apache-solr-ref-guide-4.6.pdf

Other Schema Elements
This section describes several other important elements of schema.xml.
Unique Key
The uniqueKey element specifies which field is a unique identifier for documents. Although uniqueKey is not required, it is nearly always
warranted by your application design. For example, uniqueKey should be used if you will ever update a document in the index.
You can define the unique key field by naming it:
id
Starting with Solr 4, schema defaults and copyFields cannot be used to populate the uniqueKey field. You also can't use
UUIDUpdateProcessorFactory to have uniqueKey values generated automatically.
Further, the operation will fail if the uniqueKey field is used, but is multivalued (or inherits the multivalueness from the fieldtype). However,
uniqueKey will continue to work, as long as the field is properly used.
Default Search Field
If you are using the Lucene query parser, queries that don't specify a field name will use the defaultSearchField. The DisMax and Extended
DisMax query parsers do not use this value.
Use of the defaultSearchField element is deprecated in Solr versions 3.6 and higher. Instead, you should use the df
request parameter. At some point, the defaultSearchField element may be removed.
For more information about query parsers, see the section on Query Syntax and Parsing.
Query Parser Default Operator
In queries with multiple terms, Solr can either return results where all conditions are met or where one or more conditions are met. The operator
controls this behavior. An operator of AND means that all conditions must be fulfilled, while an operator of OR means that one or more conditions
must be true.
In schema.xml, the solrQueryParser element controls what operator is used if an operator is not specified in the query. The default operator
setting only applies to the Lucene query parser, not the DisMax or Extended DisMax query parsers, which internally hard-code their operators to
OR.
The query parser default operator parameter has been deprecated in Solr versions 3.6 and higher. You are instead encouraged
to specify the query parser q.op parameter in your request handler.
Similarity
Similarity is a Lucene class used to score a document in searching. This class can be changed in order to provide a more custom sorting. With
Solr 4, you can configure a different similarity for each field, meaning that scoring a document will differ depending on what's in each field.
However, you can still configure a global similarity is configured in the schema.xml file, where an implicit instance of
DefaultSimilarityFactory is used.
A global declaration can be used to specify a custom similarity implementation that you want Solr to use when dealing with your
index. A similarity can be specified either by referring directly to the name of a class with a no-argument constructor:
or by referencing a SimilarityFactory implementation, which may take optional initialization parameters:
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