mirror of https://github.com/apache/lucene.git
LUCENE-2946: doc 4.0 postings format
git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/trunk@1332241 13f79535-47bb-0310-9956-ffa450edef68
This commit is contained in:
Robert Muir
parent
f2a682a587
commit
30bc808366
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@ -27,11 +27,222 @@ import org.apache.lucene.codecs.FieldsProducer;
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import org.apache.lucene.codecs.PostingsFormat;
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import org.apache.lucene.codecs.PostingsReaderBase;
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import org.apache.lucene.codecs.PostingsWriterBase;
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import org.apache.lucene.index.DocsEnum; // javadocs
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import org.apache.lucene.index.FieldInfo.IndexOptions; // javadocs
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import org.apache.lucene.index.FieldInfos; // javadocs
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import org.apache.lucene.index.SegmentInfo;
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import org.apache.lucene.index.SegmentReadState;
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import org.apache.lucene.index.SegmentWriteState;
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import org.apache.lucene.store.DataOutput; // javadocs
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import org.apache.lucene.util.CodecUtil; // javadocs
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import org.apache.lucene.util.fst.FST; // javadocs
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/** Default codec.
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/**
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* Lucene 4.0 Postings format.
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* <p>
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* Files:
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* <ul>
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* <li><tt>.tim</tt>: <a href="#Termdictionary">Term Dictionary</a></li>
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* <li><tt>.tip</tt>: <a href="#Termindex">Term Index</a></li>
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* <li><tt>.frq</tt>: <a href="#Frequencies">Frequencies</a></li>
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* <li><tt>.prx</tt>: <a href="#Positions">Positions</a></li>
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* </ul>
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* </p>
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* <p>
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* <a name="Termdictionary" id="Termdictionary"></a>
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* <h3>Term Dictionary</h3>
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* <p>The .tim file contains the list of terms in each field, in UTF-8 order,
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* along with per-term statistics (such as docfreq) and pointers to the frequencies,
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* positions, and skip data in the .frq and .prx files.
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* </p>
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* <p>The .tim is arranged in blocks: with blocks containing either terms or
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* sub-blocks.</p>
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* <p>NOTE: The term dictionary can plug into different postings implementations:
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* for example the postings writer/reader are actually responsible for encoding
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* and decoding the MetadataBlock.</p>
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* <ul>
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* <!-- TODO: expand on this, its not really correct and doesnt explain sub-blocks etc -->
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* <li>TermsDict (.tim) --> Header, DirOffset, PostingsHeader, SkipInterval,
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* MaxSkipLevels, SkipMinimum, Block<sup>NumBlocks</sup>,
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* FieldSummary</li>
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* <li>Block --> SuffixBlock, StatsBlock, MetadataBlock</li>
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* <li>SuffixBlock --> EntryCount, SuffixLength, Byte<sup>SuffixLength</sup></li>
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* <li>StatsBlock --> StatsLength, <DocFreq, TotalTermFreq><sup>EntryCount</sup></li>
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* <li>MetadataBlock --> MetaLength, <FreqDelta, SkipDelta?, ProxDelta?><sup>EntryCount</sup></li>
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* <li>FieldSummary --> NumFields, <FieldNumber, NumTerms, RootCodeLength, Byte<sup>RootCodeLength</sup>,
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* SumDocFreq, DocCount><sup>NumFields</sup></li>
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* <li>Header,PostingsHeader --> {@link CodecUtil#writeHeader CodecHeader}</li>
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* <li>DirOffset --> {@link DataOutput#writeLong Uint64}</li>
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* <li>SkipInterval,MaxSkipLevels,SkipMinimum --> {@link DataOutput#writeInt Uint32}</li>
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* <li>EntryCount,SuffixLength,StatsLength,DocFreq,MetaLength,SkipDelta,NumFields,
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* FieldNumber,RootCodeLength,DocCount --> {@link DataOutput#writeVInt VInt}</li>
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* <li>TotalTermFreq,FreqDelta,ProxDelta,NumTerms,SumTotalTermFreq,SumDocFreq -->
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* {@link DataOutput#writeVLong VLong}</li>
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* </ul>
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* <p>Notes:</p>
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* <ul>
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* <li>Header is a {@link CodecUtil#writeHeader CodecHeader} storing the version information
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* for the BlockTree implementation. On the other hand, PostingsHeader stores the version
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* information for the postings reader/writer.</li>
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* <li>DirOffset is a pointer to the FieldSummary section.</li>
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* <li>SkipInterval is the fraction of TermDocs stored in skip tables. It is used to accelerate
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* {@link DocsEnum#advance(int)}. Larger values result in smaller indexes, greater
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* acceleration, but fewer accelerable cases, while smaller values result in bigger indexes,
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* less acceleration (in case of a small value for MaxSkipLevels) and more accelerable cases.
