Package org.apache.lucene.misc.index

Class BPIndexReorderer

  • public final class BPIndexReorderer
extends Object
    Implementation of "recursive graph bisection", also called "bipartite graph partitioning" and often abbreviated BP, an approach to doc ID assignment that aims at reducing the sum of the log gap between consecutive postings. While originally targeted at reducing the size of postings, this algorithm has been observed to also speed up queries significantly by clustering documents that have similar sets of terms together.

    This algorithm was initially described by Dhulipala et al. in "Compressing graphs and inverted indexes with recursive graph bisection". This implementation takes advantage of some optimizations suggested by Mackenzie et al. in "Tradeoff Options for Bipartite Graph Partitioning".

    Typical usage would look like this: 

     LeafReader reader; // reader to reorder
 Directory targetDir; // Directory where to write the reordered index

 Directory targetDir = FSDirectory.open(targetPath);
 BPIndexReorderer reorderer = new BPIndexReorderer();
 ForkJoinPool pool = new ForkJoinPool(Runtime.getRuntime().availableProcessors(),
     p -> new ForkJoinWorkerThread(p) {}, null, false);
 reorderer.setForkJoinPool(pool);
 reorderer.setFields(Collections.singleton("body"));
 CodecReader reorderedReaderView = reorderer.reorder(SlowCodecReaderWrapper.wrap(reader), targetDir);
 try (IndexWriter w = new IndexWriter(targetDir, new IndexWriterConfig().setOpenMode(OpenMode.CREATE))) {
   w.addIndexes(reorderedReaderView);
 }
 DirectoryReader reorderedReader = DirectoryReader.open(targetDir);

    Note: This is a slow operation that consumes O(maxDoc + numTerms * numThreads) memory.

    • Field Detail

      • DEFAULT_MIN_DOC_FREQ

        public static final int DEFAULT_MIN_DOC_FREQ
        Minimum required document frequency for terms to be considered: 4,096.
        See Also:
        Constant Field Values

      • DEFAULT_MIN_PARTITION_SIZE

        public static final int DEFAULT_MIN_PARTITION_SIZE
        Minimum size of partitions. The algorithm will stop recursing when reaching partitions below this number of documents: 32.
        See Also:
        Constant Field Values

      • DEFAULT_MAX_ITERS

        public static final int DEFAULT_MAX_ITERS
        Default maximum number of iterations per recursion level: 20. Higher numbers of iterations typically don't help significantly.
        See Also:
        Constant Field Values

    • Constructor Detail

      • BPIndexReorderer

        public BPIndexReorderer()
        Constructor.

    • Method Detail

      • setMinDocFreq

        public void setMinDocFreq​(int minDocFreq)
        Set the minimum document frequency for terms to be considered, 4096 by default.

      • setMaxDocFreq

        public void setMaxDocFreq​(float maxDocFreq)
        Set the maximum document frequency for terms to be considered, as a ratio of maxDoc. This is useful because very frequent terms (stop words) add significant overhead to the reordering logic while not being very relevant for ordering. This value must be in (0, 1]. Default value is 1.

      • setMinPartitionSize

        public void setMinPartitionSize​(int minPartitionSize)
        Set the minimum partition size, when the algorithm stops recursing, 32 by default.

      • setMaxIters

        public void setMaxIters​(int maxIters)
        Set the maximum number of iterations on each recursion level, 20 by default. Experiments suggests that values above 20 do not help much. However, values below 20 can be used to trade effectiveness for faster reordering.

      • setForkJoinPool

        public void setForkJoinPool​(ForkJoinPool forkJoinPool)
        Set the ForkJoinPool to run graph partitioning concurrently.

        NOTE: A value of null can be used to run in the current thread, which is the default.

      • setRAMBudgetMB

        public void setRAMBudgetMB​(double ramBudgetMB)
        Set the amount of RAM that graph partitioning is allowed to use. More RAM allows running faster. If not enough RAM is provided, a BPIndexReorderer.NotEnoughRAMException will be thrown. This is 10% of the total heap size by default.

      • setFields

        public void setFields​(Set<String> fields)
        Sets the fields to use to perform partitioning. A null value indicates that all indexed fields should be used.