# Package org.apache.lucene.index

Code to maintain and access indices.

See: Description

- Interface Summary
  Interface | Description
  --- | ---
  ImpactsSource | Source of `Impacts`.
  IndexableField | Represents a single field for indexing.
  IndexableFieldType | Describes the properties of a field.
  IndexReader.CacheHelper | A utility class that gives hooks in order to help build a cache based on the data that is contained in this index.
  IndexReader.ClosedListener | A listener that is called when a resource gets closed.
  IndexWriter.IndexReaderWarmer | If `DirectoryReader.open(IndexWriter)` has been called (ie, this writer is in near real-time mode), then after a merge completes, this class can be invoked to warm the reader on the newly merged segment, before the merge commits.
  MergePolicy.MergeContext | This interface represents the current context of the merge selection process.
  PointValues.IntersectVisitor | We recurse the BKD tree, using a provided instance of this to guide the recursion.
  QueryTimeout | Base for query timeout implementations, which will provide a `shouldExit()` method, used with `ExitableDirectoryReader`.
  TwoPhaseCommit | An interface for implementations that support 2-phase commit.

- Class Summary
  Class | Description
  --- | ---
  AutomatonTermsEnum | A FilteredTermsEnum that enumerates terms based upon what is accepted by a DFA.
  BaseCompositeReader<R extends IndexReader> | Base class for implementing `CompositeReader`s based on an array of sub-readers.
  BaseTermsEnum | A base TermsEnum that adds default implementations for `BaseTermsEnum.attributes()` `BaseTermsEnum.termState()` `BaseTermsEnum.seekExact(BytesRef)` `BaseTermsEnum.seekExact(BytesRef, TermState)` In some cases, the default implementation may be slow and consume huge memory, so subclass SHOULD have its own implementation if possible.
  BinaryDocValues | A per-document numeric value.
  CheckIndex | Basic tool and API to check the health of an index and write a new segments file that removes reference to problematic segments.
  CheckIndex.Options | Run-time configuration options for CheckIndex commands.
  CheckIndex.Status | Returned from `CheckIndex.checkIndex()` detailing the health and status of the index.
  CheckIndex.Status.DocValuesStatus | Status from testing DocValues
  CheckIndex.Status.FieldInfoStatus | Status from testing field infos.
  CheckIndex.Status.FieldNormStatus | Status from testing field norms.
  CheckIndex.Status.IndexSortStatus | Status from testing index sort
  CheckIndex.Status.LiveDocStatus | Status from testing livedocs
  CheckIndex.Status.PointsStatus | Status from testing PointValues
  CheckIndex.Status.SegmentInfoStatus | Holds the status of each segment in the index.
  CheckIndex.Status.StoredFieldStatus | Status from testing stored fields.
  CheckIndex.Status.TermIndexStatus | Status from testing term index.
  CheckIndex.Status.TermVectorStatus | Status from testing stored fields.
  CheckIndex.VerifyPointsVisitor | Walks the entire N-dimensional points space, verifying that all points fall within the last cell's boundaries.
  CodecReader | LeafReader implemented by codec APIs.
  CompositeReader | Instances of this reader type can only be used to get stored fields from the underlying LeafReaders, but it is not possible to directly retrieve postings.
  CompositeReaderContext | `IndexReaderContext` for `CompositeReader` instance.
  ConcurrentMergeScheduler | A `MergeScheduler` that runs each merge using a separate thread.
  DirectoryReader | DirectoryReader is an implementation of `CompositeReader` that can read indexes in a `Directory`.
  DocIDMerger<T extends DocIDMerger.Sub> | Utility class to help merging documents from sub-readers according to either simple concatenated (unsorted) order, or by a specified index-time sort, skipping deleted documents and remapping non-deleted documents.
  DocIDMerger.Sub | Represents one sub-reader being merged
  DocValues | This class contains utility methods and constants for DocValues
  EmptyDocValuesProducer | Abstract base class implementing a `DocValuesProducer` that has no doc values.
  ExitableDirectoryReader | The `ExitableDirectoryReader` wraps a real index `DirectoryReader` and allows for a `QueryTimeout` implementation object to be checked periodically to see if the thread should exit or not.
  ExitableDirectoryReader.ExitableFilterAtomicReader | Wrapper class for another FilterAtomicReader.
  ExitableDirectoryReader.ExitableSubReaderWrapper | Wrapper class for a SubReaderWrapper that is used by the ExitableDirectoryReader.
  ExitableDirectoryReader.ExitableTerms | Wrapper class for another Terms implementation that is used by ExitableFields.
  ExitableDirectoryReader.ExitableTermsEnum | Wrapper class for TermsEnum that is used by ExitableTerms for implementing an exitable enumeration of terms.
  FieldInfo | Access to the Field Info file that describes document fields and whether or not they are indexed.
  FieldInfos | Collection of `FieldInfo`s (accessible by number or by name).
  FieldInvertState | This class tracks the number and position / offset parameters of terms being added to the index.
  Fields | Provides a `Terms` index for fields that have it, and lists which fields do.
