mirror of https://github.com/apache/lucene.git
LUCENE-2946: new fileformats page (WIP)
git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1332650 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Robert Muir
parent
981c324e58
commit
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<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
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</head>
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<body>
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Default codec for Lucene 4.0 indexes.
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Lucene 4.0 file format.
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<h1>Apache Lucene - Index File Formats</h1>
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<div>
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<ul>
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<li><a href="#Index_File_Formats">Index File Formats</a></li>
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<li><a href="#Definitions">Definitions</a>
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<ul>
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<li><a href="#Inverted_Indexing">Inverted Indexing</a></li>
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<li><a href="#Types_of_Fields">Types of Fields</a></li>
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<li><a href="#Segments">Segments</a></li>
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<li><a href="#Document_Numbers">Document Numbers</a></li>
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</ul>
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</li>
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<li><a href="#Overview">Overview</a></li>
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<li><a href="#File_Naming">File Naming</a></li>
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<li><a href="#file-names">Summary of File Extensions</a></li>
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<ul>
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<li><a href="#Lock_File">Lock File</a></li>
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<li><a href="#Limitations">Limitations</a></li>
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</ul>
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</div>
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<a name="Index_File_Formats"></a>
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<h2>Index File Formats</h2>
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<div>
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<p>This document defines the index file formats used in this version of Lucene.
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If you are using a different version of Lucene, please consult the copy of
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<code>docs/</code> that was distributed with
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the version you are using.</p>
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<p>Apache Lucene is written in Java, but several efforts are underway to write
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<a href="http://wiki.apache.org/lucene-java/LuceneImplementations">versions of
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Lucene in other programming languages</a>. If these versions are to remain
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compatible with Apache Lucene, then a language-independent definition of the
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Lucene index format is required. This document thus attempts to provide a
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complete and independent definition of the Apache Lucene file formats.</p>
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<p>As Lucene evolves, this document should evolve. Versions of Lucene in
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different programming languages should endeavor to agree on file formats, and
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generate new versions of this document.</p>
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<p>Compatibility notes are provided in this document, describing how file
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formats have changed from prior versions.</p>
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<p>In version 2.1, the file format was changed to allow lock-less commits (ie,
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no more commit lock). The change is fully backwards compatible: you can open a
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pre-2.1 index for searching or adding/deleting of docs. When the new segments
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file is saved (committed), it will be written in the new file format (meaning
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no specific "upgrade" process is needed). But note that once a commit has
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occurred, pre-2.1 Lucene will not be able to read the index.</p>
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<p>In version 2.3, the file format was changed to allow segments to share a
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single set of doc store (vectors & stored fields) files. This allows for
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faster indexing in certain cases. The change is fully backwards compatible (in
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the same way as the lock-less commits change in 2.1).</p>
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<p>In version 2.4, Strings are now written as true UTF-8 byte sequence, not
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Java's modified UTF-8. See issue LUCENE-510 for details.</p>
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<p>In version 2.9, an optional opaque Map<String,String> CommitUserData
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may be passed to IndexWriter's commit methods (and later retrieved), which is
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recorded in the segments_N file. See issue LUCENE-1382 for details. Also,
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diagnostics were added to each segment written recording details about why it
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was written (due to flush, merge; which OS/JRE was used; etc.). See issue
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LUCENE-1654 for details.</p>
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<p>In version 3.0, compressed fields are no longer written to the index (they
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can still be read, but on merge the new segment will write them, uncompressed).
