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

This modules support index-time and query-time joins.

See:
Description

Class Summary
JoinUtil | Utility for query time joining using `TermsQuery` and `TermsCollector`.
RawTermFilter | Expert: creates a filter accepting all documents containing the provided term, disregarding deleted documents.
ToChildBlockJoinQuery | Just like `ToParentBlockJoinQuery`, except this query joins in reverse: you provide a Query matching parent documents and it joins down to child documents.
ToParentBlockJoinCollector | Collects parent document hits for a Query containing one more more BlockJoinQuery clauses, sorted by the specified parent Sort.
ToParentBlockJoinQuery | This query requires that you index children and parent docs as a single block, using the `IndexWriter.addDocuments(java.util.Collection)` or `IndexWriter.updateDocuments(org.apache.lucene.index.Term, java.util.Collection)` API.

Enum Summary
ToParentBlockJoinQuery.ScoreMode | How to aggregate multiple child hit scores into a single parent score.

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

This modules support 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(java.util.Collection)`. 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 `Filter` 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 only care about the parent documents matching the query, you can use any collector to collect the parent hits, but if you'd also like to see which child documents match for each parent document, use the `ToParentBlockJoinCollector` to collect the hits. Once the search is done, you retrieve a `TopGroups` instance from the `ToParentBlockJoinCollector.getTopGroups(org.apache.lucene.search.join.ToParentBlockJoinQuery, org.apache.lucene.search.Sort, int, int, int, boolean)` method.

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

## Search-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.
- `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, String, org.apache.lucene.search.Query, org.apache.lucene.search.IndexSearcher)` :

      String fromField = "from"; // Name of the from field
      String toField = "to"; // Name of the to field
      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, toField, fromQuery, fromSearcher);
      TopDocs topDocs = toSearcher.search(joinQuery, 10); // Note: toSearcher can be the same as the fromSearcher
      // Render topDocs...

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