Apache Solr Reference Guide Covering Apache Solr 6.0

movie_directors: id, name, has_oscar, ...
To filter movies by directors that have won an Oscar using a Solr join on the movie_directors collection, you
can send the following filter query to the movies collection:
fq={!join from=id fromIndex=movie_directors to=director_id}has_oscar:true
Notice that the query criteria of the filter ( has_oscar:true) is based on a field in the collection specified using
fromIndex. Keep in mind that you cannot return fields from the fromIndex collection using join queries, you
can only use the fields for filtering results in the "to" collection (movies).
Next, let's understand how these collections need to be deployed in your cluster. Imagine the movies collection
is deployed to a four node SolrCloud cluster and has two shards with a replication factor of two. Specifically, the
movies collection has replicas on the following four nodes:
node 1: movies_shard1_replica1
node 2: movies_shard1_replica2
node 3: movies_shard2_replica1
node 4: movies_shard2_replica2
To use the movie_directors collection in Solr join queries with the movies collection, it needs to have a replica
on each of the four nodes. In other words, movie_directors must have one shard and replication factor of four:
node 1: movie_directors_shard1_replica1
node 2: movie_directors_shard1_replica2
node 3: movie_directors_shard1_replica3
node 4: movie_directors_shard1_replica4
At query time, the JoinQParser will access the local replica of the movie_directors collection to perform the
join. If a local replica is not available or active, then the query will fail. At this point, it should be clear that since
you're limited to a single shard and the data must be replicated across all nodes where it is needed, this
approach works better with smaller data sets where there is a one-to-many relationship between the from
collection and the to collection. Moreover, if you add a replica to the to collection, then you also need to add a
replica for the from collection.
For more information about join queries, see the Solr Wiki page on Joins. Erick Erickson has also written a blog
post about join performance called Solr and Joins, hosted by SearchHub.org.
Lucene Query Parser
The LuceneQParser extends the QParserPlugin by parsing Solr's variant on the Lucene QueryParser
syntax. This is effectively the same query parser that is used in Lucene. It uses the operators q.op, the default
operator ("OR" or "AND") and df, the default field name.
Example:
{!lucene q.op=AND df=text}myfield:foo +bar -baz
For more information about the syntax for the Lucene Query Parser, see the Classic QueryParser javadocs.
Max Score Query Parser
The MaxScoreQParser extends the LuceneQParser but returns the Max score from the clauses. It does this
Apache Solr Reference Guide 6.0
288