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Date: Thu, 13 May 2010 10:24:42 -0400 (EDT)
From: "Yonik Seeley (JIRA)" <jira@apache.org>
To: dev@lucene.apache.org
Subject: [jira] Commented: (LUCENE-2458) queryparser shouldn't generate
 phrasequeries based on term count
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    [ https://issues.apache.org/jira/browse/LUCENE-2458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12867143#action_12867143 ] 

Yonik Seeley commented on LUCENE-2458:
--------------------------------------

bq. Instead the queryparser should only form phrasequeries when you use double quotes, just like the documentation says.

We're conflating high level user syntax and the underlying implementation.

'text:Ready' says "search for the word 'ready' in the field 'text'"... the fact that an underlying term query of 'text:readi' (after lowercasing, stemming, etc) is not incorrect, it's simply the closest match to what the user is asking for given the details of analysis.  Likewise, a user query of 'text:ak-47'  may end up as a phrase query of "ak 47" because that's the closest representation in the index (the user doesn't necessarily know that the analysis of the field splits on dashes).

Likewise, a user query of text:"foo bar" is a high level way of saying "search for the word foo immediately followed by the word bar".  It is *not* saying "make a Lucene phrase query object with 2 terms".  Synonyms, common grams, or other analysis methods may in fact turn this into a single term query.

> queryparser shouldn't generate phrasequeries based on term count
> ----------------------------------------------------------------
>
>                 Key: LUCENE-2458
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2458
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: QueryParser
>            Reporter: Robert Muir
>            Priority: Critical
>
> The current method in the queryparser to generate phrasequeries is wrong:
> The Query Syntax documentation (http://lucene.apache.org/java/3_0_1/queryparsersyntax.html) states:
> {noformat}
> A Phrase is a group of words surrounded by double quotes such as "hello dolly".
> {noformat}
> But as we know, this isn't actually true.
> Instead the terms are first divided on whitespace, then the analyzer term count is used as some sort of "heuristic" to determine if its a phrase query or not.
> This assumption is a disaster for languages that don't use whitespace separation: CJK, compounding European languages like German, Finnish, etc. It also
> makes it difficult for people to use n-gram analysis techniques. In these cases you get bad relevance (MAP improves nearly *10x* if you use a PositionFilter at query-time to "turn this off" for chinese).
> For even english, this undocumented behavior is bad. Perhaps in some cases its being abused as some heuristic to "second guess" the tokenizer and piece back things it shouldn't have split, but for large collections, doing things like generating phrasequeries because StandardTokenizer split a compound on a dash can cause serious performance problems. Instead people should analyze their text with the appropriate methods, and QueryParser should only generate phrase queries when the syntax asks for one.
> The PositionFilter in contrib can be seen as a workaround, but its pretty obscure and people are not familiar with it. The result is we have bad out-of-box behavior for many languages, and bad performance for others on some inputs.
> I propose instead that we change the grammar to actually look for double quotes to determine when to generate a phrase query, consistent with the documentation.

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