org.apache.lucene.search

## Class MultiTermQuery

- java.lang.Object
- - org.apache.lucene.search.Query
  - - org.apache.lucene.search.MultiTermQuery

- Direct Known Subclasses:
    AutomatonQuery, FuzzyQuery

      public abstract class MultiTermQuery
      extends Query

  An abstract `Query` that matches documents containing a subset of terms provided by a `FilteredTermsEnum` enumeration.

  This query cannot be used directly; you must subclass it and define `getTermsEnum(Terms,AttributeSource)` to provide a `FilteredTermsEnum` that iterates through the terms to be matched.

  NOTE: if `setRewriteMethod(org.apache.lucene.search.MultiTermQuery.RewriteMethod)` is either `CONSTANT_SCORE_BOOLEAN_REWRITE` or `SCORING_BOOLEAN_REWRITE`, you may encounter a `BooleanQuery.TooManyClauses` exception during searching, which happens when the number of terms to be searched exceeds `BooleanQuery.getMaxClauseCount()`. Setting `setRewriteMethod(org.apache.lucene.search.MultiTermQuery.RewriteMethod)` to `CONSTANT_SCORE_REWRITE` prevents this.

  The recommended rewrite method is `CONSTANT_SCORE_REWRITE`: it doesn't spend CPU computing unhelpful scores, and is the most performant rewrite method given the query. If you need scoring (like `FuzzyQuery`, use `MultiTermQuery.TopTermsScoringBooleanQueryRewrite` which uses a priority queue to only collect competitive terms and not hit this limitation. Note that org.apache.lucene.queryparser.classic.QueryParser produces MultiTermQueries using `CONSTANT_SCORE_REWRITE` by default.

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class and Description
    --- | ---
    `static class` | `MultiTermQuery.RewriteMethod` Abstract class that defines how the query is rewritten.
    `static class` | `MultiTermQuery.TopTermsBlendedFreqScoringRewrite` A rewrite method that first translates each term into `BooleanClause.Occur.SHOULD` clause in a BooleanQuery, but adjusts the frequencies used for scoring to be blended across the terms, otherwise the rarest term typically ranks highest (often not useful eg in the set of expanded terms in a FuzzyQuery).
    `static class` | `MultiTermQuery.TopTermsBoostOnlyBooleanQueryRewrite` A rewrite method that first translates each term into `BooleanClause.Occur.SHOULD` clause in a BooleanQuery, but the scores are only computed as the boost.
    `static class` | `MultiTermQuery.TopTermsScoringBooleanQueryRewrite` A rewrite method that first translates each term into `BooleanClause.Occur.SHOULD` clause in a BooleanQuery, and keeps the scores as computed by the query.

  - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `static MultiTermQuery.RewriteMethod` | `CONSTANT_SCORE_BOOLEAN_REWRITE` Like `SCORING_BOOLEAN_REWRITE` except scores are not computed.
    `static MultiTermQuery.RewriteMethod` | `CONSTANT_SCORE_REWRITE` A rewrite method that first creates a private Filter, by visiting each term in sequence and marking all docs for that term.
    `protected String` | `field`
    `protected MultiTermQuery.RewriteMethod` | `rewriteMethod`
    `static MultiTermQuery.RewriteMethod` | `SCORING_BOOLEAN_REWRITE` A rewrite method that first translates each term into `BooleanClause.Occur.SHOULD` clause in a BooleanQuery, and keeps the scores as computed by the query.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `MultiTermQuery(String field)` Constructs a query matching terms that cannot be represented with a single Term.

  - ### Method Summary

    All Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `equals(Object other)` Override and implement query instance equivalence properly in a subclass.
    `String` | `getField()` Returns the field name for this query
    `MultiTermQuery.RewriteMethod` | `getRewriteMethod()`
    `TermsEnum` | `getTermsEnum(Terms terms)` Constructs an enumeration that expands the pattern term.
    `protected abstract TermsEnum` | `getTermsEnum(Terms terms, AttributeSource atts)` Construct the enumeration to be used, expanding the pattern term.
    `int` | `hashCode()` Override and implement query hash code properly in a subclass.
    `Query` | `rewrite(IndexReader reader)` To rewrite to a simpler form, instead return a simpler enum from `getTermsEnum(Terms, AttributeSource)`.
    `void` | `setRewriteMethod(MultiTermQuery.RewriteMethod method)` Sets the rewrite method to be used when executing the query.