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* </li>
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* <li>MaxSkipLevels is the max. number of skip levels stored for each term in the .frq file. A
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* low value results in smaller indexes but less acceleration, a larger value results in
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* slightly larger indexes but greater acceleration. See format of .frq file for more
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* information about skip levels.</li>
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* <li>SkipMinimum is the minimum document frequency a term must have in order to write any
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* skip data at all.</li>
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* <li>DocFreq is the count of documents which contain the term.</li>
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* <li>TotalTermFreq is the total number of occurrences of the term.</li>
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* <li>FreqDelta determines the position of this term's TermFreqs within the .frq
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* file. In particular, it is the difference between the position of this term's
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* data in that file and the position of the previous term's data (or zero, for
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* the first term in the block).</li>
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* <li>ProxDelta determines the position of this term's TermPositions within the
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* .prx file. In particular, it is the difference between the position of this
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* term's data in that file and the position of the previous term's data (or zero,
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* for the first term in the block. For fields that omit position data, this will
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* be 0 since prox information is not stored.</li>
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* <li>SkipDelta determines the position of this term's SkipData within the .frq
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* file. In particular, it is the number of bytes after TermFreqs that the
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* SkipData starts. In other words, it is the length of the TermFreq data.
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* SkipDelta is only stored if DocFreq is not smaller than SkipMinimum.</li>
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* <li>FieldNumber is the fields number from {@link FieldInfos}. (.fnm)</li>
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* <li>NumTerms is the number of unique terms for the field.</li>
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* <li>RootCode points to the root block for the field.</li>
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* <li>SumDocFreq is the total number of postings, the number of term-document pairs across
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* the entire field.</li>
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* <li>DocCount is the number of documents that have at least one posting for this field.</li>
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* </ul>
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* <a name="Termindex" id="Termindex"></a>
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* <h3>Term Index</h3>
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* <p>The .tip file contains an index into the term dictionary, so that it can be
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* accessed randomly.</p>
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* <ul>
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* <li>TermsIndex (.tip) --> Header, <IndexStartFP><sup>NumFields</sup>,
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* FSTIndex<sup>NumFields</sup></li>
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* <li>Header --> {@link CodecUtil#writeHeader CodecHeader}</li>
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* <li>IndexStartFP --> {@link DataOutput#writeVLong VLong}</li>
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* <!-- TODO: better describe FST output here -->
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* <li>FSTIndex --> {@link FST FST<byte[]>}</li>
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* </ul>
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* <p>Notes:</p>
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* <ul>
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* <li>The .tip file contains a separate FST for each field. Each field's IndexStartFP points
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* to its FST.</li>
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* </ul>
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* <a name="Frequencies" id="Frequencies"></a>
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* <h3>Frequencies</h3>
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* <p>The .frq file contains the lists of documents which contain each term, along
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* with the frequency of the term in that document (except when frequencies are
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* omitted: {@link IndexOptions#DOCS_ONLY}).</p>
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* <ul>
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* <li>FreqFile (.frq) --> <TermFreqs, SkipData> <sup>TermCount</sup></li>
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* <li>TermFreqs --> <TermFreq> <sup>DocFreq</sup></li>
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* <li>TermFreq --> DocDelta[, Freq?]</li>
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* <li>SkipData --> <<SkipLevelLength, SkipLevel>
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* <sup>NumSkipLevels-1</sup>, SkipLevel> <SkipDatum></li>
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* <li>SkipLevel --> <SkipDatum> <sup>DocFreq/(SkipInterval^(Level +
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* 1))</sup></li>
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* <li>SkipDatum -->
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* DocSkip,PayloadLength?,OffsetLength?,FreqSkip,ProxSkip,SkipChildLevelPointer?</li>
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* <li>DocDelta,Freq,DocSkip,PayloadLength,OffsetLength,FreqSkip,ProxSkip --> {@link DataOutput#writeVInt VInt}</li>
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* <li>SkipChildLevelPointer --> {@link DataOutput#writeVLong VLong}</li>
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* </ul>
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* <p>TermFreqs are ordered by term (the term is implicit, from the term dictionary).</p>
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* <p>TermFreq entries are ordered by increasing document number.</p>
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* <p>DocDelta: if frequencies are indexed, this determines both the document
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* number and the frequency. In particular, DocDelta/2 is the difference between
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* this document number and the previous document number (or zero when this is the
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* first document in a TermFreqs). When DocDelta is odd, the frequency is one.
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* When DocDelta is even, the frequency is read as another VInt. If frequencies
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* are omitted, DocDelta contains the gap (not multiplied by 2) between document
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* numbers and no frequency information is stored.</p>
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* <p>For example, the TermFreqs for a term which occurs once in document seven
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* and three times in document eleven, with frequencies indexed, would be the
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* following sequence of VInts:</p>
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* <p>15, 8, 3</p>
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* <p>If frequencies were omitted ({@link IndexOptions#DOCS_ONLY}) it would be this
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* sequence of VInts instead:</p>
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* <p>7,4</p>
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* <p>DocSkip records the document number before every SkipInterval <sup>th</sup>
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* document in TermFreqs. If payloads are disabled for the term's field, then
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* DocSkip represents the difference from the previous value in the sequence. If
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* payloads are enabled for the term's field, then DocSkip/2 represents the
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* difference from the previous value in the sequence. If payloads are enabled and
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* DocSkip is odd, then PayloadLength is stored indicating the length of the last
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* payload before the SkipInterval<sup>th</sup> document in TermPositions. If offsets
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* are enabled, then OffsetLength is stored indicating the length of the last offset
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* (endOffset-startOffset).
