Lucene 5.0
term vectors format.
Very similarly to Lucene87StoredFieldsFormat, this format is based
on compressed chunks of data, with document-level granularity so that a
document can never span across distinct chunks. Moreover, data is made as
compact as possible:
- textual data is compressed using the very light,
LZ4 compression algorithm,
- binary data is written using fixed-size blocks of
packed ints.
Term vectors are stored using two files
- a data file where terms, frequencies, positions, offsets and payloads
are stored,
- an index file, loaded into memory, used to locate specific documents in
the data file.
Looking up term vectors for any document requires at most 1 disk seek.
File formats
-
A vector data file (extension .tvd). This file stores terms,
frequencies, positions, offsets and payloads for every document. Upon writing
a new segment, it accumulates data into memory until the buffer used to store
terms and payloads grows beyond 4KB. Then it flushes all metadata, terms
and positions to disk using LZ4
compression for terms and payloads and
blocks of packed ints for positions.
Here is a more detailed description of the field data file format:
- VectorData (.tvd) --> <Header>, PackedIntsVersion, ChunkSize, <Chunk>ChunkCount, ChunkCount, DirtyChunkCount, Footer
- Header -->
IndexHeader
- PackedIntsVersion -->
PackedInts.VERSION_CURRENT as a VInt
- ChunkSize is the number of bytes of terms to accumulate before flushing, as a
VInt
- ChunkCount is not known in advance and is the number of chunks necessary to store all document of the segment
- Chunk --> DocBase, ChunkDocs, < NumFields >, < FieldNums >, < FieldNumOffs >, < Flags >,
< NumTerms >, < TermLengths >, < TermFreqs >, < Positions >, < StartOffsets >, < Lengths >,
< PayloadLengths >, < TermAndPayloads >
- DocBase is the ID of the first doc of the chunk as a
VInt
- ChunkDocs is the number of documents in the chunk
- NumFields --> DocNumFieldsChunkDocs
- DocNumFields is the number of fields for each doc, written as a
VInt if ChunkDocs==1 and as a PackedInts array otherwise
- FieldNums --> FieldNumDeltaTotalDistincFields, a delta-encoded list of the sorted unique field numbers present in the chunk
- FieldNumOffs --> FieldNumOffTotalFields, as a
PackedInts array
- FieldNumOff is the offset of the field number in FieldNums
- TotalFields is the total number of fields (sum of the values of NumFields)
- Flags --> Bit < FieldFlags >
- Bit is a single bit which when true means that fields have the same options for every document in the chunk
- FieldFlags --> if Bit==1: FlagTotalDistinctFields else FlagTotalFields
- Flag: a 3-bits int where:
- the first bit means that the field has positions
- the second bit means that the field has offsets
- the third bit means that the field has payloads
- NumTerms --> FieldNumTermsTotalFields
- FieldNumTerms: the number of terms for each field, using
blocks of 64 packed ints
- TermLengths --> PrefixLengthTotalTerms SuffixLengthTotalTerms
- TotalTerms: total number of terms (sum of NumTerms)
- PrefixLength: 0 for the first term of a field, the common prefix with the previous term otherwise using
blocks of 64 packed ints
- SuffixLength: length of the term minus PrefixLength for every term using
blocks of 64 packed ints
- TermFreqs --> TermFreqMinus1TotalTerms
- TermFreqMinus1: (frequency - 1) for each term using
blocks of 64 packed ints
- Positions --> PositionDeltaTotalPositions
- TotalPositions is the sum of frequencies of terms of all fields that have positions
- PositionDelta: the absolute position for the first position of a term, and the difference with the previous positions for following positions using
blocks of 64 packed ints
- StartOffsets --> (AvgCharsPerTermTotalDistinctFields) StartOffsetDeltaTotalOffsets
- TotalOffsets is the sum of frequencies of terms of all fields that have offsets
- AvgCharsPerTerm: average number of chars per term, encoded as a float on 4 bytes. They are not present if no field has both positions and offsets enabled.
- StartOffsetDelta: (startOffset - previousStartOffset - AvgCharsPerTerm * PositionDelta). previousStartOffset is 0 for the first offset and AvgCharsPerTerm is 0 if the field has no positions using
blocks of 64 packed ints
- Lengths --> LengthMinusTermLengthTotalOffsets
- LengthMinusTermLength: (endOffset - startOffset - termLength) using
blocks of 64 packed ints
- PayloadLengths --> PayloadLengthTotalPayloads
- TotalPayloads is the sum of frequencies of terms of all fields that have payloads
- PayloadLength is the payload length encoded using
blocks of 64 packed ints
- TermAndPayloads --> LZ4-compressed representation of < FieldTermsAndPayLoads >TotalFields
- FieldTermsAndPayLoads --> Terms (Payloads)
- Terms: term bytes
- Payloads: payload bytes (if the field has payloads)
- ChunkCount --> the number of chunks in this file
- DirtyChunkCount --> the number of prematurely flushed chunks in this file
- Footer -->
CodecFooter
-
An index file (extension .tvx).