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## org.apache.lucene.index
Class LogMergePolicy

    java.lang.Object
      org.apache.lucene.index.MergePolicy
          org.apache.lucene.index.LogMergePolicy

Direct Known Subclasses:
  LogByteSizeMergePolicy, LogDocMergePolicy

    public abstract class LogMergePolicy

extends MergePolicy

This class implements a `MergePolicy` that tries to merge segments into levels of exponentially increasing size, where each level has < mergeFactor segments in it. Whenever a given levle has mergeFactor segments or more in it, they will be merged.

This class is abstract and requires a subclass to define the `size(org.apache.lucene.index.SegmentInfo)` method which specifies how a segment's size is determined. `LogDocMergePolicy` is one subclass that measures size by document count in the segment. `LogByteSizeMergePolicy` is another subclass that measures size as the total byte size of the file(s) for the segment.

Nested Class Summary

Nested classes/interfaces inherited from class org.apache.lucene.index.MergePolicy
`MergePolicy.MergeAbortedException, MergePolicy.MergeException, MergePolicy.MergeSpecification, MergePolicy.OneMerge`

Field Summary
`static int` | `DEFAULT_MAX_MERGE_DOCS` Default maximum segment size.
`static int` | `DEFAULT_MERGE_FACTOR` Default merge factor, which is how many segments are merged at a time
`static double` | `LEVEL_LOG_SPAN` Defines the allowed range of log(size) for each level.

Constructor Summary
`LogMergePolicy()`

Method Summary
`void` | `close()` Release all resources for the policy.
`MergePolicy.MergeSpecification` | `findMerges(org.apache.lucene.index.SegmentInfos infos, IndexWriter writer)` Checks if any merges are now necessary and returns a `MergePolicy.MergeSpecification` if so.
`MergePolicy.MergeSpecification` | `findMergesForOptimize(org.apache.lucene.index.SegmentInfos infos, IndexWriter writer, int maxNumSegments, Set segmentsToOptimize)` Returns the merges necessary to optimize the index.
`int` | `getMaxMergeDocs()` Returns the largest segment (measured by document count) that may be merged with other segments.
`int` | `getMergeFactor()` Returns the number of segments that are merged at once and also controls the total number of segments allowed to accumulate in the index.
`boolean` | `getUseCompoundDocStore()` Returns true if newly flushed and newly merge doc store segment files (term vectors and stored fields) are written in compound file format.
`boolean` | `getUseCompoundFile()` Returns true if newly flushed and newly merge segments are written in compound file format.
`void` | `setMaxMergeDocs(int maxMergeDocs)` Determines the largest segment (measured by document count) that may be merged with other segments.
`void` | `setMergeFactor(int mergeFactor)` Determines how often segment indices are merged by addDocument().
`void` | `setUseCompoundDocStore(boolean useCompoundDocStore)` Sets whether compound file format should be used for newly flushed and newly merged doc store segment files (term vectors and stored fields).
`void` | `setUseCompoundFile(boolean useCompoundFile)` Sets whether compound file format should be used for newly flushed and newly merged segments.
`protected abstract long` | `size(org.apache.lucene.index.SegmentInfo info)`
`boolean` | `useCompoundDocStore(org.apache.lucene.index.SegmentInfos infos)` Returns true if the doc store files should use the compound file format.
`boolean` | `useCompoundFile(org.apache.lucene.index.SegmentInfos infos, org.apache.lucene.index.SegmentInfo info)` Returns true if a newly flushed (not from merge) segment should use the compound file format.

Methods inherited from class java.lang.Object
`clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

Field Detail

### LEVEL_LOG_SPAN

    public static final double LEVEL_LOG_SPAN

  Defines the allowed range of log(size) for each level. A level is computed by taking the max segment log size, minuse LEVEL_LOG_SPAN, and finding all segments falling within that range.

  See Also:
    Constant Field Values

### DEFAULT_MERGE_FACTOR

    public static final int DEFAULT_MERGE_FACTOR

  Default merge factor, which is how many segments are merged at a time

  See Also:
    Constant Field Values

### DEFAULT_MAX_MERGE_DOCS

    public static final int DEFAULT_MAX_MERGE_DOCS

  Default maximum segment size. A segment of this size or larger will never be merged. @see setMaxMergeDocs

  See Also:
    Constant Field Values

Constructor Detail

### LogMergePolicy

    public LogMergePolicy()

Method Detail

### getMergeFactor

    public int getMergeFactor()

  Returns the number of segments that are merged at once and also controls the total number of segments allowed to accumulate in the index.