    - ### Methods inherited from class org.apache.lucene.search.Query

      `classHash, createWeight, sameClassAs, toString, toString, visit`

    - ### Methods inherited from class java.lang.Object

      `clone, finalize, getClass, notify, notifyAll, wait, wait, wait`

- - ### Field Detail

    - #### field

          protected final String field

    - #### rewriteMethod

          protected MultiTermQuery.RewriteMethod rewriteMethod

    - #### CONSTANT_SCORE_REWRITE

          public static final MultiTermQuery.RewriteMethod CONSTANT_SCORE_REWRITE

      A rewrite method that first creates a private Filter, by visiting each term in sequence and marking all docs for that term. Matching documents are assigned a constant score equal to the query's boost.

      This method is faster than the BooleanQuery rewrite methods when the number of matched terms or matched documents is non-trivial. Also, it will never hit an errant `BooleanQuery.TooManyClauses` exception.

      See Also:
        `setRewriteMethod(org.apache.lucene.search.MultiTermQuery.RewriteMethod)`

    - #### SCORING_BOOLEAN_REWRITE

          public static final MultiTermQuery.RewriteMethod SCORING_BOOLEAN_REWRITE

      A rewrite method that first translates each term into `BooleanClause.Occur.SHOULD` clause in a BooleanQuery, and keeps the scores as computed by the query. Note that typically such scores are meaningless to the user, and require non-trivial CPU to compute, so it's almost always better to use `CONSTANT_SCORE_REWRITE` instead.

      NOTE: This rewrite method will hit `BooleanQuery.TooManyClauses` if the number of terms exceeds `BooleanQuery.getMaxClauseCount()`.

      See Also:
        `setRewriteMethod(org.apache.lucene.search.MultiTermQuery.RewriteMethod)`

    - #### CONSTANT_SCORE_BOOLEAN_REWRITE

          public static final MultiTermQuery.RewriteMethod CONSTANT_SCORE_BOOLEAN_REWRITE

      Like `SCORING_BOOLEAN_REWRITE` except scores are not computed. Instead, each matching document receives a constant score equal to the query's boost.

      NOTE: This rewrite method will hit `BooleanQuery.TooManyClauses` if the number of terms exceeds `BooleanQuery.getMaxClauseCount()`.

      See Also:
        `setRewriteMethod(org.apache.lucene.search.MultiTermQuery.RewriteMethod)`

  - ### Constructor Detail

    - #### MultiTermQuery

          public MultiTermQuery(String field)

      Constructs a query matching terms that cannot be represented with a single Term.

  - ### Method Detail

    - #### getField

          public final String getField()

      Returns the field name for this query

    - #### getTermsEnum

          protected abstract TermsEnum getTermsEnum(Terms terms,
                                                    AttributeSource atts)
                                             throws IOException

      Construct the enumeration to be used, expanding the pattern term. This method should only be called if the field exists (ie, implementations can assume the field does exist). This method should not return null (should instead return `TermsEnum.EMPTY` if no terms match). The TermsEnum must already be positioned to the first matching term. The given `AttributeSource` is passed by the `MultiTermQuery.RewriteMethod` to share information between segments, for example `TopTermsRewrite` uses it to share maximum competitive boosts
      Throws:
        `IOException`

    - #### getTermsEnum

          public final TermsEnum getTermsEnum(Terms terms)
                                       throws IOException

      Constructs an enumeration that expands the pattern term. This method should only be called if the field exists (ie, implementations can assume the field does exist). This method never returns null. The returned TermsEnum is positioned to the first matching term.
      Throws:
        `IOException`

    - #### rewrite

          public final Query rewrite(IndexReader reader)
                              throws IOException

      To rewrite to a simpler form, instead return a simpler enum from `getTermsEnum(Terms, AttributeSource)`. For example, to rewrite to a single term, return a `SingleTermsEnum`
      Overrides:
        `rewrite` in class `Query`
      Throws:
        `IOException`

    - #### getRewriteMethod

          public MultiTermQuery.RewriteMethod getRewriteMethod()

      See Also:
        `setRewriteMethod(org.apache.lucene.search.MultiTermQuery.RewriteMethod)`

    - #### setRewriteMethod

          public void setRewriteMethod(MultiTermQuery.RewriteMethod method)

      Sets the rewrite method to be used when executing the query. You can use one of the four core methods, or implement your own subclass of `MultiTermQuery.RewriteMethod`.

    - #### hashCode

          public int hashCode()

      Description copied from class: `Query`
      Override and implement query hash code properly in a subclass. This is required so that `QueryCache` works properly.
      Specified by:
        `hashCode` in class `Query`
      See Also:
        `Query.equals(Object)`

    - #### equals

          public boolean equals(Object other)

      Description copied from class: `Query`
      Override and implement query instance equivalence properly in a subclass. This is required so that `QueryCache` works properly. Typically a query will be equal to another only if it's an instance of the same class and its document-filtering properties are identical that other instance. Utility methods are provided for certain repetitive code.
      Specified by:
        `equals` in class `Query`
      See Also:
        `Query.sameClassAs(Object)`, `Query.classHash()`

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