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From "Marvin Humphrey (JIRA)" <>
Subject [jira] Commented: (LUCENE-2458) queryparser shouldn't generate phrasequeries based on term count
Date Wed, 12 May 2010 15:32:41 GMT


Marvin Humphrey commented on LUCENE-2458:

I have mixed feelings about this for English.  It's a weakness of our engine
that we do not take position of terms within a query string into account.  At
times I've tried to modify the scoring hierarchy to improve the situation, but
I gave up because it was too difficult.  This behavior of QueryParser is a
sneaky way of getting around that limitation by turning stuff which should
almost certainly be treated as phrase queries as such.  It's the one place 
where we actually exploit position data within the query string.

Mike's "wi-fi" example, though, wouldn't suffer that badly.  The terms "wi"
and "fi" are unlikely to occur much outside the context of 'wi-fi/wi fi/wifi'.
And treating "wi-fi" as a phrase still won't conflate results with "wifi" as
it would ideally.  

The example I would use doesn't typically apply to Lucene.  Lucene's
StandardAnalyzer tokenizes URLs as wholes, but KinoSearch's analogous analyzer
breaks them up into individual components.  As described in another recent
thread, this allows a search for '' to match a document which
contains the URL ''.  It would suck if all of
a sudden a search for '' started matching every document that
contained 'com'. 

You could, and theoretically should, address this problem with sophisticated
analysis.  But it does make it harder to write a good Analyzer.  You make it
more important to solve what Yonik calls the 'e space mail' problem by making
it worse.

> queryparser shouldn't generate phrasequeries based on term count
> ----------------------------------------------------------------
>                 Key: LUCENE-2458
>                 URL:
>             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 (
> {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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