org.apache.lucene.search
Class DisjunctionMaxQuery

java.lang.Object
  extended by org.apache.lucene.search.Query
      extended by org.apache.lucene.search.DisjunctionMaxQuery
All Implemented Interfaces:
Serializable, Cloneable

public class DisjunctionMaxQuery
extends Query

A query that generates the union of documents produced by its subqueries, and that scores each document with the maximum score for that document as produced by any subquery, plus a tie breaking increment for any additional matching subqueries. This is useful when searching for a word in multiple fields with different boost factors (so that the fields cannot be combined equivalently into a single search field). We want the primary score to be the one associated with the highest boost, not the sum of the field scores (as BooleanQuery would give). If the query is "albino elephant" this ensures that "albino" matching one field and "elephant" matching another gets a higher score than "albino" matching both fields. To get this result, use both BooleanQuery and DisjunctionMaxQuery: for each term a DisjunctionMaxQuery searches for it in each field, while the set of these DisjunctionMaxQuery's is combined into a BooleanQuery. The tie breaker capability allows results that include the same term in multiple fields to be judged better than results that include this term in only the best of those multiple fields, without confusing this with the better case of two different terms in the multiple fields.

See Also:
Serialized Form

Nested Class Summary
protected  class DisjunctionMaxQuery.DisjunctionMaxWeight
          Expert: the Weight for DisjunctionMaxQuery, used to normalize, score and explain these queries.
 
Constructor Summary
DisjunctionMaxQuery(Collection disjuncts, float tieBreakerMultiplier)
          Creates a new DisjunctionMaxQuery
DisjunctionMaxQuery(float tieBreakerMultiplier)
          Creates a new empty DisjunctionMaxQuery.
 
Method Summary
 void add(Collection disjuncts)
          Add a collection of disjuncts to this disjunction via Iterable
 void add(Query query)
          Add a subquery to this disjunction
 Object clone()
          Create a shallow copy of us -- used in rewriting if necessary
 Weight createWeight(Searcher searcher)
          Expert: Constructs an appropriate Weight implementation for this query.
 boolean equals(Object o)
          Return true iff we represent the same query as o
 void extractTerms(Set terms)
          Expert: adds all terms occurring in this query to the terms set.
 int hashCode()
          Compute a hash code for hashing us
 Iterator iterator()
          An Iterator over the disjuncts
 Query rewrite(IndexReader reader)
          Optimize our representation and our subqueries representations
 String toString(String field)
          Prettyprint us.
 
Methods inherited from class org.apache.lucene.search.Query
combine, getBoost, getSimilarity, mergeBooleanQueries, setBoost, toString, weight
 
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
 

Constructor Detail

DisjunctionMaxQuery

public DisjunctionMaxQuery(float tieBreakerMultiplier)
Creates a new empty DisjunctionMaxQuery. Use add() to add the subqueries.

Parameters:
tieBreakerMultiplier - the score of each non-maximum disjunct for a document is multiplied by this weight and added into the final score. If non-zero, the value should be small, on the order of 0.1, which says that 10 occurrences of word in a lower-scored field that is also in a higher scored field is just as good as a unique word in the lower scored field (i.e., one that is not in any higher scored field.

DisjunctionMaxQuery

public DisjunctionMaxQuery(Collection disjuncts,
                           float tieBreakerMultiplier)
Creates a new DisjunctionMaxQuery

Parameters:
disjuncts - a Collection of all the disjuncts to add
tieBreakerMultiplier - the weight to give to each matching non-maximum disjunct
Method Detail

add

public void add(Query query)
Add a subquery to this disjunction

Parameters:
query - the disjunct added

add

public void add(Collection disjuncts)
Add a collection of disjuncts to this disjunction via Iterable


iterator

public Iterator iterator()
An Iterator over the disjuncts


createWeight

public Weight createWeight(Searcher searcher)
                    throws IOException
Description copied from class: Query
Expert: Constructs an appropriate Weight implementation for this query.

Only implemented by primitive queries, which re-write to themselves.

Overrides:
createWeight in class Query
Throws:
IOException

rewrite

public Query rewrite(IndexReader reader)
              throws IOException
Optimize our representation and our subqueries representations

Overrides:
rewrite in class Query
Parameters:
reader - the IndexReader we query
Returns:
an optimized copy of us (which may not be a copy if there is nothing to optimize)
Throws:
IOException

clone

public Object clone()
Create a shallow copy of us -- used in rewriting if necessary

Overrides:
clone in class Query
Returns:
a copy of us (but reuse, don't copy, our subqueries)

extractTerms

public void extractTerms(Set terms)
Description copied from class: Query
Expert: adds all terms occurring in this query to the terms set. Only works if this query is in its rewritten form.

Overrides:
extractTerms in class Query

toString

public String toString(String field)
Prettyprint us.

Specified by:
toString in class Query
Parameters:
field - the field to which we are applied
Returns:
a string that shows what we do, of the form "(disjunct1 | disjunct2 | ... | disjunctn)^boost"

equals

public boolean equals(Object o)
Return true iff we represent the same query as o

Overrides:
equals in class Query
Parameters:
o - another object
Returns:
true iff o is a DisjunctionMaxQuery with the same boost and the same subqueries, in the same order, as us

hashCode

public int hashCode()
Compute a hash code for hashing us

Overrides:
hashCode in class Query
Returns:
the hash code


Copyright © 2000-2009 Apache Software Foundation. All Rights Reserved.