On 27/02/2013 10:28, Uwe Schindler wrote:
Hi Paul,
QueryParser and MTQ's rewrite method have nothing to do with each other. The rewrite method is (explained as
simple as possible) a class that is responsible to "rewrite" a MultiTermQuery to another query type
(generally a query that allows to add "Term" instances, e.g. BooleanQuery of TermQuery or
DisjunctionMaxQuery of Terms). The rewrite method takes the "filtered" terms enum provided by the
query and creates a combined query out of it. Lucene ships with some already implemented rewrite methods
based on abstract classes that handle the most common cases:
- ScoringRewrite handles the case where you want to collect the terms from the termsenum
and place them as "clauses" in a top level query (e.g. a scoring BooleanQuery).
You have to implement 2 abstract methods that produce the top-level query and create the
clauses, that can be added to the top-level query. This class is generic to the top-level
query, as the clauses can only be added to the correct top-level query. To make this work
without casting, all methods are redefined to take the generics classes. So addClause()
takes the generic top level query and a term. The rewrite method by itself returns the
top level query
- TopTermsRewrite is similar, but has a major difference: It has almost same
API, but the internal implementation of this class is different: It never hits
the Boolean Max Clause Count, because the collected terms are ordered in a
priority queue and only the top-ranking terms are added to the resulting
top-level query. This class is also generified against the top-level query.
Rewrite returns an instance of the top-level query.
- The very base class MultiTermQuery.RewriteMethod is most flexible but has no
concrete implementation. It is used to rewrite a MTQ to a query that is not a
composite top-level one with a number of terms, e.g. a filter that’s handled in
a totally different stage of rewriting.
You can use the same MTQ rewrite for different MTQ types, e.g. you can rewrite
a FuzzyQuery to a simple ConstantScore Query or a DisjunctionMaxQuery - but
only the second one makes sense. On the other hand it makes no sense to rewrite
Prefix and Wildcard using TopTermsRewrite, as those queries have terms enums
withouth term boosts (only Fuzzy assigns a boost to every term depending on
levensthein distance).
Things to note:
A rewrite method in MTQ would never rewrite to another MTQ like PrefixQuery - it could do
this, but only in the lowest base class (see above)! -> If you rely on that, your code
has a major problem. In that case the correct behavior would be to create a completely
"own"oal.search.Query (that not extends MTQ) and implement a standard rewrite
logic. This query could of course rewrite to MTQ's like Fuzzy or Prefix. IndexSearcher
rewrites the query until it is completely rewritten, so your custom query would create a
PrefixQuery which itself rewrites to something else.
QueryParser is just a factory for queries, its not related to MTQ. It only has an option
to set a "default" method for common queries. But as you have a custom
QueryParser, you can return the queries, configured like you want, to the caller.
Uwe
Hi Uwe
Okay, think I have it now. Now have a working rewrite method for Fuzzy
Queries
public static class FuzzyTermRewrite<Q extends DisjunctionMaxQuery>
extends TopTermsRewrite<Query> {
public FuzzyTermRewrite(int size) {
super(size);
}
@Override
protected int getMaxSize() {
return BooleanQuery.getMaxClauseCount();
}
@Override
protected DisjunctionMaxQuery getTopLevelQuery() {
return new DisjunctionMaxQuery(0.1f);
}
@Override
protected void addClause(Query topLevel, Term term, int
docCount, float boost, TermContext states) {
final Query tq = new ConstantScoreQuery(new TermQuery(term,
states));
tq.setBoost(boost);
((DisjunctionMaxQuery)topLevel).add(tq);
}
}
and now writing a separate class for Prefix Queries so it does actually
modify the idf
Paul
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