a wide range of parameters • Future Work • Instead of using the regular query evaluator, we could use the techniques proposed for ranked query evaluation • Views may contain non-monotonic operators such as group-by

Efficient Keyword Search over Virtual XML Views