An extractor policy is a named dictionary entity that determines the characteristics of a semantic index that is created using the policy.

Each extractor policy refers, directly or indirectly, to an instance of an extractor type. An extractor policy with a direct reference to an extractor type instance can be used to compose other extractor policies that include additional RDF graphs for ontologies.

The following example creates a basic extractor policy created using the GATE extractor type:

begin
  sem_rdfctx.create_policy (policy_name => 'SEM_EXTR',
                            extractor   => mdsys.gatenlp_extractor());
end;
/

The following example creates a dependent extractor policy that combines the metadata extracted by the policy in the preceding example with a user-defined RDF graph named geo_ontology:

begin
  sem_rdfctx.create_policy (policy_name => 'SEM_EXTR_PLUS_GEOONT',
                            base_policy => 'SEM_EXTR',
                            user_models => SEM_MODELS ('geo_ontology'));
end;
/

You can use an extractor policy to create one or more semantic indexes on columns that store unstructured documents, as explained in Semantically Indexing Documents.