Apache Solr Reference Guide Covering Apache Solr 6.0

Similarity
Similarity is a Lucene class used to score a document in searching.
Each collection has one "global" Similarity, and by default Solr uses an implicit SchemaSimilarityFactory w
hich allows individual field types to be configured with a "per-type" specific Similarity and implicitly uses BM25Sim
ilarity for any field type which does not have an explicit Similarity.
This default behavior can be overridden by declaring a top level element in your schema.xml,
outside of any single field type. This similarity declaration can either refer directly to the name of a class with a
no-argument constructor, such as in this example showing BM25Similarity:
or by referencing a SimilarityFactory implementation, which may take optional initialization parameters:
P
L
H2
7
In most cases, specifying global level similarity like this will cause an error if your schema.xml also includes
field type specific declarations. One key exception to this is that you may explicitly declare a S
chemaSimilarityFactory and specify what that default behavior will be for all field types that do not declare
an explicit Similarity using the name of field type (specified by defaultSimFromFieldType) that is configured
with a specific similarity:
text_dfr
I(F)
B
H3
900
SPL
DF
H2
In the example above IBSimilarityFactory (using the Information-Based model) will be used for any fields
of type text_ib, while DFRSimilarityFactory (divergence from random) will be used for any fields of type t
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