  FilterBinaryDocValues | Delegates all methods to a wrapped `BinaryDocValues`.
  FilterCodecReader | A `FilterCodecReader` contains another CodecReader, which it uses as its basic source of data, possibly transforming the data along the way or providing additional functionality.
  FilterDirectoryReader | A FilterDirectoryReader wraps another DirectoryReader, allowing implementations to transform or extend it.
  FilterDirectoryReader.SubReaderWrapper | Factory class passed to FilterDirectoryReader constructor that allows subclasses to wrap the filtered DirectoryReader's subreaders.
  FilteredTermsEnum | Abstract class for enumerating a subset of all terms.
  FilterLeafReader | A `FilterLeafReader` contains another LeafReader, which it uses as its basic source of data, possibly transforming the data along the way or providing additional functionality.
  FilterLeafReader.FilterFields | Base class for filtering `Fields` implementations.
  FilterLeafReader.FilterPostingsEnum | Base class for filtering `PostingsEnum` implementations.
  FilterLeafReader.FilterTerms | Base class for filtering `Terms` implementations.
  FilterLeafReader.FilterTermsEnum | Base class for filtering `TermsEnum` implementations.
  FilterMergePolicy | A wrapper for `MergePolicy` instances.
  FilterNumericDocValues | Delegates all methods to a wrapped `NumericDocValues`.
  Impact | Per-document scoring factors.
  Impacts | Information about upcoming impacts, ie.
  ImpactsEnum | Extension of `PostingsEnum` which also provides information about upcoming impacts.
  IndexCommit | Expert: represents a single commit into an index as seen by the `IndexDeletionPolicy` or `IndexReader`.
  IndexDeletionPolicy | Expert: policy for deletion of stale `index commits`.
  IndexFileNames | This class contains useful constants representing filenames and extensions used by lucene, as well as convenience methods for querying whether a file name matches an extension (`matchesExtension`), as well as generating file names from a segment name, generation and extension ( `fileNameFromGeneration`, `segmentFileName`).
  IndexReader | IndexReader is an abstract class, providing an interface for accessing a point-in-time view of an index.
  IndexReader.CacheKey | A cache key identifying a resource that is being cached on.
  IndexReaderContext | A struct like class that represents a hierarchical relationship between `IndexReader` instances.
  IndexUpgrader | This is an easy-to-use tool that upgrades all segments of an index from previous Lucene versions to the current segment file format.
  IndexWriter | An `IndexWriter` creates and maintains an index.
  IndexWriter.DocStats | DocStats for this index
  IndexWriterConfig | Holds all the configuration that is used to create an `IndexWriter`.
  KeepOnlyLastCommitDeletionPolicy | This `IndexDeletionPolicy` implementation that keeps only the most recent commit and immediately removes all prior commits after a new commit is done.
  LeafMetaData | Provides read-only metadata about a leaf.
  LeafReader | `LeafReader` is an abstract class, providing an interface for accessing an index.
  LeafReaderContext | `IndexReaderContext` for `LeafReader` instances.
  LiveIndexWriterConfig | Holds all the configuration used by `IndexWriter` with few setters for settings that can be changed on an `IndexWriter` instance "live".
  LogByteSizeMergePolicy | This is a `LogMergePolicy` that measures size of a segment as the total byte size of the segment's files.
  LogDocMergePolicy | This is a `LogMergePolicy` that measures size of a segment as the number of documents (not taking deletions into account).
  LogMergePolicy | This class implements a `MergePolicy` that tries to merge segments into levels of exponentially increasing size, where each level has fewer segments than the value of the merge factor.
  MappedMultiFields | A `Fields` implementation that merges multiple Fields into one, and maps around deleted documents.
  MergePolicy | Expert: a MergePolicy determines the sequence of primitive merge operations.
  MergePolicy.MergeSpecification | A MergeSpecification instance provides the information necessary to perform multiple merges.
  MergePolicy.OneMerge | OneMerge provides the information necessary to perform an individual primitive merge operation, resulting in a single new segment.
  MergePolicy.OneMergeProgress | Progress and state for an executing merge.
  MergeRateLimiter | This is the `RateLimiter` that `IndexWriter` assigns to each running merge, to give `MergeScheduler`s ionice like control.
  MergeScheduler | Expert: `IndexWriter` uses an instance implementing this interface to execute the merges selected by a `MergePolicy`.
  MergeState | Holds common state used during segment merging.
  MergeState.DocMap | A map of doc IDs.
  MultiBits | Concatenates multiple Bits together, on every lookup.
  MultiDocValues | A wrapper for CompositeIndexReader providing access to DocValues.
  MultiDocValues.MultiSortedDocValues | Implements SortedDocValues over n subs, using an OrdinalMap
  MultiDocValues.MultiSortedSetDocValues | Implements MultiSortedSetDocValues over n subs, using an OrdinalMap
  MultiFields | Provides a single `Fields` term index view over an `IndexReader`.
  MultiLeafReader | Utility methods for working with a `IndexReader` as if it were a `LeafReader`.
