RDF Aggregate Queries and Views

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RDF Aggregate Queries and Views

• Edward Hung, Yu Deng, V.S. Subrahmanian
• University of Maryland, College Park

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Maintenance of RDF Aggregate Views

• Introduction of RDF and RDQL
• RDQL Extension for Aggregate Views
• Aggregate View Maintenance Algorithms AMX
• Implementation and Experiments
• Related Work

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Publication

• Edward Hung, Yu Deng, V.S. Subrahmanian, "RDF
  Aggregate Queries and Views", to appear in the
  Proc. of the 21st International Conference on
  Data Engineering (ICDE), Tokyo, Japan, 2005.

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Introduction

• Resource Description Framework (RDF)
• W3C Recommendation
• Represents metadata about resources identifiable
  on the web (by Uniform Resource Identifier (URI))
• Triple (Resource, Property, Value)
• (Artist, rdftype, rdfsClass)
• (Painter, rdftype, rdfsClass)
• (Painter, rdfssubClassOf, Artist)

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• lt?xml version"1.0"?gt
• lt!DOCTYPE rdfRDF lt!ENTITY xsd
  "http//www.w3.org/2001/XMLSchema"gtgt
• ltrdfRDF xmlnsrdf"http//www.w3.org/1999/02/22-
  rdf-syntax-ns" xmlnsrdfs"http//www.w3.org/20
  00/01/rdf-schema" xmlbase"http//www.auctions
  chema.com/schema1"gt
• ltrdfsClass rdfID"Artist"/gt
• ltrdfsClass rdfID"Painter"gtltrdfssubClassOf
  rdfresource"Artist"/gtlt/rdfsClassgt
• ltrdfsDatatype rdfabout"xsdstring"/gt
• ltrdfProperty rdfID"fname"gt
• ltrdfsdomain rdfresource"Artist"/gt
• ltrdfsrange rdfresource"xsdstring"/gt
• lt/rdfPropertygt
• lt/rdfRDFgt
• lt?xml version"1.0"?gt
• lt!DOCTYPE rdfRDF lt!ENTITY xsd
  "http//www.w3.org/2001/XMLSchema"gtgt
• ltrdfRDF xmlnsrdf "http//www.w3.org/1999/02/22
  -rdf-syntax-ns"
• xmlnsns1"http//www.auctionschema.com/schema
  1"gt
• ltrdfDescription rdfabout"http//www.artist.n
  etguyrose"gt
• ltrdftype rdfresource"ns1Painter"/gt
• ltns1fname rdfdatatype"xsdstring"gt Guy
  lt/ns1fnamegt

RDF Schema
RDF Instance
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• lt?xml version"1.0"?gt
• lt!DOCTYPE rdfRDF lt!ENTITY xsd
  "http//www.w3.org/2001/XMLSchema"gtgt
• ltrdfRDF xmlnsrdf"http//www.w3.org/1999/02/22-
  rdf-syntax-ns" xmlnsrdfs"http//www.w3.org/20
  00/01/rdf-schema" xmlbase"http//www.auctions
  chema.com/schema1"gt
• ltrdfsClass rdfID"Artist"/gt
• ltrdfsClass rdfID"Painter"gtltrdfssubClassOf
  rdfresource"Artist"/gtlt/rdfsClassgt
• ltrdfsDatatype rdfabout"xsdstring"/gt
• ltrdfProperty rdfID"fname"gt
• ltrdfsdomain rdfresource"Artist"/gt
• ltrdfsrange rdfresource"xsdstring"/gt
• lt/rdfPropertygt
• lt/rdfRDFgt
• lt?xml version"1.0"?gt
• lt!DOCTYPE rdfRDF lt!ENTITY xsd
  "http//www.w3.org/2001/XMLSchema"gtgt
• ltrdfRDF xmlnsrdf "http//www.w3.org/1999/02/22
  -rdf-syntax-ns"
• xmlnsns1"http//www.auctionschema.com/schema
  1"gt
• ltrdfDescription rdfabout"http//www.artist.n
  etguyrose"gt
• ltrdftype rdfresource"ns1Painter"/gt
• ltns1fname rdfdatatype"xsdstring"gt Guy
  lt/ns1fnamegt

fname
Artist
String
subClassOf
Painter
fname
r1
Guy
r1 http//www.artist.netguyrose
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RDQL RDF Query Language

• SELECT?highprice
• WHERE (?artist, ltns1lnamegt, "Rose"),
• (?artist, ltns1fnamegt, "Guy"),
• (?artist, ltns1createsgt, ?artifact),
• (?artifact, ltns1estimatedgt, ?price),
• (?price, ltns1highgt, ?highprice),
• (?artifact, ltns1presentedgt, ?date)
• AND 2004-04-01 lt ?date lt 2004-04-30
• USING ns1 FOR http//www.auctionschema.com/schema1
  gt

view pattern
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RDQL Extension for Aggregates and Views

