queryparser shouldn't generate phrasequeries based on term count
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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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