  MultiPostingsEnum | Exposes `PostingsEnum`, merged from `PostingsEnum` API of sub-segments.
  MultiPostingsEnum.EnumWithSlice | Holds a `PostingsEnum` along with the corresponding `ReaderSlice`.
  MultiReader | A `CompositeReader` which reads multiple indexes, appending their content.
  MultiTerms | Exposes flex API, merged from flex API of sub-segments.
  MultiTermsEnum | Exposes `TermsEnum` API, merged from `TermsEnum` API of sub-segments.
  NoDeletionPolicy | An `IndexDeletionPolicy` which keeps all index commits around, never deleting them.
  NoMergePolicy | A `MergePolicy` which never returns merges to execute.
  NoMergeScheduler | A `MergeScheduler` which never executes any merges.
  NumericDocValues | A per-document numeric value.
  OneMergeWrappingMergePolicy | A wrapping merge policy that wraps the `MergePolicy.OneMerge` objects returned by the wrapped merge policy.
  OrdinalMap | Maps per-segment ordinals to/from global ordinal space, using a compact packed-ints representation.
  OrdTermState | An ordinal based `TermState`
  ParallelCompositeReader | An `CompositeReader` which reads multiple, parallel indexes.
  ParallelLeafReader | An `LeafReader` which reads multiple, parallel indexes.
  PersistentSnapshotDeletionPolicy | A `SnapshotDeletionPolicy` which adds a persistence layer so that snapshots can be maintained across the life of an application.
  PointValues | Access to indexed numeric values.
  PostingsEnum | Iterates through the postings.
  PrefixCodedTerms | Prefix codes term instances (prefixes are shared).
  PrefixCodedTerms.Builder | Builds a PrefixCodedTerms: call add repeatedly, then finish.
  PrefixCodedTerms.TermIterator | An iterator over the list of terms stored in a `PrefixCodedTerms`.
  QueryTimeoutImpl | An implementation of `QueryTimeout` that can be used by the `ExitableDirectoryReader` class to time out and exit out when a query takes a long time to rewrite.
  ReaderManager | Utility class to safely share `DirectoryReader` instances across multiple threads, while periodically reopening.
  ReaderSlice | Subreader slice from a parent composite reader.
  ReaderUtil | Common util methods for dealing with `IndexReader`s and `IndexReaderContext`s.
  SegmentCommitInfo | Embeds a [read-only] SegmentInfo and adds per-commit fields.
  SegmentInfo | Information about a segment such as its name, directory, and files related to the segment.
  SegmentInfos | A collection of segmentInfo objects with methods for operating on those segments in relation to the file system.
  SegmentInfos.FindSegmentsFile<T> | Utility class for executing code that needs to do something with the current segments file.
  SegmentReader | IndexReader implementation over a single segment.
  SegmentReadState | Holder class for common parameters used during read.
  SegmentWriteState | Holder class for common parameters used during write.
  SerialMergeScheduler | A `MergeScheduler` that simply does each merge sequentially, using the current thread.
  SimpleMergedSegmentWarmer | A very simple merged segment warmer that just ensures data structures are initialized.
  SingleTermsEnum | Subclass of FilteredTermsEnum for enumerating a single term.
  SlowCodecReaderWrapper | Wraps arbitrary readers for merging.
  SlowImpactsEnum | `ImpactsEnum` that doesn't index impacts but implements the API in a legal way.
  SnapshotDeletionPolicy | An `IndexDeletionPolicy` that wraps any other `IndexDeletionPolicy` and adds the ability to hold and later release snapshots of an index.
  SoftDeletesDirectoryReaderWrapper | This reader filters out documents that have a doc values value in the given field and treat these documents as soft deleted.
  SoftDeletesRetentionMergePolicy | This `MergePolicy` allows to carry over soft deleted documents across merges.
  SortedDocValues | A per-document byte[] with presorted values.
  SortedNumericDocValues | A list of per-document numeric values, sorted according to `Long.compare(long, long)`.
  SortedSetDocValues | A multi-valued version of `SortedDocValues`.
  StandardDirectoryReader | Default implementation of `DirectoryReader`.
  StoredFieldVisitor | Expert: provides a low-level means of accessing the stored field values in an index.
  Term | A Term represents a word from text.
  Terms | Access to the terms in a specific field.
  TermsEnum | Iterator to seek (`TermsEnum.seekCeil(BytesRef)`, `TermsEnum.seekExact(BytesRef)`) or step through (`BytesRefIterator.next()` terms to obtain frequency information (`TermsEnum.docFreq()`), `PostingsEnum` or `PostingsEnum` for the current term (`TermsEnum.postings(org.apache.lucene.index.PostingsEnum)`.
  TermState | Encapsulates all required internal state to position the associated `TermsEnum` without re-seeking.
  TermStates | Maintains a `IndexReader` `TermState` view over `IndexReader` instances containing a single term.
  TieredMergePolicy | Merges segments of approximately equal size, subject to an allowed number of segments per tier.
  TieredMergePolicy.MergeScore | Holds score and explanation for a single candidate merge.
  TwoPhaseCommitTool | A utility for executing 2-phase commit on several objects.
  UpgradeIndexMergePolicy | This `MergePolicy` is used for upgrading all existing segments of an index when calling `IndexWriter.forceMerge(int)`.

