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https://issues.apache.org/jira/browse/LUCENE-8753?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16839429#comment-16839429
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Bruno Roustant commented on LUCENE-8753:
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Beyond the performance aspects, we developed UniformSplit to be extensible. To
give an idea of how it can be extended, I have added a new PR#676: SharedTerms
UniformSplit.
The use-case is when there are many fields. We want to take advantage of the
FST property to share the terms between all the fields, by replacing one FST
per field by a single FST containing the shared terms. In this case each term
is stored only once in the block file, and its block line contains the
TermState for each different field for which the term occurs.
term A -> field1 TermState, field2 TermState, field3 TermState
term B -> field3 TermState, field5 TermState
The FST is compact and this posting format also unlocks the possibility to
cache when the same term is searched in many fields (but this is not part of
this PR).
My goal here is to showcase the extensibility of this posting format. This
extension is in a separate sub-package sharedterms and is quite concise. (the
only tricky part is the custom merge to merge efficiently two segments by
accessing directly the sharedterms posting format)
> New PostingFormat - UniformSplit
> --------------------------------
>
> Key: LUCENE-8753
> URL: https://issues.apache.org/jira/browse/LUCENE-8753
> Project: Lucene - Core
> Issue Type: Improvement
> Components: core/codecs
> Affects Versions: 8.0
> Reporter: Bruno Roustant
> Assignee: David Smiley
> Priority: Major
> Attachments: Uniform Split Technique.pdf, luceneutil.benchmark.txt
>
> Time Spent: 20m
> Remaining Estimate: 0h
>
> This is a proposal to add a new PostingsFormat called "UniformSplit" with 4
> objectives:
> - Clear design and simple code.
> - Easily extensible, for both the logic and the index format.
> - Light memory usage with a very compact FST.
> - Focus on efficient TermQuery, PhraseQuery and PrefixQuery performance.
> (the pdf attached explains visually the technique in more details)
> The principle is to split the list of terms into blocks and use a FST to
> access the block, but not as a prefix trie, rather with a seek-floor pattern.
> For the selection of the blocks, there is a target average block size (number
> of terms), with an allowed delta variation (10%) to compare the terms and
> select the one with the minimal distinguishing prefix.
> There are also several optimizations inside the block to make it more
> compact and speed up the loading/scanning.
> The performance obtained is interesting with the luceneutil benchmark,
> comparing UniformSplit with BlockTree. Find it in the first comment and also
> attached for better formatting.
> Although the precise percentages vary between runs, three main points:
> - TermQuery and PhraseQuery are improved.
> - PrefixQuery and WildcardQuery are ok.
> - Fuzzy queries are clearly less performant, because BlockTree is so
> optimized for them.
> Compared to BlockTree, FST size is reduced by 15%, and segment writing time
> is reduced by 20%. So this PostingsFormat scales to lots of docs, as
> BlockTree.
> This initial version passes all Lucene tests. Use “ant test
> -Dtests.codec=UniformSplitTesting” to test with this PostingsFormat.
> Subjectively, we think we have fulfilled our goal of code simplicity. And we
> have already exercised this PostingsFormat extensibility to create a
> different flavor for our own use-case.
> Contributors: Juan Camilo Rodriguez Duran, Bruno Roustant, David Smiley
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