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https://issues.apache.org/jira/browse/LUCENE-2458?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12869971#action_12869971
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Robert Muir commented on LUCENE-2458:
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bq. But I don't think we can improve that much (the logic is must implement is
fundamentally hairy).
I do have some suggestions for the future (we could do when we can get rid of
the deprecated behavior).
But I couldn't implement anything below, because this change I am working on is
already hairy enough itself :)
The problem with getFieldQuery is that it does too much. One of the things it
does is a bunch of query rewrites.
I think a lot of this is silly, and I think if you search on "foo" its ok for
the QP to call newPhraseQuery and return a 1-term PhraseQuery.
This is, after all, what the query was, and it should be closer to a simple
parser.
PhraseQuery already has its own code to later rewrite() to a single TermQuery
in this case.
The other problem with this method is that it is trying to abuse a
CachingTokenFilter to work with a TokenStream in 'extra dimensions' that it
doesnt have.
In my opinion, instead of doing this, it should just put the terms from the
TokenStream in a more suitable data structure that allows the code to be less
hairy,
instead of using a CachingTokenFilter and reset()'ing it.
> 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
> Attachments: LUCENE-2458.patch
>
>
> 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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