- Enum Summary
  Enum | Description
  --- | ---
  DocValuesType | DocValues types.
  FilteredTermsEnum.AcceptStatus | Return value, if term should be accepted or the iteration should `END`.
  IndexOptions | Controls how much information is stored in the postings lists.
  IndexWriterConfig.OpenMode | Specifies the open mode for `IndexWriter`.
  MergePolicy.OneMergeProgress.PauseReason | Reason for pausing the merge thread.
  MergeTrigger | MergeTrigger is passed to `MergePolicy.findMerges(MergeTrigger, SegmentInfos, MergePolicy.MergeContext)` to indicate the event that triggered the merge.
  PointValues.Relation | Used by `PointValues.intersect(org.apache.lucene.index.PointValues.IntersectVisitor)` to check how each recursive cell corresponds to the query.
  StoredFieldVisitor.Status | Enumeration of possible return values for `StoredFieldVisitor.needsField(org.apache.lucene.index.FieldInfo)`.
  TermsEnum.SeekStatus | Represents returned result from `TermsEnum.seekCeil(org.apache.lucene.util.BytesRef)`.

- Exception Summary
  Exception | Description
  --- | ---
  CorruptIndexException | This exception is thrown when Lucene detects an inconsistency in the index.
  ExitableDirectoryReader.ExitingReaderException | Exception that is thrown to prematurely terminate a term enumeration.
  IndexFormatTooNewException | This exception is thrown when Lucene detects an index that is newer than this Lucene version.
  IndexFormatTooOldException | This exception is thrown when Lucene detects an index that is too old for this Lucene version
  IndexNotFoundException | Signals that no index was found in the Directory.
  MergePolicy.MergeAbortedException | Thrown when a merge was explicitly aborted because `IndexWriter.abortMerges()` was called.
  MergePolicy.MergeException | Exception thrown if there are any problems while executing a merge.
  TwoPhaseCommitTool.CommitFailException | Thrown by `TwoPhaseCommitTool.execute(TwoPhaseCommit...)` when an object fails to commit().
  TwoPhaseCommitTool.PrepareCommitFailException | Thrown by `TwoPhaseCommitTool.execute(TwoPhaseCommit...)` when an object fails to prepareCommit().

