Indexing Semistructured Data

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 select DB.Movie.Title where
DB.Movie.Actor.Name Harrison Ford Example
2 select T from DB.Movie M, M.Title T where
exists 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 lt17, 2gt, lt21, 2gt.

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
• select T
• from DB.Movie M, M.Title T
• where exists 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
select X from A.B X where exists Y in X.C Y 5
Bottom-up Value Index A.B.C 5 Top-down
A.B Intersect
15
Conclusions

• Presents Lores 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.