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See issue LUCENE-1960 for details.</p>
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<p>In version 3.1, segments records the code version that created them. See
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LUCENE-2720 for details. Additionally segments track explicitly whether or not
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they have term vectors. See LUCENE-2811 for details.</p>
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<p>In version 3.2, numeric fields are written as natively to stored fields
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file, previously they were stored in text format only.</p>
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<p>In version 3.4, fields can omit position data while still indexing term
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frequencies.</p>
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<p>In version 4.0, the format of the inverted index became extensible via
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the {@link org.apache.lucene.codecs.Codec Codec} api. Fast per-document storage
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({@link org.apache.lucene.index.DocValues DocValues}) was introduced. Normalization
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factors need no longer be a single byte, they can be any DocValues
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{@link org.apache.lucene.index.DocValues.Type type}. Terms need not be unicode
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strings, they can be any byte sequence. Term offsets can optionally be indexed
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into the postings lists.</p>
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</div>
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<a name="Definitions" id="Definitions"></a>
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<h2>Definitions</h2>
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<div>
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<p>The fundamental concepts in Lucene are index, document, field and term.</p>
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<p>An index contains a sequence of documents.</p>
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<ul>
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<li>
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<p>A document is a sequence of fields.</p>
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</li>
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<li>
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<p>A field is a named sequence of terms.</p>
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</li>
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<li>A term is a sequence of bytes.</li>
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</ul>
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<p>The same sequence of bytes in two different fields is considered a different
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term. Thus terms are represented as a pair: the string naming the field, and the
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bytes within the field.</p>
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<a name="Inverted_Indexing"></a>
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<h3>Inverted Indexing</h3>
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<p>The index stores statistics about terms in order to make term-based search
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more efficient. Lucene's index falls into the family of indexes known as an
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<i>inverted index.</i> This is because it can list, for a term, the documents
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that contain it. This is the inverse of the natural relationship, in which
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documents list terms.</p>
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<a name="Types_of_Fields"></a>
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<h3>Types of Fields</h3>
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<p>In Lucene, fields may be <i>stored</i>, in which case their text is stored
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in the index literally, in a non-inverted manner. Fields that are inverted are
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called <i>indexed</i>. A field may be both stored and indexed.</p>
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<p>The text of a field may be <i>tokenized</i> into terms to be indexed, or the
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text of a field may be used literally as a term to be indexed. Most fields are
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tokenized, but sometimes it is useful for certain identifier fields to be
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indexed literally.</p>
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<p>See the {@link org.apache.lucene.document.Field Field}
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java docs for more information on Fields.</p>
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<a name="Segments" id="Segments"></a>
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<h3>Segments</h3>
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<p>Lucene indexes may be composed of multiple sub-indexes, or <i>segments</i>.
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Each segment is a fully independent index, which could be searched separately.
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Indexes evolve by:</p>
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<ol>
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<li>
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<p>Creating new segments for newly added documents.</p>
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</li>
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<li>
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<p>Merging existing segments.</p>
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</li>
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</ol>
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<p>Searches may involve multiple segments and/or multiple indexes, each index
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potentially composed of a set of segments.</p>
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<a name="Document_Numbers"></a>
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<h3>Document Numbers</h3>
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<p>Internally, Lucene refers to documents by an integer <i>document number</i>.
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The first document added to an index is numbered zero, and each subsequent
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document added gets a number one greater than the previous.</p>
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<p><br></p>
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<p>Note that a document's number may change, so caution should be taken when
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storing these numbers outside of Lucene. In particular, numbers may change in
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the following situations:</p>
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<ul>
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<li>
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<p>The numbers stored in each segment are unique only within the segment, and
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must be converted before they can be used in a larger context. The standard
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technique is to allocate each segment a range of values, based on the range of
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numbers used in that segment. To convert a document number from a segment to an
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external value, the segment's <i>base</i> document number is added. To convert
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an external value back to a segment-specific value, the segment is identified
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by the range that the external value is in, and the segment's base value is
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subtracted. For example two five document segments might be combined, so that
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the first segment has a base value of zero, and the second of five. Document
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three from the second segment would have an external value of eight.</p>
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</li>
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<li>
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<p>When documents are deleted, gaps are created in the numbering. These are
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eventually removed as the index evolves through merging. Deleted documents are
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dropped when segments are merged. A freshly-merged segment thus has no gaps in
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its numbering.</p>
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</li>
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</ul>
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</div>
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<a name="Overview" id="Overview"></a>
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<h2>Overview</h2>
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<div>
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<p>Each segment index maintains the following:</p>
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<ul>
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<li>
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<p>{@link org.apache.lucene.codecs.lucene40.Lucene40FieldInfosFormat Field names}.
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This contains the set of field names used in the index.</p>
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</li>
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<li>
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<p>{@link org.apache.lucene.codecs.lucene40.Lucene40StoredFieldsFormat Stored Field values}.
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This contains, for each document, a list of attribute-value pairs, where the attributes
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are field names. These are used to store auxiliary information about the document, such as
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its title, url, or an identifier to access a database. The set of stored fields are what is
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returned for each hit when searching. This is keyed by document number.</p>
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</li>
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<li>
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<p>{@link org.apache.lucene.codecs.lucene40.Lucene40PostingsFormat Term dictionary}.