## Package org.apache.lucene.index Description

Code to maintain and access indices.

## Table Of Contents

1. Index APIs
   - IndexWriter
   - IndexReader
   - Segments and docids
2. Field types
   - Postings
   - Stored Fields
   - DocValues
   - Points
3. Postings APIs
   - Fields
   - Terms
   - Documents
   - Positions
4. Index Statistics
   - Term-level
   - Field-level
   - Segment-level
   - Document-level

## Index APIs

### IndexWriter

`IndexWriter` is used to create an index, and to add, update and delete documents. The IndexWriter class is thread safe, and enforces a single instance per index. Creating an IndexWriter creates a new index or opens an existing index for writing, in a `Directory`, depending on the configuration in `IndexWriterConfig`. A Directory is an abstraction that typically represents a local file-system directory (see various implementations of `FSDirectory`), but it may also stand for some other storage, such as RAM.

### IndexReader

`IndexReader` is used to read data from the index, and supports searching. Many thread-safe readers may be `DirectoryReader.open(org.apache.lucene.store.Directory)` concurrently with a single (or no) writer. Each reader maintains a consistent "point in time" view of an index and must be explicitly refreshed (see `DirectoryReader.openIfChanged(org.apache.lucene.index.DirectoryReader)`) in order to incorporate writes that may occur after it is opened.

### Segments and docids

Lucene's index is composed of segments, each of which contains a subset of all the documents in the index, and is a complete searchable index in itself, over that subset. As documents are written to the index, new segments are created and flushed to directory storage. Segments are immutable; updates and deletions may only create new segments and do not modify existing ones. Over time, the writer merges groups of smaller segments into single larger ones in order to maintain an index that is efficient to search, and to reclaim dead space left behind by deleted (and updated) documents.

