Package org.apache.lucene.codecs

Class BlockTreeTermsWriter

  • All Implemented Interfaces:
    java.io.Closeable, java.lang.AutoCloseable
    public class BlockTreeTermsWriter
extends FieldsConsumer
    Block-based terms index and dictionary writer.

    Writes terms dict and index, block-encoding (column stride) each term's metadata for each set of terms between two index terms.

    Files:

    Term Dictionary

    The .tim file contains the list of terms in each field along with per-term statistics (such as docfreq) and per-term metadata (typically pointers to the postings list for that term in the inverted index).

    The .tim is arranged in blocks: with blocks containing a variable number of entries (by default 25-48), where each entry is either a term or a reference to a sub-block.

    NOTE: The term dictionary can plug into different postings implementations: the postings writer/reader are actually responsible for encoding and decoding the Postings Metadata and Term Metadata sections.

    • TermsDict (.tim) --> Header, PostingsHeader, NodeBlockNumBlocks, FieldSummary, DirOffset
    • NodeBlock --> (OuterNode | InnerNode)
    • OuterNode --> EntryCount, SuffixLength, ByteSuffixLength, StatsLength, < TermStats >EntryCount, MetaLength, <TermMetadata>EntryCount
    • InnerNode --> EntryCount, SuffixLength[,Sub?], ByteSuffixLength, StatsLength, < TermStats ? >EntryCount, MetaLength, <TermMetadata ? >EntryCount
    • TermStats --> DocFreq, TotalTermFreq
    • FieldSummary --> NumFields, <FieldNumber, NumTerms, RootCodeLength, ByteRootCodeLength, SumTotalTermFreq?, SumDocFreq, DocCount>NumFields
    • Header --> CodecHeader
    • DirOffset --> Uint64
    • EntryCount,SuffixLength,StatsLength,DocFreq,MetaLength,NumFields, FieldNumber,RootCodeLength,DocCount --> VInt
    • TotalTermFreq,NumTerms,SumTotalTermFreq,SumDocFreq --> VLong

    Notes:

    • Header is a CodecHeader storing the version information for the BlockTree implementation.
    • DirOffset is a pointer to the FieldSummary section.
    • DocFreq is the count of documents which contain the term.
    • TotalTermFreq is the total number of occurrences of the term. This is encoded as the difference between the total number of occurrences and the DocFreq.
    • FieldNumber is the fields number from FieldInfos. (.fnm)
    • NumTerms is the number of unique terms for the field.
    • RootCode points to the root block for the field.
    • SumDocFreq is the total number of postings, the number of term-document pairs across the entire field.
    • DocCount is the number of documents that have at least one posting for this field.
    • PostingsHeader and TermMetadata are plugged into by the specific postings implementation: these contain arbitrary per-file data (such as parameters or versioning information) and per-term data (such as pointers to inverted files).
    • For inner nodes of the tree, every entry will steal one bit to mark whether it points to child nodes(sub-block). If so, the corresponding TermStats and TermMetaData are omitted

    Term Index

    The .tip file contains an index into the term dictionary, so that it can be accessed randomly. The index is also used to determine when a given term cannot exist on disk (in the .tim file), saving a disk seek.

    • TermsIndex (.tip) --> Header, FSTIndexNumFields <IndexStartFP>NumFields, DirOffset
    • Header --> CodecHeader
    • DirOffset --> Uint64
    • IndexStartFP --> VLong
    • FSTIndex --> FST<byte[]>

    Notes:

    • The .tip file contains a separate FST for each field. The FST maps a term prefix to the on-disk block that holds all terms starting with that prefix. Each field's IndexStartFP points to its FST.
    • DirOffset is a pointer to the start of the IndexStartFPs for all fields
    • It's possible that an on-disk block would contain too many terms (more than the allowed maximum (default: 48)). When this happens, the block is sub-divided into new blocks (called "floor blocks"), and then the output in the FST for the block's prefix encodes the leading byte of each sub-block, and its file pointer.
    See Also:
    BlockTreeTermsReader

    • Constructor Detail

      • BlockTreeTermsWriter

        public BlockTreeTermsWriter​(SegmentWriteState state,
                            PostingsWriterBase postingsWriter,
                            int minItemsInBlock,
                            int maxItemsInBlock)
                     throws java.io.IOException
        Create a new writer. The number of items (terms or sub-blocks) per block will aim to be between minItemsPerBlock and maxItemsPerBlock, though in some cases the blocks may be smaller than the min.
        Throws:
        java.io.IOException

    • Method Detail

      • close

        public void close()
           throws java.io.IOException
        Description copied from class: FieldsConsumer
        Called when we are done adding everything.
        Specified by:
        close in interface java.lang.AutoCloseable
        Specified by:
        close in interface java.io.Closeable
        Specified by:
        close in class FieldsConsumer
        Throws:
        java.io.IOException