If you do not specify a thesaurus by name in a query, by default, the thesaurus operators use a thesaurus named DEFAULT. However, Oracle Text does not provide a DEFAULT thesaurus.
As a result, if you want to use a default thesaurus for the thesaurus operators, you must create a thesaurus named DEFAULT. You can create the thesaurus through any of the thesaurus creation methods supported by Oracle Text:
CTX_THES.CREATE_THESAURUS (PL/SQL)
CTX_THES.IMPORT_THESAURUS (PL/SQL)
ctxload utility
Oracle Text Reference to learn more about using ctxload and the CTX_THES package, and "ctxload Utility" in this chapter