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* FreqSkip and ProxSkip record the position of every SkipInterval <sup>th</sup>
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* entry in FreqFile and ProxFile, respectively. File positions are relative to
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* the start of TermFreqs and Positions, to the previous SkipDatum in the
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* sequence.</p>
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* <p>For example, if DocFreq=35 and SkipInterval=16, then there are two SkipData
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* entries, containing the 15 <sup>th</sup> and 31 <sup>st</sup> document numbers
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* in TermFreqs. The first FreqSkip names the number of bytes after the beginning
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* of TermFreqs that the 16 <sup>th</sup> SkipDatum starts, and the second the
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* number of bytes after that that the 32 <sup>nd</sup> starts. The first ProxSkip
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* names the number of bytes after the beginning of Positions that the 16
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* <sup>th</sup> SkipDatum starts, and the second the number of bytes after that
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* that the 32 <sup>nd</sup> starts.</p>
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* <p>Each term can have multiple skip levels. The amount of skip levels for a
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* term is NumSkipLevels = Min(MaxSkipLevels,
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* floor(log(DocFreq/log(SkipInterval)))). The number of SkipData entries for a
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* skip level is DocFreq/(SkipInterval^(Level + 1)), whereas the lowest skip level
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* is Level=0.<br>
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* Example: SkipInterval = 4, MaxSkipLevels = 2, DocFreq = 35. Then skip level 0
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* has 8 SkipData entries, containing the 3<sup>rd</sup>, 7<sup>th</sup>,
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* 11<sup>th</sup>, 15<sup>th</sup>, 19<sup>th</sup>, 23<sup>rd</sup>,
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* 27<sup>th</sup>, and 31<sup>st</sup> document numbers in TermFreqs. Skip level
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* 1 has 2 SkipData entries, containing the 15<sup>th</sup> and 31<sup>st</sup>
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* document numbers in TermFreqs.<br>
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* The SkipData entries on all upper levels > 0 contain a SkipChildLevelPointer
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* referencing the corresponding SkipData entry in level-1. In the example has
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* entry 15 on level 1 a pointer to entry 15 on level 0 and entry 31 on level 1 a
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* pointer to entry 31 on level 0.
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* </p>
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* <a name="Positions" id="Positions"></a>
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* <h3>Positions</h3>
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* <p>The .prx file contains the lists of positions that each term occurs at
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* within documents. Note that fields omitting positional data do not store
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* anything into this file, and if all fields in the index omit positional data
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* then the .prx file will not exist.</p>
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* <ul>
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* <li>ProxFile (.prx) --> <TermPositions> <sup>TermCount</sup></li>
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* <li>TermPositions --> <Positions> <sup>DocFreq</sup></li>
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* <li>Positions --> <PositionDelta,PayloadLength?,OffsetDelta?,OffsetLength?,PayloadData?> <sup>Freq</sup></li>
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* <li>PositionDelta,OffsetDelta,OffsetLength,PayloadLength --> {@link DataOutput#writeVInt VInt}</li>
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* <li>PayloadData --> {@link DataOutput#writeByte byte}<sup>PayloadLength</sup></li>
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* </ul>
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* <p>TermPositions are ordered by term (the term is implicit, from the term dictionary).</p>
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* <p>Positions entries are ordered by increasing document number (the document
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* number is implicit from the .frq file).</p>
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* <p>PositionDelta is, if payloads are disabled for the term's field, the
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* difference between the position of the current occurrence in the document and
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* the previous occurrence (or zero, if this is the first occurrence in this
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* document). If payloads are enabled for the term's field, then PositionDelta/2
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* is the difference between the current and the previous position. If payloads
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* are enabled and PositionDelta is odd, then PayloadLength is stored, indicating
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* the length of the payload at the current term position.</p>
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* <p>For example, the TermPositions for a term which occurs as the fourth term in
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* one document, and as the fifth and ninth term in a subsequent document, would
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* be the following sequence of VInts (payloads disabled):</p>
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* <p>4, 5, 4</p>
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* <p>PayloadData is metadata associated with the current term position. If
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* PayloadLength is stored at the current position, then it indicates the length
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* of this Payload. If PayloadLength is not stored, then this Payload has the same
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* length as the Payload at the previous position.</p>
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* <p>OffsetDelta/2 is the difference between this position's startOffset from the
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* previous occurrence (or zero, if this is the first occurrence in this document).
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* If OffsetDelta is odd, then the length (endOffset-startOffset) differs from the
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* previous occurrence and an OffsetLength follows. Offset data is only written for
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* {@link IndexOptions#DOCS_AND_FREQS_AND_POSITIONS_AND_OFFSETS}.</p>
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*
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* @lucene.experimental */
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// TODO: this class could be created by wrapping
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