RE: Force MultiFieldQueryParser always to use PrefixQuery

[Anders Lybecker]

Hi,

I thought about appending a * at the end of each word like <word>*. I did
not like the idea and it becomes cumbersome when using the complex
QueryParser syntax.

I also investigated the option of overriding the getFieldQuery via
inheritance, but I doubt it works at the method does more than just return a
TermQuery. For instance it also returns PhraseQuery, MultiPhraseQuery and
BooleanQuery.

My solution to the problem is post-processing of the returned query from the
MultiFieldQueryParser.

I iterate over the query graph and replaces each TermQuery with a
PrefixQuery.

This is how I do it. Beware! It's C# code, but I'm pretty sure that you will
be able to interpret it anyway :-)

    public static class QueryModifier
    {
        public static Query AppendWildcard(Query q)
        {
            BooleanQuery boolQuery = q as BooleanQuery;

            if (boolQuery != null)
            {
                Iterate(ref boolQuery);

                return boolQuery;
            }
            TermQuery termQuery = q as TermQuery;

            if (termQuery != null)
                return new PrefixQuery(termQuery.GetTerm());

            return q;
        }

        private static void Iterate(ref BooleanQuery boolQuery)
        {
            BooleanClause[] boolClauses = boolQuery.GetClauses();

            BooleanClause clause;

            for (int i = 0; i < boolClauses.Length; i++)
            {
                clause = boolClauses[i];

                Query clauseQuery = clause.GetQuery();

                BooleanQuery boolClauseQuery = clauseQuery as BooleanQuery;

                if (boolClauseQuery != null)
                {
                    Iterate(ref boolClauseQuery);

                    boolClauses[i] = new BooleanClause(boolClauseQuery,
boolClauses[i].GetOccur()); ;
                }

                TermQuery termClauseQuery = clauseQuery as TermQuery;

                if (termClauseQuery != null)
                {
                    // Change TermQuery -> PrefixQuery to wildcard search
like <word>*
                    PrefixQuery prefixQuery = new
PrefixQuery(termClauseQuery.GetTerm());

                    boolClauses[i] = new BooleanClause(prefixQuery,
boolClauses[i].GetOccur());
                }
            }

            boolQuery = new BooleanQuery(boolQuery.IsCoordDisabled());
            foreach (BooleanClause claus in boolClauses)
            {
                boolQuery.Add(claus);
            }
        }
    }

Thanks for the suggestions.

:-)
Anders Lybecker

-----Original Message-----
From: Shai Erera [mailto:[EMAIL PROTECTED] 
Sent: 22. november 2007 17:04
To: java-user@lucene.apache.org
Subject: Re: Force MultiFieldQueryParser always to use PrefixQuery

Hi

I wrote the following class:

    public class AlwaysPrefixMultiFieldQP extends MultiFieldQueryParser {

        public MyQP(String[] fields, Analyzer analyzer) {
            super(fields, analyzer);
        }

        protected Query getFieldQuery(String field, String queryText) throws
ParseException {
            if (field != null) {
                return new PrefixQuery(new Term(field, queryText));
            }
            return super.getFieldQuery(field, queryText);
        }
    }

What it does is override getFieldQuery. If the field is not null, it creates
a new PrefixQuery. Following is a sample code:
       AlwaysPrefixMultiFieldQP m = new AlwaysPrefixMultiFieldQP(new
String[] { "field" }, analyzer);
       Query q = m.parse("sof was");
       System.out.println(q);

This code prints: (field:sof*) (field:was*), which is I believe what you
need.

As for splitting the query (pre processing) - this is not recommended. It
may work great for space separated languages, however may produce poor
results for Asian languages (for example). The way I propose above makes use
of the Analyzer you use, thus guarantees you append a '*' to all the words
in the query.

Hope this helps.

On Nov 22, 2007 3:35 PM, Erick Erickson <[EMAIL PROTECTED]> wrote:

> The simplest way would be to pre-process the query. That
> is, just split on words and add the '*' as appropriate.
>
> Erick
>
> On Nov 21, 2007 2:16 PM, Anders Lybecker <[EMAIL PROTECTED]> wrote:
>
> > How do I force the MultiFieldQueryParser to interpret a string like
> > "dock boat" as "dock* boat*" and therefore use PrefixQuery instead of
> > TemQuery?
> >
> > The customer wants always to search with <word>* as default when
> entering
> > <word>
> >
> > :-)
> > Anders Lybecker
> >
> > ---------------------------------------------------------------------
> > To unsubscribe, e-mail: [EMAIL PROTECTED]
> > For additional commands, e-mail: [EMAIL PROTECTED]
> >
> >
>



-- 
Regards,

Shai Erera

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