### setMergeFactor

    public void setMergeFactor(int mergeFactor)

  Determines how often segment indices are merged by addDocument(). With smaller values, less RAM is used while indexing, and searches on unoptimized indices are faster, but indexing speed is slower. With larger values, more RAM is used during indexing, and while searches on unoptimized indices are slower, indexing is faster. Thus larger values (> 10) are best for batch index creation, and smaller values (< 10) for indices that are interactively maintained.

### useCompoundFile

    public boolean useCompoundFile(org.apache.lucene.index.SegmentInfos infos,
                                   org.apache.lucene.index.SegmentInfo info)

  Description copied from class: `MergePolicy`
  Returns true if a newly flushed (not from merge) segment should use the compound file format.

### setUseCompoundFile

    public void setUseCompoundFile(boolean useCompoundFile)

  Sets whether compound file format should be used for newly flushed and newly merged segments.

### getUseCompoundFile

    public boolean getUseCompoundFile()

  Returns true if newly flushed and newly merge segments are written in compound file format. @see #setUseCompoundFile

### useCompoundDocStore

    public boolean useCompoundDocStore(org.apache.lucene.index.SegmentInfos infos)

  Description copied from class: `MergePolicy`
  Returns true if the doc store files should use the compound file format.

### setUseCompoundDocStore

    public void setUseCompoundDocStore(boolean useCompoundDocStore)

  Sets whether compound file format should be used for newly flushed and newly merged doc store segment files (term vectors and stored fields).

### getUseCompoundDocStore

    public boolean getUseCompoundDocStore()

  Returns true if newly flushed and newly merge doc store segment files (term vectors and stored fields) are written in compound file format. @see #setUseCompoundDocStore

### close

    public void close()

  Description copied from class: `MergePolicy`
  Release all resources for the policy.

### size

    protected abstract long size(org.apache.lucene.index.SegmentInfo info)
                          throws IOException

  Throws:
    `IOException`

### findMergesForOptimize

    public MergePolicy.MergeSpecification findMergesForOptimize(org.apache.lucene.index.SegmentInfos infos,
                                                                IndexWriter writer,
                                                                int maxNumSegments,
                                                                Set segmentsToOptimize)
                                                         throws IOException

  Returns the merges necessary to optimize the index. This merge policy defines "optimized" to mean only one segment in the index, where that segment has no deletions pending nor separate norms, and it is in compound file format if the current useCompoundFile setting is true. This method returns multiple merges (mergeFactor at a time) so the `MergeScheduler` in use may make use of concurrency.

  Parameters:
    `infos` - the total set of segments in the index
    `writer` - IndexWriter instance
    `maxNumSegments` - requested maximum number of segments in the index (currently this is always 1)
    `segmentsToOptimize` - contains the specific SegmentInfo instances that must be merged away. This may be a subset of all SegmentInfos.
  Throws:
    `IOException`

### findMerges

    public MergePolicy.MergeSpecification findMerges(org.apache.lucene.index.SegmentInfos infos,
                                                     IndexWriter writer)
                                              throws IOException

  Checks if any merges are now necessary and returns a `MergePolicy.MergeSpecification` if so. A merge is necessary when there are more than `setMergeFactor(int)` segments at a given level. When multiple levels have too many segments, this method will return multiple merges, allowing the `MergeScheduler` to use concurrency.

  Parameters:
    `infos` - the total set of segments in the index
    `writer` - IndexWriter instance
  Throws:
    `IOException`

### setMaxMergeDocs

    public void setMaxMergeDocs(int maxMergeDocs)

  Determines the largest segment (measured by document count) that may be merged with other segments. Small values (e.g., less than 10,000) are best for interactive indexing, as this limits the length of pauses while indexing to a few seconds. Larger values are best for batched indexing and speedier searches.

  The default value is `Integer.MAX_VALUE`.

  The default merge policy (`LogByteSizeMergePolicy`) also allows you to set this limit by net size (in MB) of the segment, using `LogByteSizeMergePolicy.setMaxMergeMB(double)`.

### getMaxMergeDocs

    public int getMaxMergeDocs()

  Returns the largest segment (measured by document count) that may be merged with other segments.

  See Also:
    `setMaxMergeDocs(int)`

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