Package org.apache.lucene.misc.index

# Class BPIndexReorderer

java.lang.Object
org.apache.lucene.misc.index.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.

## Nested Class Summary

Nested Classes
Modifier and Type
Class
Description
`static class`
`BPIndexReorderer.NotEnoughRAMException`
Exception that is thrown when not enough RAM is available.

## Field Summary

Fields
Modifier and Type
Field
Description
`static final int`
`DEFAULT_MAX_ITERS`
Default maximum number of iterations per recursion level: 20.
`static final int`
`DEFAULT_MIN_DOC_FREQ`
Minimum required document frequency for terms to be considered: 4,096.
`static final int`
`DEFAULT_MIN_PARTITION_SIZE`
Minimum size of partitions.

## Constructor Summary

Constructors
Constructor
Description
`BPIndexReorderer()`
Constructor.

## Method Summary

Modifier and Type
Method
Description
`Sorter.DocMap`
`computeDocMap(CodecReader reader, Directory tempDir, Executor executor)`
Expert: Compute the `Sorter.DocMap` that holds the new doc ID numbering.
`CodecReader`
`reorder(CodecReader reader, Directory tempDir, Executor executor)`
Reorder the given `CodecReader` into a reader that tries to minimize the log gap between consecutive documents in postings, which usually helps improve space efficiency and query evaluation efficiency.
`void`
`setFields(Set<String> fields)`
Sets the fields to use to perform partitioning.
`void`
`setMaxDocFreq(float maxDocFreq)`
Set the maximum document frequency for terms to be considered, as a ratio of `maxDoc`.
`void`
`setMaxIters(int maxIters)`
Set the maximum number of iterations on each recursion level, 20 by default.
`void`
`setMinDocFreq(int minDocFreq)`
Set the minimum document frequency for terms to be considered, 4096 by default.
`void`
`setMinPartitionSize(int minPartitionSize)`
Set the minimum partition size, when the algorithm stops recursing, 32 by default.
`void`
`setRAMBudgetMB(double ramBudgetMB)`
Set the amount of RAM that graph partitioning is allowed to use.

### Methods inherited from class java.lang.Object

`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

## Field Details

### 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 Details

### BPIndexReorderer

public BPIndexReorderer()
Constructor.

## Method Details

### 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.

### 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.

### computeDocMap

public Sorter.DocMap computeDocMap(CodecReader reader, Directory tempDir, Executor executor) throws IOException
Expert: Compute the `Sorter.DocMap` that holds the new doc ID numbering. This is exposed to enable integration into `BPReorderingMergePolicy`, `reorder(CodecReader, Directory, Executor)` should be preferred in general.

Throws:
  `IOException`

### reorder

public CodecReader reorder(CodecReader reader, Directory tempDir, Executor executor) throws IOException
Reorder the given `CodecReader` into a reader that tries to minimize the log gap between consecutive documents in postings, which usually helps improve space efficiency and query evaluation efficiency. Note that the returned `CodecReader` is slow and should typically be used in a call to `IndexWriter.addIndexes(CodecReader...)`.

The provided `Executor` can be used to perform reordering concurrently. A value of `null` indicates that reordering should be performed in the current thread.

NOTE: The provided `Executor` must not reject tasks.

Throws:
  `BPIndexReorderer.NotEnoughRAMException` - if not enough RAM is provided
  `IOException`