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Indexing Semistructured Data

Indexing Semistructured Data. J. McHugh, J. Widom, S. Abiteboul, Q. Luo, and A. Rajaraman Stanford University January 1998 http://www-db.stanford.edu/lore/. EECS 684 02/21/2000 Presented by Weiming Zhou . Outline. Introduction - Data Model - Query Language

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Indexing Semistructured Data

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  1. Indexing Semistructured Data J. McHugh, J. Widom, S. Abiteboul, Q. Luo, and A. Rajaraman Stanford University January 1998 http://www-db.stanford.edu/lore/ EECS 684 02/21/2000 Presented by Weiming Zhou

  2. Outline • Introduction • - Data Model • - Query Language • Indexes in Lore • Query plans using indexes • Conclusions

  3. Data Model - Object Exchange Model (OEM)

  4. The Lorel Query Language (Lorel) Example 1 selectDB.Movie.Title whereDB.Movie.Actor.Name = “Harrison Ford” Example 2 selectT fromDB.Movie M, M.Title T whereexists A in M.Actor : exists N in A.Name : N = “Harrison Ford”

  5. Indexes In Lore • Value index • Text index • Link index • Path index • Edge index

  6. Value index Similar to attribute indexes in Relational DBMS Example Suppose we create a Value index for DB.Movie.Year If we perform a lookup for DB.Movie.Year = “1956”, Result: &12.

  7. Text Index • An information-retrieval style keyword search. • Restricted by incoming labels. • Locates string values containing specific words. • Useful for strings containing a significant amount of text. • Implementation: • Inverted lists - map a given word w and label l to a list of atomic values with incoming edge l that contain word w. • Example: • Lookup for all objects with an atomic string value containing the • word “Ford" and an incoming edge Name. • Results: {<&17, 2>, <&21, 2>}.

  8. Link Index • Locates parents of a given object. • Serves as back-pointers • Implementation • Extendible hashing • One Link Index for the entire database graph • Example • The Link Index lookup for object &17 returns parent object &6, and the lookup for object &21 returns object &13.

  9. Path Index Locate all objects reachable by a given labeled path. Provided by DataGuide. Example select DB.Movie.Title Using the Path Index to directly locate all objects reachable via DB.Movie.Title. Results: &5; &9; &14.

  10. Edge Index All parent-child pairs connected via a specified label. Example Look up label “Year” in Edge Index Results: &2-&7, &3-&12

  11. Query Plans Using Indexes • Top-Down • Bottom-Up • Hybrid • Example • selectT • fromDB.Movie M, M.Title T • whereexists A in M.Actor : exists N in A.Name • : N = “Harrison Ford”

  12. Top-Down Query Plan Exhaustive Top-down traversals DB.Movie.Actor.Name = “Harrison Ford” &17, &21 Link Index &17  &2, &21  &4 DB.Movie.Title &5, &14

  13. Bottom-Up Query Plan Look up Value Index DB.Movie.Actor.Name = “Harrison Ford” &17, &21 Link Index &17  &2, &21  &4 DB.Movie.Title &5, &14

  14. Hybrid Query Plan selectX fromA.B X whereexists Y in X.C : Y =5 Bottom-up: Value Index A.B.C = “5” Top-down: A.B Intersect

  15. Conclusions • Presents Lore’s indexing structures: Value Index, Text Index, Link Index, Path Index and Edge Index. • Query plans using indexes • Preliminary performance results: • at least an order of magnitude improvement when indexes are used for query processing.

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