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https://issues.apache.org/jira/browse/LUCENE-2458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12869280#action_12869280
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Michael McCandless commented on LUCENE-2458:
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OK mulling some more on this one...
Even for english, the QP hack (pre-splitting on whitespace, then
turning any text that analyzers to multiple tokens into a
PhraseQuery), doesn't work right.
EG, say I want ice-cream, ice cream and icecream to mean the same
thing. Really I should do this (handling compounds) during indexing
-- I'll get better relevance and performance. But say for some reason
I'm doing it at search time...
I would want an analyzer that detects all three forms and in turn
expands to all three forms in the query.
But there's no way to do this today, because QP pre-splits on
whitespace, for ice cream the analyzer would separately receive ice
and cream, so it never has a chance to detect this form of the
compound.
So... first, I think we should fix QP to not pre-split on whitespace.
QP really should be as language neutral as possible. It should only
split on syntax chars, and send the whole string in between syntax
chars to the analyzer.
And, second, the QP should not create PhraseQuery when it sees
multiple tokens come back. This obliterates the OOTB experience for
non-whitespace languages. And, it doesn't work right for
english... so I think we should deprecate the option and default it to
"off".
Really the contrib queryparser is a better fit for doing rewrites like
this: it's able to operate on the abstract query tree, and can easily
do things like rewriting the query to add phrase queries...
> 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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