Package org.apache.lucene.index.pruning

Class CarmelTopKTermPruningPolicy

  • public class CarmelTopKTermPruningPolicy
extends TermPruningPolicy
    Pruning policy with a search quality parameterized guarantee - configuration of this policy allows to specify two parameters: k and ε such that:

    For any OR query with r terms, the score of each of the top k results in the original index, should be "practically the same" as the score that document in the pruned index: the scores difference should not exceed r * ε.

    See the following paper for more details about this method: Static index pruning for information retrieval systems, D. Carmel at al, ACM SIGIR 2001 .

    The claim of this pruning technique is, quoting from the above paper:

    Prune the index in such a way that a human "cannot distinguish the difference" between the results of a search engine whose index is pruned and one whose index is not pruned.

    For indexes with a large number of terms this policy might be too slow. In such situations, the uniform pruning approach in CarmelUniformTermPruningPolicy will be faster, though it might produce inferior search quality, as that policy does not pose a theoretical guarantee on resulted search quality.

    TODO implement also CarmelTermPruningDeltaTopPolicy

    • Field Detail

      • DEFAULT_TOP_K

        public static final int DEFAULT_TOP_K
        Default number of guaranteed top K scores
        See Also:
        Constant Field Values

      • DEFAULT_R

        public static final int DEFAULT_R
        Default number of query terms
        See Also:
        Constant Field Values

      • DEFAULT_EPSILON

        public static final float DEFAULT_EPSILON
        Default largest meaningless score difference
        See Also:
        Constant Field Values

    • Constructor Detail

      • CarmelTopKTermPruningPolicy

        public CarmelTopKTermPruningPolicy​(IndexReader in,
                                   Map<String,​Integer> fieldFlags,
                                   int k,
                                   float epsilon,
                                   int r,
                                   Similarity sim)
        Constructor with specific settings
        Parameters:
        in - reader for original index
        k - number of guaranteed top scores. Each top K results in the pruned index is either also an original top K result or its original score is indistinguishable from some original top K result.
        epsilon - largest meaningless score difference Results whose scores difference is smaller or equal to epsilon are considered indistinguishable.
        r - maximal number of terms in a query for which search quaility in pruned index is guaranteed
        sim - similarity to use when selecting top docs fir each index term. When null, DefaultSimilarity is used.

    • Method Detail

      • pruneTermEnum

        public boolean pruneTermEnum​(TermEnum te)
                      throws IOException
        Description copied from class: TermPruningPolicy
        Pruning of all postings for a term (invoked once per term).
        Specified by:
        pruneTermEnum in class TermPruningPolicy
        Parameters:
        te - positioned term enum.
        Returns:
        true if all postings for this term should be removed, false otherwise.
        Throws:
        IOException

      • pruneAllPositions

        public boolean pruneAllPositions​(TermPositions termPositions,
                                 Term t)
                          throws IOException
        Description copied from class: TermPruningPolicy
        Prune all postings per term (invoked once per term per doc)
        Specified by:
        pruneAllPositions in class TermPruningPolicy
        Parameters:
        termPositions - positioned term positions. Implementations MUST NOT advance this by calling TermPositions methods that advance either the position pointer (next, skipTo) or term pointer (seek).
        t - current term
        Returns:
        true if the current posting should be removed, false otherwise.
        Throws:
        IOException

      • pruneTermVectorTerms

        public int pruneTermVectorTerms​(int docNumber,
                                String field,
                                String[] terms,
                                int[] freqs,
                                TermFreqVector tfv)
                         throws IOException
        Description copied from class: TermPruningPolicy
        Pruning of individual terms in term vectors.
        Specified by:
        pruneTermVectorTerms in class TermPruningPolicy
        Parameters:
        docNumber - document number
        field - field name
        terms - array of terms
        freqs - array of term frequencies
        tfv - the original term frequency vector
        Returns:
        0 if no terms are to be removed, positive number to indicate how many terms need to be removed. The same number of entries in the terms array must be set to null to indicate which terms to remove.
        Throws:
        IOException

      • pruneSomePositions

        public int pruneSomePositions​(int docNum,
                              int[] positions,
                              Term curTerm)
        Description copied from class: TermPruningPolicy
        Prune some postings per term (invoked once per term per doc).
        Specified by:
        pruneSomePositions in class TermPruningPolicy
        Parameters:
        docNum - current document number
        positions - original term positions in the document (and indirectly term frequency)
        curTerm - current term
        Returns:
        0 if no postings are to be removed, or positive number to indicate how many postings need to be removed. The same number of entries in the positions array must be set to -1 to indicate which positions to remove.