Class UniformSplitTermsWriter
- java.lang.Object
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- org.apache.lucene.codecs.FieldsConsumer
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- org.apache.lucene.codecs.uniformsplit.UniformSplitTermsWriter
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- All Implemented Interfaces:
Closeable
,AutoCloseable
- Direct Known Subclasses:
STUniformSplitTermsWriter
public class UniformSplitTermsWriter extends FieldsConsumer
A block-based terms index and dictionary that assigns terms to nearly uniform length blocks. This technique is called Uniform Split.The block construction is driven by two parameters,
targetNumBlockLines
anddeltaNumLines
. Each block size (number of terms) istargetNumBlockLines
+-deltaNumLines
. The algorithm computes the minimal distinguishing prefix (MDP) between each term and its previous term (alphabetically ordered). Then it selects in the neighborhood of thetargetNumBlockLines
, and within thedeltaNumLines
, the term with the minimal MDP. This term becomes the first term of the next block and its MDP is the block key. This block key is added to the terms dictionary trie.We call dictionary the trie structure in memory, and block file the disk file containing the block lines, with one term and its corresponding term state details per line.
When seeking a term, the dictionary seeks the floor leaf of the trie for the searched term and jumps to the corresponding file pointer in the block file. There, the block terms are scanned for the exact searched term.
The terms inside a block do not need to share a prefix. Only the block key is used to find the block from the dictionary trie. And the block key is selected because it is the locally smallest MDP. This makes the dictionary trie very compact.
An interesting property of the Uniform Split technique is the very linear balance between memory usage and lookup performance. By decreasing the target block size, the block scan becomes faster, and since there are more blocks, the dictionary trie memory usage increases. Additionally, small blocks are faster to read from disk. A good sweet spot for the target block size is 32 with delta of 3 (10%) (default values). This can be tuned in the constructor.
There are additional optimizations:
- Each block has a header that allows the lookup to jump directly to the middle term with a fast comparison. This reduces the linear scan by 2 for a small disk size increase.
- Each block term is incrementally encoded according to its previous term. This both reduces the disk size and speeds up the block scan.
- All term line details (the terms states) are written after all terms. This allows faster term scan without needing to decode the term states.
- All file pointers are base-encoded. Their value is relative to the block base file pointer (not to the previous file pointer), this allows to read the term state of any term independently.
Blocks can be compressed or encrypted with an optional
BlockEncoder
provided in theconstructor
.The
block file
contains all the term blocks for each field sequentially. It also contains the fields metadata at the end of the file.The
dictionary file
contains the trie (FST
bytes) for each field sequentially.- WARNING: This API is experimental and might change in incompatible ways in the next release.
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Field Summary
Fields Modifier and Type Field Description protected BlockEncoder
blockEncoder
protected IndexOutput
blockOutput
static int
DEFAULT_DELTA_NUM_LINES
Default value for the maximum allowed delta variation of the block size (delta of the number of terms per block).static int
DEFAULT_TARGET_NUM_BLOCK_LINES
Default value for the target block size (number of terms per block).protected int
deltaNumLines
protected IndexOutput
dictionaryOutput
protected FieldInfos
fieldInfos
protected FieldMetadata.Serializer
fieldMetadataWriter
protected static int
MAX_NUM_BLOCK_LINES
Upper limit of the block size (maximum number of terms per block).protected int
maxDoc
protected PostingsWriterBase
postingsWriter
protected int
targetNumBlockLines
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Constructor Summary
Constructors Modifier Constructor Description UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, int targetNumBlockLines, int deltaNumLines, BlockEncoder blockEncoder)
protected
UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, int targetNumBlockLines, int deltaNumLines, BlockEncoder blockEncoder, FieldMetadata.Serializer fieldMetadataWriter, String codecName, int versionCurrent, String termsBlocksExtension, String dictionaryExtension)
UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, BlockEncoder blockEncoder)
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Method Summary
All Methods Static Methods Instance Methods Concrete Methods Modifier and Type Method Description void
close()
protected static void
validateSettings(int targetNumBlockLines, int deltaNumLines)
Validates theconstructor
settings.void
write(Fields fields, NormsProducer normsProducer)
protected void
writeDictionary(IndexDictionary.Builder dictionaryBuilder)
Writes the dictionary index (FST) to disk.protected void
writeEncodedFieldsMetadata(ByteBuffersDataOutput fieldsOutput)
protected void
writeFieldsMetadata(int fieldsNumber, ByteBuffersDataOutput fieldsOutput)
protected int
writeFieldTerms(BlockWriter blockWriter, DataOutput fieldsOutput, TermsEnum termsEnum, FieldInfo fieldInfo, NormsProducer normsProducer)
protected BlockTermState
writePostingLine(TermsEnum termsEnum, FieldMetadata fieldMetadata, NormsProducer normsProducer)
Writes the posting values for the current term in the givenTermsEnum
and updates theFieldMetadata
stats.protected void
writeUnencodedFieldsMetadata(ByteBuffersDataOutput fieldsOutput)
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Methods inherited from class org.apache.lucene.codecs.FieldsConsumer
merge
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Field Detail
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DEFAULT_TARGET_NUM_BLOCK_LINES
public static final int DEFAULT_TARGET_NUM_BLOCK_LINES