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A dictionary containing all of the terms used in all of the
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indexed fields of all of the documents. The dictionary also contains the number
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of documents which contain the term, and pointers to the term's frequency and
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proximity data.</p>
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</li>
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<li>
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<p>{@link org.apache.lucene.codecs.lucene40.Lucene40PostingsFormat Term Frequency data}.
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For each term in the dictionary, the numbers of all the
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documents that contain that term, and the frequency of the term in that
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document, unless frequencies are omitted (IndexOptions.DOCS_ONLY)</p>
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</li>
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<li>
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<p>{@link org.apache.lucene.codecs.lucene40.Lucene40PostingsFormat Term Proximity data}.
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For each term in the dictionary, the positions that the
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term occurs in each document. Note that this will not exist if all fields in
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all documents omit position data.</p>
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</li>
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<li>
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<p>Normalization factors. For each field in each document, a value is stored
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that is multiplied into the score for hits on that field.</p>
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</li>
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<li>
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<p>{@link org.apache.lucene.codecs.lucene40.Lucene40TermVectorsFormat Term Vectors}.
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For each field in each document, the term vector (sometimes
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called document vector) may be stored. A term vector consists of term text and
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term frequency. To add Term Vectors to your index see the
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{@link org.apache.lucene.document.Field Field} constructors</p>
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</li>
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<li>
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<p>Per-document values. Like stored values, these are also keyed by document
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number, but are generally intended to be loaded into main memory for fast
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access. Whereas stored values are generally intended for summary results from
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searches, per-document values are useful for things like scoring factors.
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</li>
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<li>
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<p>{@link org.apache.lucene.codecs.lucene40.Lucene40LiveDocsFormat Deleted documents}.
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An optional file indicating which documents are deleted.</p>
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</li>
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</ul>
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<p>Details on each of these are provided in their linked pages.</p>
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</div>
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<a name="File_Naming"></a>
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<h2>File Naming</h2>
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<div>
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<p>All files belonging to a segment have the same name with varying extensions.
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The extensions correspond to the different file formats described below. When
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using the Compound File format (default in 1.4 and greater) these files are
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collapsed into a single .cfs file (see below for details)</p>
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<p>Typically, all segments in an index are stored in a single directory,
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although this is not required.</p>
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<p>As of version 2.1 (lock-less commits), file names are never re-used (there
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is one exception, "segments.gen", see below). That is, when any file is saved
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to the Directory it is given a never before used filename. This is achieved
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using a simple generations approach. For example, the first segments file is
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segments_1, then segments_2, etc. The generation is a sequential long integer
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represented in alpha-numeric (base 36) form.</p>
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</div>
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<a name="file-names" id="file-names"></a>
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<h2>Summary of File Extensions</h2>
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<div>
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<p>The following table summarizes the names and extensions of the files in
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Lucene:</p>
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<table cellspacing="1" cellpadding="4">
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<tr>
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<th>Name</th>
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<th>Extension</th>
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<th>Brief Description</th>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40SegmentInfosFormat Segments File}</td>
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<td>segments.gen, segments_N</td>
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<td>Stores information about segments</td>
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</tr>
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<tr>
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<td><a href="#Lock_File">Lock File</a></td>
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<td>write.lock</td>
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<td>The Write lock prevents multiple IndexWriters from writing to the same
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file.</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.store.CompoundFileDirectory Compound File}</td>
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<td>.cfs, .cfe</td>
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<td>An optional "virtual" file consisting of all the other index files for
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systems that frequently run out of file handles.</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40FieldInfosFormat Fields}</td>
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<td>.fnm</td>
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<td>Stores information about the fields</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40StoredFieldsFormat Field Index}</td>
