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From "Shai Erera (JIRA)" <>
Subject [jira] Commented: (LUCENE-2458) queryparser shouldn't generate phrasequeries based on term count
Date Sun, 23 May 2010 04:38:16 GMT


Shai Erera commented on LUCENE-2458:

FWIW, I agree w/ Mark. I don't think it's a bug, but more of a user option. Whether it should
be specified by a setter, or an extension of QP - I have no strong feelings for either of
them, so either would be fine by me.

And for what's it's also worth, we've once worked w/ a Japanese linguist, who suggested that
we always convert queries like [abcd] to [abcd "abcd"] or just ["abcd"] because if someone
had already bothered to write them like that, then phrase matching should contribute to the
rank of the documents. IMO, if someone had gone even further by writing [field:abcd], then
even if the query should be [field:a field:b field:c field:d], executing the query [field:"abcd"]
is still important and better.

So .. I'm not trying to argue what should be the default behavior, because that is subject
to personal flavor and apps requirements -- only to emphasize that there are many user cases
out there, and we should cater for such scenarios.

The extension way is already supported, right? So perhaps we just need to document the current
behavior, and not change anything? Or, introduce a setter, that will do the simple thing -
either keep it as a phrase or break it down to terms. More sophisticated scenarios can be
dealt through extension.

> queryparser shouldn't generate phrasequeries based on term count
> ----------------------------------------------------------------
>                 Key: LUCENE-2458
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: QueryParser
>            Reporter: Robert Muir
>            Assignee: Robert Muir
>            Priority: Blocker
>             Fix For: 3.1, 4.0
>         Attachments: LUCENE-2458.patch, LUCENE-2458.patch
> 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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