org.apache.lucene.codecs.lucene410 (Lucene 4.10.1 API)

Index Structure Overview

Each segment index maintains the following:

• Segment info. This contains metadata about a segment, such as the number of documents, what files it uses,
• Field names. This contains the set of field names used in the index.
• Stored Field values. This contains, for each document, a list of attribute-value pairs, where the attributes are field names. These are used to store auxiliary information about the document, such as its title, url, or an identifier to access a database. The set of stored fields are what is returned for each hit when searching. This is keyed by document number.
• Term dictionary. A dictionary containing all of the terms used in all of the indexed fields of all of the documents. The dictionary also contains the number of documents which contain the term, and pointers to the term's frequency and proximity data.
• Term Frequency data. For each term in the dictionary, the numbers of all the documents that contain that term, and the frequency of the term in that document, unless frequencies are omitted (IndexOptions.DOCS_ONLY)
• Term Proximity data. For each term in the dictionary, the positions that the term occurs in each document. Note that this will not exist if all fields in all documents omit position data.
• Normalization factors. For each field in each document, a value is stored that is multiplied into the score for hits on that field.
• Term Vectors. For each field in each document, the term vector (sometimes called document vector) may be stored. A term vector consists of term text and term frequency. To add Term Vectors to your index see the Field constructors
• Per-document values. Like stored values, these are also keyed by document number, but are generally intended to be loaded into main memory for fast access. Whereas stored values are generally intended for summary results from searches, per-document values are useful for things like scoring factors.
• Deleted documents. An optional file indicating which documents are deleted.

Details on each of these are provided in their linked pages.

File Naming

All files belonging to a segment have the same name with varying extensions. The extensions correspond to the different file formats described below. When using the Compound File format (default in 1.4 and greater) these files (except for the Segment info file, the Lock file, and Deleted documents file) are collapsed into a single .cfs file (see below for details)

Typically, all segments in an index are stored in a single directory, although this is not required.

As of version 2.1 (lock-less commits), file names are never re-used (there is one exception, "segments.gen", see below). That is, when any file is saved to the Directory it is given a never before used filename. This is achieved using a simple generations approach. For example, the first segments file is segments_1, then segments_2, etc. The generation is a sequential long integer represented in alpha-numeric (base 36) form.

Summary of File Extensions

The following table summarizes the names and extensions of the files in Lucene:

Name	Extension	Brief Description
Segments File	segments.gen, segments_N	Stores information about a commit point
Lock File	write.lock	The Write lock prevents multiple IndexWriters from writing to the same file.
Segment Info	.si	Stores metadata about a segment
Compound File	.cfs, .cfe	An optional "virtual" file consisting of all the other index files for systems that frequently run out of file handles.
Fields	.fnm	Stores information about the fields
Field Index	.fdx	Contains pointers to field data
Field Data	.fdt	The stored fields for documents
Term Dictionary	.tim	The term dictionary, stores term info
Term Index	.tip	The index into the Term Dictionary
Frequencies	.doc	Contains the list of docs which contain each term along with frequency
Positions	.pos	Stores position information about where a term occurs in the index
Payloads	.pay	Stores additional per-position metadata information such as character offsets and user payloads
Norms	.nvd, .nvm	Encodes length and boost factors for docs and fields
Per-Document Values	.dvd, .dvm	Encodes additional scoring factors or other per-document information.
Term Vector Index	.tvx	Stores offset into the document data file
Term Vector Documents	.tvd	Contains information about each document that has term vectors
Term Vector Fields	.tvf	The field level info about term vectors
Deleted Documents	.del	Info about what files are deleted

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