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Proposal for nested document support in Lucene
 

Proposal for nested document support in Lucene

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    Proposal for nested document support in Lucene Proposal for nested document support in Lucene Presentation Transcript

    • Nested Documents in Lucene
      High-performance support for parent/child document relations
      mark@searcharea.co.uk
    • Problem:
      The Lucene data model is based on Documents, Fields and Terms. However many real-world data structures cannot be properly represented when collapsed into a single Lucene document.
      Single
      Lucene
      document
    • Problem: “Cross-matching”
      When two or more data structures of the same type are jumbled up into a single Lucene field, matching logic becomes confused e.g. >1 qualification in a resume
      John
      Name
      John
      A1 in Maths
      A1, E1
      Grade
      E1 in Science
      Subject
      Maths, Science
      !
      False match for query:
      Grade:A1 AND Subject:Science
    • Unacceptable solution #1
      One modeling approach is to store related items in the same field and use proximity operators in queries
      Name
      John
      A1 Maths….E1 Science
      GradeAndSubject
      John
      Example query:
      “GradeAndSubject:”A1 Science”~2
      A1 in Maths
      E1 in Science
      !
      Slow
      !
      Not scalable with number of fields
      • Loss of fieldnames as context in query
      • Proximity distances must grow.
      • Only one choice of Analyzer for given field
    • Unacceptable solution #2
      Use numbered fieldnames to group related structures
      Name
      John
      Example query:
      ( Grade1:A1 AND
      Subject1:Science)
      OR
      (Grade2:A1 AND
      Subject2:Science )

      A1
      Grade1
      Maths
      Subject1
      E1
      Grade2
      John
      Subject2
      Science
      A1 in Maths
      E1 in Science
      !
      Slow
      !
      Not scalable with number of nested structures
      • More numbered fieldnames = more query complexity and more unique tokens in index
    • Solution: Nested documents
      The existing Lucene codebase can be used to simply store multiple “nested” documents to represent arbitrarily complex structures. Related documents are just added in sequence
      John
      Name
      John
      A1 in Maths
      A1
      E1
      Grade
      Grade
      E1 in Science
      Subject
      Maths
      Subject
      Science
      ?
      But how to query?....
    • Solution: Nested Document Queries
      Nested documents need to be queried using new NestedDocumentQuery class which understands document relationships
      John
      Name
      A1
      E1
      Grade
      Grade
      docType
      resume
      Subject
      Maths
      Subject
      Science
      New NestedDocumentQuery
      • Executes child search using any arbitrary Lucene Query object e.g. Boolean, fuzzy, numeric etc
      • Reports any matches as a match on the parent document not the child
      • Super-fast evaluation of joins between child and parent
      • Requires an indexed field to identify parent documents
      ?
    • Solution: Example Query
      Find resume of person called “John” with A1 grade in Maths
      John
      Name
      E1
      A1
      resume
      Grade
      docType
      Grade
      Subject
      Science
      Subject
      Maths
      The NestedDocumentQuery wrapper simply translates the stream of reported matches from the child-level query criteria into matches on the parent for evaluation of all the parent-level logic
    • Solution: Join speed
      Unlike a database, the cost of a join (child to parent) is blisteringly fast
      3) Find first prior set bit e.g. position #356,670
      100000100000000100000001000000010000001000010000000001000000100000100001
      2) Index directly into cached BitSet at position #356,675
      1) Match reported on document #356,675
      ParentQuery
      4) Attribute match to doc #356,670
      NestedDocumentQuery
      ChildQuery
      The BitSet for defining parents is obtained from a Filter and can be cached aggressively with minimal memory cost (one bit per document in the index)
    • Other advantages
      Parent-child document relationships can also be used to limit child results from any one parent (e.g. efficiently control the max number of pages returned from any one website)
      Nesting levels can be arbitrarily deep
      Very powerful multi-child queries possible e.g. find people likely to know person X using resume’s employment histories (multiple employer names/urls and related date-ranges)
    • “Lucene is not a database”, but…..
      Structure matters
      Many data sources are a mix of structured and unstructured content (e.g. microformats). This is unlikely to change. Lucene has historically been about unstructured text but has steadily been adding structured capability (Trie, spatial, facets) and become a great solution for hybrid data. However support for modeling and querying non-trivial data structures is missing currently.
      Relationships matter
      This proposal is not to recreate the full capabilities of a SQL database with arbitrary relationships. However we can benefit greatly from providing simple parent-child relationships
      We have some unique capabilities
      Parent-child joins are very fast
      Unlike SQL we can return partial, relevance-ranked matches
      Probably more akin to XML databases than SQL databases
    • Next steps
      Existing code/unit tests can be released to Lucene project if there is sufficient interest. This software has been deployed in production on large datasets.
      The matching approach is reliant on parents and children being held in the same Lucene index segment. Additional control is needed to enforce this more rigorously - either by
      Adding more user-control over IndexWritersegment creation where applications understand/control parent-child dependencies OR
      Making Lucene aware of parent-child relationships e.g. new method Document.add(Document)
      Query parser support
      XML Query Parser support is available
      End-user Query parser could add new syntax e.g. +candidateLocale:UK +child(grade:A1 AND subject:music)
    • Thoughts?
      Feedback encouraged on dev@lucene.apache.org