Each document is identified by a 32-bit number, its "docid," and is composed of a collection of Field values of diverse types (postings, stored fields, doc values, and points). Docids come in two flavors: global and per-segment. A document's global docid is just the sum of its per-segment docid and that segment's base docid offset. External, high-level APIs only handle global docids, but internal APIs that reference a `LeafReader`, which is a reader for a single segment, deal in per-segment docids.

Docids are assigned sequentially within each segment (starting at 0). Thus the number of documents in a segment is the same as its maximum docid; some may be deleted, but their docids are retained until the segment is merged. When segments merge, their documents are assigned new sequential docids. Accordingly, docid values must always be treated as internal implementation, not exposed as part of an application, nor stored or referenced outside of Lucene's internal APIs.

## Field Types

Lucene supports a variety of different document field data structures. Lucene's core, the inverted index, is comprised of "postings." The postings, with their term dictionary, can be thought of as a map that provides efficient lookup given a `Term` (roughly, a word or token), to (the ordered list of) `Document`s containing that Term. Codecs may additionally record `impacts` alongside postings in order to be able to skip over low-scoring documents at search time. Postings do not provide any way of retrieving terms given a document, short of scanning the entire index.

Stored fields are essentially the opposite of postings, providing efficient retrieval of field values given a docid. All stored field values for a document are stored together in a block. Different types of stored field provide high-level datatypes such as strings and numbers on top of the underlying bytes. Stored field values are usually retrieved by the searcher using an implementation of `StoredFieldVisitor`.

`DocValues` fields are what are sometimes referred to as columnar, or column-stride fields, by analogy to relational database terminology, in which documents are considered as rows, and fields, columns. DocValues fields store values per-field: a value for every document is held in a single data structure, providing for rapid, sequential lookup of a field-value given a docid. These fields are used for efficient value-based sorting, and for faceting, but they are not useful for filtering.

`PointValues` represent numeric values using a kd-tree data structure. Efficient 1- and higher dimensional implementations make these the choice for numeric range and interval queries, and geo-spatial queries.

## Postings APIs

### Fields

`Fields` is the initial entry point into the postings APIs, this can be obtained in several ways:

     // access indexed fields for an index segment
     Fields fields = reader.fields();
     // access term vector fields for a specified document
     Fields fields = reader.getTermVectors(docid);

Fields implements Java's Iterable interface, so it's easy to enumerate the list of fields:

     // enumerate list of fields
     for (String field : fields) {
       // access the terms for this field
       Terms terms = fields.terms(field);
     }

### Terms

`Terms` represents the collection of terms within a field, exposes some metadata and statistics, and an API for enumeration.

     // metadata about the field
     System.out.println("positions? " + terms.hasPositions());
     System.out.println("offsets? " + terms.hasOffsets());
     System.out.println("payloads? " + terms.hasPayloads());
     // iterate through terms
     TermsEnum termsEnum = terms.iterator(null);
     BytesRef term = null;
     while ((term = termsEnum.next()) != null) {
       doSomethingWith(termsEnum.term());
     }

`TermsEnum` provides an iterator over the list of terms within a field, some statistics about the term, and methods to access the term's documents and positions.

     // seek to a specific term
     boolean found = termsEnum.seekExact(new BytesRef("foobar"));
     if (found) {
       // get the document frequency
       System.out.println(termsEnum.docFreq());
       // enumerate through documents
       PostingsEnum docs = termsEnum.postings(null, null);
       // enumerate through documents and positions
       PostingsEnum docsAndPositions = termsEnum.postings(null, null, PostingsEnum.FLAG_POSITIONS);
     }

### Documents

`PostingsEnum` is an extension of `DocIdSetIterator`that iterates over the list of documents for a term, along with the term frequency within that document.

     int docid;
     while ((docid = docsEnum.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
       System.out.println(docid);
       System.out.println(docsEnum.freq());
      }