• CREATEVIEW AS
• SELECT max(?highprice)
• WHERE (?artist, ltns1lnamegt, "Rose"),
• (?artist, ltns1fnamegt, "Guy"),
• (?artist, ltns1createsgt, ?artifact),
• (?artifact, ltns1estimatedgt, ?price),
• (?price, ltns1highgt, ?highprice),
• (?artifact, ltns1presentedgt, ?date)
• AND 2004-04-01 lt ?date lt 2004-04-30
• USING ns1 FOR http//www.auctionschema.com/schema1
  gt

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• We are expanding the syntax of RDQL so that it
  allows constants in SELECT clauses which
  equivalently creates new resources and properties
  using the constants.
• For example, the previous query can be modified
  as follows
• CREATEVIEW AS
• SELECT ltns1works_by_guyrosegt, ltns1maxpricegt,
  max(?highprice)
• WHERE (?artist, ltns1lnamegt, "Rose"),
• (?artist, ltns1fnamegt, "Guy"),
• (?artist, ltns1createsgt, ?artifact),
• (?artifact, ltns1estimatedgt, ?price),
• (?price, ltns1highgt, ?highprice),
• (?artifact, ltns1presentedgt, ?date)
• AND 2004-04-01 lt ?date lt 2004-04-30
• USING ns1 FOR http//www.auctionschema.com/schema1
  gt
• The result is a valid RDF statement
  (ltns1works_by_guyrosegt,ltns1maxpricegt,800000"
  ns1USD)

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Aggregate View Maintenance

• Relational Approach
• Store all triples in a relational table with
  schema (Resource, Property, Value)
• OR
• Store resources and values of the same property
  in a separate relational table with schema
  (Resource, Value)
• self-joins (triples in where-clause) 1
• Large number of delta rules during relational
  view maintenance ? expensive

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Aggregate View Maintenance

• Graph-structured DB (GSDB) Zhuge, Garcia-Molina,
  ICDE 1998
• GSDB assumes a rooted graph model while RDF is a
  general graph
• A GSDB view contains a set of nodes while our RDF
  views can contain nodes, edges, or any
  combinations.

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Aggregate View Maintenance

• Our Approach
• Localized search in RDF graphs
• breadth-first search starting at the
  inserted/deleted edge
• auxiliary data are needed for certain aggregate
  views
• min, max, avg

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Compute Aggregates Algorithm CAA
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view pattern
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BAG
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BAG 800000
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BAG 800000, 500000
SELECT max(?highprice)
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Aggregate View Maintenance Algorithms AMX

• AMI Insertion
• AMD Deletion
• AMT Triple Modification
• AMR Resource Modification

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BAG 800000, 500000
Update Insertion
paints
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BAG 800000, 500000
paints
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BAG 800000, 500000, 60000
SELECT max(?highprice)
paints
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AMI for Insertion
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Distributive Aggregate Function

• An aggregate function f is distributive w.r.t a
  source update operation if and only if after such
  an operation, the updated value of the function
  can be computed based on its old value and the
  value(s) of the source update without reference
  to the source.
• More formally, f is distributive w.r.t. an update
  operation U if and only if there exists a
  function g such that f(I') g(f(I), v) where
  f(I) is the aggregate value, I' is the updated
  instance after the update operation U(I, v), and
  v is the value(s) used in the update (e.g., the
  new value to add, the old value to remove, etc).

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Distributive Aggregate Function

• Examples of distributive aggregate functions
• count, sum, average w.r.t. insertion, deletion
  and update
• For average, we will need an additional attribute
  size which stores the size of S (in line 3 of
  CAA) in order to compute the correct updated
  value (or, we can use sum, count to calculate it)
• max and min are distributive w.r.t. insertion,
  but not deletion and update
• Auxiliary data computed from the source (such as
  S) can help to maintain non-distributive
  aggregate functions to avoid the need to refer to
  the source.

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TMaintainI
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BAG 800000, 500000, 60000
Update Deletion
paints
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BAG 800000, 500000, 60000
paints
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BAG 500000, 60000
SELECT max(?highprice)
paints
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AMD for Deletion
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TMaintainD
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Implementation and Experiment

• Implemented in Java
• Jena RDQL Engine of HP
• Comparison with Relational Approach (standard
  view maintenance algorithm on relational tables)
• Counting Algorithm in Gupta et al. "Maintaining
  Views Incrementally", SIGMOD 1993
• Dataset Chef Moz Project RDF dump
• Data stored in memory

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Other Related Work

• Voltz et al. DBFUSION02
• the first to introduce a view mechanism for RDF
  data
• Their views require that
• the results contain class instances (i.e., a
  subject or object variable), or
• the result itself has the pattern of RDF
  statement (i.e., a triple containing subject,
  predicate and object).
• Magkanaraki et al ISWC03
• proposed RVL, a view definition language that can
  also create virtual RDF schemas and restructure
  class and property hierarchies such that new
  resources, property values, classes and property
  types can be created.
• None of these works specifically address (i)
  aggregates in RDF or (ii) the problem of
  maintaining aggregate RDF views.

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Summary

• RDQL Extension for Views and Aggregates
• Compute Aggregates Algorithm CAA
• Aggregate View Maintenance Algorithms AMX