# Package org.apache.lucene.search.join

Support for index-time and query-time joins.

See: Description

- Interface Summary
  Interface | Description
  --- | ---
  BitSetProducer | A producer of `BitSet`s per segment.

- Class Summary
  Class | Description
  --- | ---
  BlockJoinSelector | Select a value from a block of documents.
  CheckJoinIndex | Utility class to check a block join index.
  JoinUtil | Utility for query time joining.
  ParentChildrenBlockJoinQuery | A query that returns all the matching child documents for a specific parent document indexed together in the same block.
  QueryBitSetProducer | A `BitSetProducer` that wraps a query and caches matching `BitSet`s per segment.
  ToChildBlockJoinQuery | Just like `ToParentBlockJoinQuery`, except this query joins in reverse: you provide a Query matching parent documents and it joins down to child documents.
  ToParentBlockJoinQuery | This query requires that you index children and parent docs as a single block, using the `IndexWriter.addDocuments()` or `IndexWriter.updateDocuments()` API.
  ToParentBlockJoinSortField | A special sort field that allows sorting parent docs based on nested / child level fields.

- Enum Summary
  Enum | Description
  --- | ---
  BlockJoinSelector.Type | Type of selection to perform.
  ScoreMode | How to aggregate multiple child hit scores into a single parent score.

## Package org.apache.lucene.search.join Description

Support for index-time and query-time joins.

## Index-time joins

The index-time joining support joins while searching, where joined documents are indexed as a single document block using `IndexWriter.addDocuments()`. This is useful for any normalized content (XML documents or database tables). In database terms, all rows for all joined tables matching a single row of the primary table must be indexed as a single document block, with the parent document being last in the group.

When you index in this way, the documents in your index are divided into parent documents (the last document of each block) and child documents (all others). You provide a `BitSetProducer` that identifies the parent documents, as Lucene does not currently record any information about doc blocks.

At search time, use `ToParentBlockJoinQuery` to remap/join matches from any child `Query` (ie, a query that matches only child documents) up to the parent document space. The resulting query can then be used as a clause in any query that matches parent.

If you care about what child documents matched for each parent document, then use the `ParentChildrenBlockJoinQuery` query to per matched parent document retrieve the child documents that caused to match the parent document in first place. This query should be used after your main query has been executed. For each hit execute the the `ParentChildrenBlockJoinQuery` query

       TopDocs results = searcher.search(mainQuery, 10);
       for (int i = 0; i < results.scoreDocs.length; i++) {
         ScoreDoc scoreDoc = results.scoreDocs[i];

         // Run ParentChildrenBlockJoinQuery to figure out the top matching child docs:
         ParentChildrenBlockJoinQuery parentChildrenBlockJoinQuery =
           new ParentChildrenBlockJoinQuery(parentFilter, childQuery, scoreDoc.doc);
         TopDocs topChildResults = searcher.search(parentChildrenBlockJoinQuery, 3);
         // Process top child hits...
       }

To map/join in the opposite direction, use `ToChildBlockJoinQuery`. This wraps any query matching parent documents, creating the joined query matching only child documents.

## Query-time joins

The query time joining is index term based and implemented as two pass search. The first pass collects all the terms from a fromField that match the fromQuery. The second pass returns all documents that have matching terms in a toField to the terms collected in the first pass.

Query time joining has the following input:

- `fromField`: The from field to join from.
- `fromQuery`: The query executed to collect the from terms. This is usually the user specified query.
- `multipleValuesPerDocument`: Whether the fromField contains more than one value per document
- `scoreMode`: Defines how scores are translated to the other join side. If you don't care about scoring use `ScoreMode.None` mode. This will disable scoring and is therefore more efficient (requires less memory and is faster).
- `toField`: The to field to join to

Basically the query-time joining is accessible from one static method. The user of this method supplies the method with the described input and a `IndexSearcher` where the from terms need to be collected from. The returned query can be executed with the same `IndexSearcher`, but also with another `IndexSearcher`. Example usage of the `JoinUtil.createJoinQuery()` :

       String fromField = "from"; // Name of the from field
       boolean multipleValuesPerDocument = false; // Set only to true in the case when your fromField has multiple values per document in your index
       String toField = "to"; // Name of the to field
       ScoreMode scoreMode = ScoreMode.Max; // Defines how the scores are translated into the other side of the join.
       Query fromQuery = new TermQuery(new Term("content", searchTerm)); // Query executed to collect from values to join to the to values

       Query joinQuery = JoinUtil.createJoinQuery(fromField, multipleValuesPerDocument, toField, fromQuery, fromSearcher, scoreMode);
       TopDocs topDocs = toSearcher.search(joinQuery, 10); // Note: toSearcher can be the same as the fromSearcher
       // Render topDocs...

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