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<td>.fdx</td>
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<td>Contains pointers to field data</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40StoredFieldsFormat Field Data}</td>
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<td>.fdt</td>
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<td>The stored fields for documents</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40PostingsFormat Term Dictionary}</td>
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<td>.tim</td>
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<td>The term dictionary, stores term info</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40PostingsFormat Term Index}</td>
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<td>.tip</td>
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<td>The index into the Term Dictionary</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40PostingsFormat Frequencies}</td>
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<td>.frq</td>
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<td>Contains the list of docs which contain each term along with frequency</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40PostingsFormat Positions}</td>
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<td>.prx</td>
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<td>Stores position information about where a term occurs in the index</td>
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</tr>
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<tr>
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<td>Norms</td>
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<td>.nrm.cfs</td>
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<td>Encodes length and boost factors for docs and fields</td>
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</tr>
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<tr>
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<td>Per-Document Values</td>
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<td>.dv.cfs</td>
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<td>Encodes additional scoring factors or other per-document information.</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40TermVectorsFormat Term Vector Index}</td>
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<td>.tvx</td>
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<td>Stores offset into the document data file</td>
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</tr>
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<tr>
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40TermVectorsFormat Term Vector Documents}</td>
|
||||
<td>.tvd</td>
|
||||
<td>Contains information about each document that has term vectors</td>
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</tr>
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<tr>
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||||
<td>{@link org.apache.lucene.codecs.lucene40.Lucene40TermVectorsFormat Term Vector Fields}</td>
|
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<td>.tvf</td>
|
||||
<td>The field level info about term vectors</td>
|
||||
</tr>
|
||||
<tr>
|
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<td>{@link org.apache.lucene.codecs.lucene40.Lucene40LiveDocsFormat Deleted Documents}</td>
|
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<td>.del</td>
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<td>Info about what files are deleted</td>
|
||||
</tr>
|
||||
<tr>
|
||||
<td>{@link org.apache.lucene.codecs.perfield.PerFieldPostingsFormat Field formats}</td>
|
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<td>.per</td>
|
||||
<td>Enables per-field {@link org.apache.lucene.codecs.PostingsFormat PostingsFormat} configuration.</td>
|
||||
</tr>
|
||||
</table>
|
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</div>
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<a name="Lock_File" id="Lock_File"></a>
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<h2>Lock File</h2>
|
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The write lock, which is stored in the index directory by default, is named
|
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"write.lock". If the lock directory is different from the index directory then
|
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the write lock will be named "XXXX-write.lock" where XXXX is a unique prefix
|
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derived from the full path to the index directory. When this file is present, a
|
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writer is currently modifying the index (adding or removing documents). This
|
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lock file ensures that only one writer is modifying the index at a time.</p>
|
||||
<a name="Limitations" id="Limitations"></a>
|
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<h2>Limitations</h2>
|
||||
<div>
|
||||
<p>When referring to term numbers, Lucene's current implementation uses a Java
|
||||
<code>int</code> to hold the term index, which means the
|
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maximum number of unique terms in any single index segment is ~2.1 billion
|
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times the term index interval (default 128) = ~274 billion. This is technically
|
||||
not a limitation of the index file format, just of Lucene's current
|
||||
implementation.</p>
|
||||
<p>Similarly, Lucene uses a Java <code>int</code> to refer to
|
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document numbers, and the index file format uses an <code>Int32</code>
|
||||
on-disk to store document numbers. This is a limitation
|
||||
of both the index file format and the current implementation. Eventually these
|
||||
should be replaced with either <code>UInt64</code> values, or
|
||||
better yet, <code>VInt</code> values which have no limit.</p>
|
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</div>
|
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</body>
|
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</html>
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|
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File diff suppressed because it is too large
Load Diff
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|
@ -65,7 +65,7 @@
|
|||
<li><a href="changes/Changes.html">Changes</a>: List of changes in this release.</li>
|
||||
<li><a href="MIGRATE.html">Migration Guide</a>: What changed in Lucene 4; how to migrate code from Lucene 3.x.</li>
|
||||
<li><a href="JRE_VERSION_MIGRATION.html">JRE Version Migration</a>: Information about upgrading between major JRE versions.</li>
|
||||
<li><a href="fileformats.html">File Formats</a>: Guide to the index format used by Lucene.</li>
|
||||
<li><a href="core/org/apache/lucene/codecs/lucene40/package-summary.html#package_description">File Formats</a>: Guide to the index format used by Lucene.</li>
|
||||
<li><a href="core/org/apache/lucene/search/package-summary.html#package_description">Search and Scoring in Lucene</a>: Introduction to how Lucene scores documents.</li>
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<li><a href="core/org/apache/lucene/search/similarities/TFIDFSimilarity.html">Classic Scoring Formula</a>: Formula of Lucene's classic <a href="http://en.wikipedia.org/wiki/Vector_Space_Model">Vector Space</a> implementation. (look <a href="core/org/apache/lucene/search/similarities/package-summary.html#package_description">here</a> for other models)</li>
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<li><a href="queryparser/org/apache/lucene/queryparser/classic/package-summary.html#package_description">Classic QueryParser Syntax</a>: Overview of the Classic QueryParser's syntax and features.</li>
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Reference in New Issue