[jira] [Commented] (LUCENE-8753) New PostingFormat - UniformSplit

["Bruno Roustant (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/LUCENE-8753?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16881046#comment-16881046 ] 

Bruno Roustant commented on LUCENE-8753:
----------------------------------------

I have created a related Jira issue LUCENE-8906 (Lucene50PostingsReader.postings() casts BlockTermState param to private IntBlockTermState) to make the PR review advance.

If we find a solution for this issue, then UniformSplit posting format will be fully isolated in a separate package in codecs, with no intrusion anymore elsewhere.

The goal is to have it as an additional optional posting format (not to replace BlockTree) for the following use-cases: customizable by extension, shared-terms extension available, low memory on-heap footprint, best efficiency when dealing with small to medium indexes.

> 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: 3h 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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