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* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
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/**
* Code to maintain and access indices.
* Table Of Contents
*
* - Index APIs
*
*
* - Field types
*
* - Postings
* - Stored Fields
* - DocValues
* - Points
*
*
* - Postings APIs
*
*
* - Index Statistics
*
* - Term-level
* - Field-level
* - Segment-level
* - Document-level
*
*
*
*
* Index APIs
*
* IndexWriter
* {@link org.apache.lucene.index.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
* {@link org.apache.lucene.store.Directory}, depending on the configuration in {@link
* org.apache.lucene.index.IndexWriterConfig}. A Directory is an abstraction that typically
* represents a local file-system directory (see various implementations of {@link
* org.apache.lucene.store.FSDirectory}), but it may also stand for some other storage, such as
* RAM.
*
* IndexReader
* {@link org.apache.lucene.index.IndexReader} is used to read data from the index, and supports
* searching. Many thread-safe readers may be {@link org.apache.lucene.index.DirectoryReader#open}
* 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 {@link
* org.apache.lucene.index.DirectoryReader#openIfChanged}) 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 {@link org.apache.lucene.index.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 {@link org.apache.lucene.index.Term}
* (roughly, a word or token), to (the ordered list of) {@link org.apache.lucene.document.Document}s
* containing that Term. 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 {@link org.apache.lucene.index.StoredFieldVisitor}.
*
* {@link org.apache.lucene.index.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.
*
* {@link org.apache.lucene.index.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
*
*
* {@link org.apache.lucene.index.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
*
*
* {@link org.apache.lucene.index.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());
* }
*
* {@link org.apache.lucene.index.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
*
*
* {@link org.apache.lucene.index.PostingsEnum} is an extension of
* {@link org.apache.lucene.search.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
*
*
* - {@link org.apache.lucene.index.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.
*
- {@link org.apache.lucene.index.TermsEnum#totalTermFreq}: Returns the number
* of occurrences of this term across all documents. Note that this statistic
* is unavailable (returns
-1) if term frequencies were omitted
* from the index
* ({@link org.apache.lucene.index.IndexOptions#DOCS DOCS})
* for the field. Like docFreq(), it will also count occurrences that appear in
* deleted documents.
*
*
*
* Field statistics
*
*
* - {@link org.apache.lucene.index.Terms#size}: Returns the number of
* unique terms in the field. This statistic may be unavailable
* (returns
-1) for some Terms implementations such as
* {@link org.apache.lucene.index.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.
* - {@link org.apache.lucene.index.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.
*
- {@link org.apache.lucene.index.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 {@link org.apache.lucene.index.TermsEnum#docFreq}
* across all terms in the field, and like docFreq() it will also count postings
* that appear in deleted documents.
*
- {@link org.apache.lucene.index.Terms#getSumTotalTermFreq}: Returns the number
* of tokens for the field. This can be thought of as the sum of
* {@link org.apache.lucene.index.TermsEnum#totalTermFreq} across all terms in the
* field, and like totalTermFreq() it will also count occurrences that appear in
* deleted documents, and will be unavailable (returns
-1) if term
* frequencies were omitted from the index
* ({@link org.apache.lucene.index.IndexOptions#DOCS DOCS})
* for the field.
*
*
*
* Segment statistics
*
*
* - {@link org.apache.lucene.index.IndexReader#maxDoc}: Returns the number of
* documents (including deleted documents) in the index.
*
- {@link org.apache.lucene.index.IndexReader#numDocs}: Returns the number
* of live documents (excluding deleted documents) in the index.
*
- {@link org.apache.lucene.index.IndexReader#numDeletedDocs}: Returns the
* number of deleted documents in the index.
*
- {@link org.apache.lucene.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 {@link org.apache.lucene.search.similarities.Similarity} implementation will store some
* of these values (possibly in a lossy way), into the normalization value for the document in
* its {@link org.apache.lucene.search.similarities.Similarity#computeNorm} method.
*
* - {@link org.apache.lucene.index.FieldInvertState#getLength}: Returns the number of
* tokens for this field in the document. Note that this is just the number
* of times that {@link org.apache.lucene.analysis.TokenStream#incrementToken} returned
* true, and is unrelated to the values in
* {@link org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute}.
*
- {@link org.apache.lucene.index.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.
*
- {@link org.apache.lucene.index.FieldInvertState#getPosition}: Returns the accumulated
* position value for this field in the document: computed from the values of
* {@link org.apache.lucene.analysis.tokenattributes.PositionIncrementAttribute} and including
* {@link org.apache.lucene.analysis.Analyzer#getPositionIncrementGap}s across multivalued
* fields.
*
- {@link org.apache.lucene.index.FieldInvertState#getOffset}: Returns the total
* character offset value for this field in the document: computed from the values of
* {@link org.apache.lucene.analysis.tokenattributes.OffsetAttribute} returned by
* {@link org.apache.lucene.analysis.TokenStream#end}, and including
* {@link org.apache.lucene.analysis.Analyzer#getOffsetGap}s across multivalued
* fields.
*
- {@link org.apache.lucene.index.FieldInvertState#getUniqueTermCount}: Returns the number
* of unique terms encountered for this field in the document.
*
- {@link org.apache.lucene.index.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 {@link org.apache.lucene.index.LeafReader#getNumericDocValues}.
*/
package org.apache.lucene.index;