Default value for the target block size (number of terms per block).- See Also:
- Constant Field Values
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DEFAULT_DELTA_NUM_LINES
public static final int DEFAULT_DELTA_NUM_LINES
Default value for the maximum allowed delta variation of the block size (delta of the number of terms per block). The block size will be [target block size]+-[allowed delta].- See Also:
- Constant Field Values
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MAX_NUM_BLOCK_LINES
protected static final int MAX_NUM_BLOCK_LINES
Upper limit of the block size (maximum number of terms per block).- See Also:
- Constant Field Values
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fieldInfos
protected final FieldInfos fieldInfos
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postingsWriter
protected final PostingsWriterBase postingsWriter
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maxDoc
protected final int maxDoc
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targetNumBlockLines
protected final int targetNumBlockLines
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deltaNumLines
protected final int deltaNumLines
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blockEncoder
protected final BlockEncoder blockEncoder
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fieldMetadataWriter
protected final FieldMetadata.Serializer fieldMetadataWriter
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blockOutput
protected final IndexOutput blockOutput
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dictionaryOutput
protected final IndexOutput dictionaryOutput
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Constructor Detail
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UniformSplitTermsWriter
public UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, BlockEncoder blockEncoder) throws IOException
- Parameters:
blockEncoder
- Optional block encoder, may be null if none. It can be used for compression or encryption.- Throws:
IOException
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UniformSplitTermsWriter
public UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, int targetNumBlockLines, int deltaNumLines, BlockEncoder blockEncoder) throws IOException
- Parameters:
blockEncoder
- Optional block encoder, may be null if none. It can be used for compression or encryption.- Throws:
IOException
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UniformSplitTermsWriter
protected UniformSplitTermsWriter(PostingsWriterBase postingsWriter, SegmentWriteState state, int targetNumBlockLines, int deltaNumLines, BlockEncoder blockEncoder, FieldMetadata.Serializer fieldMetadataWriter, String codecName, int versionCurrent, String termsBlocksExtension, String dictionaryExtension) throws IOException
- Parameters:
targetNumBlockLines
- Target number of lines per block. Must be strictly greater than 0. The parameters can be pre-validated withvalidateSettings(int, int)
. There is one term per block line, with its corresponding details (TermState
).deltaNumLines
- Maximum allowed delta variation of the number of lines per block. Must be greater than or equal to 0 and strictly less thantargetNumBlockLines
. The block size will betargetNumBlockLines
+-deltaNumLines
. The block size must always be less than or equal toMAX_NUM_BLOCK_LINES
.blockEncoder
- Optional block encoder, may be null if none. It can be used for compression or encryption.- Throws:
IOException
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Method Detail
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validateSettings
protected static void validateSettings(int targetNumBlockLines, int deltaNumLines)
Validates theconstructor
settings.- Parameters:
targetNumBlockLines
- Target number of lines per block. Must be strictly greater than 0.deltaNumLines
- Maximum allowed delta variation of the number of lines per block. Must be greater than or equal to 0 and strictly less thantargetNumBlockLines
. Additionally,targetNumBlockLines
+deltaNumLines
must be less than or equal toMAX_NUM_BLOCK_LINES
.
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write
public void write(Fields fields, NormsProducer normsProducer) throws IOException
- Specified by:
write
in classFieldsConsumer
- Throws:
IOException
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writeFieldsMetadata
protected void writeFieldsMetadata(int fieldsNumber, ByteBuffersDataOutput fieldsOutput) throws IOException
- Throws:
IOException
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writeUnencodedFieldsMetadata
protected void writeUnencodedFieldsMetadata(ByteBuffersDataOutput fieldsOutput) throws IOException
- Throws:
IOException
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writeEncodedFieldsMetadata
protected void writeEncodedFieldsMetadata(ByteBuffersDataOutput fieldsOutput) throws IOException
- Throws:
IOException
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writeFieldTerms
protected int writeFieldTerms(BlockWriter blockWriter, DataOutput fieldsOutput, TermsEnum termsEnum, FieldInfo fieldInfo, NormsProducer normsProducer) throws IOException
- Returns:
- 1 if the field was written; 0 otherwise.
- Throws:
IOException
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writePostingLine
protected BlockTermState writePostingLine(TermsEnum termsEnum, FieldMetadata fieldMetadata, NormsProducer normsProducer) throws IOException
Writes the posting values for the current term in the givenTermsEnum
and updates theFieldMetadata
stats.- Returns:
- the written
BlockTermState
; or null if none. - Throws:
IOException
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writeDictionary
protected void writeDictionary(IndexDictionary.Builder dictionaryBuilder) throws IOException
Writes the dictionary index (FST) to disk.- Throws:
IOException
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close
public void close() throws IOException
- Specified by:
close
in interfaceAutoCloseable
- Specified by:
close
in interfaceCloseable
- Specified by:
close
in classFieldsConsumer
- Throws:
IOException
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