### Positions

PostingsEnum also allows iteration of the positions a term occurred within the document, and any additional per-position information (offsets and payload). The information available is controlled by flags passed to TermsEnum#postings

     int docid;
     PostingsEnum postings = termsEnum.postings(null, null, PostingsEnum.FLAG_PAYLOADS | PostingsEnum.FLAG_OFFSETS);
     while ((docid = postings.nextDoc()) != DocIdSetIterator.NO_MORE_DOCS) {
       System.out.println(docid);
       int freq = postings.freq();
       for (int i = 0; i < freq; i++) {
        System.out.println(postings.nextPosition());
        System.out.println(postings.startOffset());
        System.out.println(postings.endOffset());
        System.out.println(postings.getPayload());
       }
     }

## Index Statistics

### Term statistics

- `TermsEnum.docFreq()`: Returns the number of documents that contain at least one occurrence of the term. This statistic is always available for an indexed term. Note that it will also count deleted documents, when segments are merged the statistic is updated as those deleted documents are merged away.
- `TermsEnum.totalTermFreq()`: Returns the number of occurrences of this term across all documents. Like docFreq(), it will also count occurrences that appear in deleted documents.

### Field statistics

- `Terms.size()`: Returns the number of unique terms in the field. This statistic may be unavailable (returns `-1`) for some Terms implementations such as `MultiTerms`, where it cannot be efficiently computed. Note that this count also includes terms that appear only in deleted documents: when segments are merged such terms are also merged away and the statistic is then updated.
- `Terms.getDocCount()`: Returns the number of documents that contain at least one occurrence of any term for this field. This can be thought of as a Field-level docFreq(). Like docFreq() it will also count deleted documents.
- `Terms.getSumDocFreq()`: Returns the number of postings (term-document mappings in the inverted index) for the field. This can be thought of as the sum of `TermsEnum.docFreq()` across all terms in the field, and like docFreq() it will also count postings that appear in deleted documents.
- `Terms.getSumTotalTermFreq()`: Returns the number of tokens for the field. This can be thought of as the sum of `TermsEnum.totalTermFreq()` across all terms in the field, and like totalTermFreq() it will also count occurrences that appear in deleted documents.

### Segment statistics

- `IndexReader.maxDoc()`: Returns the number of documents (including deleted documents) in the index.
- `IndexReader.numDocs()`: Returns the number of live documents (excluding deleted documents) in the index.
- `IndexReader.numDeletedDocs()`: Returns the number of deleted documents in the index.
- `Fields.size()`: Returns the number of indexed fields.

### Document statistics

Document statistics are available during the indexing process for an indexed field: typically a `Similarity` implementation will store some of these values (possibly in a lossy way), into the normalization value for the document in its `Similarity.computeNorm(org.apache.lucene.index.FieldInvertState)` method.

- `FieldInvertState.getLength()`: Returns the number of tokens for this field in the document. Note that this is just the number of times that `TokenStream.incrementToken()` returned true, and is unrelated to the values in `PositionIncrementAttribute`.
- `FieldInvertState.getNumOverlap()`: Returns the number of tokens for this field in the document that had a position increment of zero. This can be used to compute a document length that discounts artificial tokens such as synonyms.
- `FieldInvertState.getPosition()`: Returns the accumulated position value for this field in the document: computed from the values of `PositionIncrementAttribute` and including `Analyzer.getPositionIncrementGap(java.lang.String)`s across multivalued fields.
- `FieldInvertState.getOffset()`: Returns the total character offset value for this field in the document: computed from the values of `OffsetAttribute` returned by `TokenStream.end()`, and including `Analyzer.getOffsetGap(java.lang.String)`s across multivalued fields.
- `FieldInvertState.getUniqueTermCount()`: Returns the number of unique terms encountered for this field in the document.
- `FieldInvertState.getMaxTermFrequency()`: Returns the maximum frequency across all unique terms encountered for this field in the document.

Additional user-supplied statistics can be added to the document as DocValues fields and accessed via `LeafReader.getNumericDocValues(java.lang.String)`.

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