9 Working With a Thesaurus

9.1.1 Thesaurus Creation and Maintenance

Thesauri and thesaurus entries can be created, modified, and deleted by all Oracle Text users with the CTXAPP role.

9.1.2 Case-sensitive Thesauri

In a case-sensitive thesaurus, terms (words and phrases) are stored exactly as entered. For example, if a term is entered in mixed-case (using either the CTX_THES package or a thesaurus load file), the thesaurus stores the entry in mixed-case.

When loading a thesaurus, you can specify that the thesaurus be loaded case-sensitive using the -thescase parameter.

When creating a thesaurus with CTX_THES.CREATE_THESAURUS, you can specify that the thesaurus created be case-sensitive.

In addition, when a case-sensitive thesaurus is specified in a query, the thesaurus lookup uses the query terms exactly as entered in the query. Therefore, queries that use case-sensitive thesauri allow for a higher level of precision in the query expansion, which helps lookup when and only when you have a case-sensitive index.

For example, a case-sensitive thesaurus is created with different entries for the distinct meanings of the terms Turkey (the country) and turkey (the type of bird). Using the thesaurus, a query for Turkey expands to include only the entries associated with Turkey.

9.1.3 Case-insensitive Thesauri

In a case-insensitive thesaurus, terms are stored in all-uppercase, regardless of the case in which they were entered.

The ctxload program loads a thesaurus case-insensitive by default.

When creating a thesaurus with CTX_THES.CREATE_THESAURUS, the thesaurus is created case-insensitive by default.

In addition, when a case-insensitive thesaurus is specified in a query, the query terms are converted to all-uppercase for thesaurus lookup. As a result, Oracle Text is unable to distinguish between terms that have different meanings when they are in mixed-case.

For example, a case-insensitive thesaurus is created with different entries for the two distinct meanings of the term TURKEY (the country or the type of bird). Using the thesaurus, a query for either Turkey or turkey is converted to TURKEY for thesaurus lookup and then expanded to include all the entries associated with both meanings.

9.1.4 Default Thesaurus

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)

• ctxload

  
  

  See Also: Oracle Text Reference to learn more about using ctxload and the CTX_THES package.

  
  

9.1.5 Supplied Thesaurus

Although Oracle Text does not provide a default thesaurus, Oracle Text does supply a thesaurus, in the form of a ctxload load file, that can be used to create a general-purpose, English-language thesaurus.

The thesaurus load file can be used to create a default thesaurus for Oracle Text or it can be used as the basis for creating thesauri tailored to a specific subject or range of subjects.

<Oracle_home_directory>
    <interMedia_Text_directory>
       sample
           thes

9.2.1 Defining Synonyms

If you have a thesaurus of computer science terms, you might define a synonym for the term XML as extensible markup language. This enables queries on either of these terms to return the same documents.

XML

SYN Extensible Markup Language

You can thus use the SYN operator to expand XML into its synonyms:

'SYN(XML)'

is expanded to:

'XML, Extensible Markup Language'

9.2.2 Defining Hierarchical Relations

If your document set is made up of news articles, you can use a thesaurus to define a hierarchy of geographical terms. Consider the following hierarchy that describes a geographical hierarchy for the U.S state of California:

California
   NT Northern California
       NT San Francisco
       NT San Jose
   NT Central Valley
       NT Fresno
   NT Southern California
       NT Los Angeles

You can thus use the NT operator to expand a query on California as follows:

'NT(California)'

expands to:

'California, Northern California, San Francisco, San Jose, Central Valley, Fresno, Southern California, Los Angeles'

The resulting hitlist shows all documents related to the U.S. state of California regions and cities.

9.3.1 Loading a Custom Thesaurus and Issuing Thesaural Queries

To build a custom thesaurus, follow these steps:

1. Create your thesaurus. See "Defining Thesaural Terms" in this chapter.

2. Load thesaurus with ctxload. For example, the following example imports a thesaurus named tech_doc from an import file named tech_thesaurus.txt:

   ctxload -user jsmith/123abc -thes -name tech_doc -file tech_thesaurus.txt 
   

1. Use THES operators to query. For example, you can find all documents that contain XML and its synonyms as defined in tech_doc:

   'SYN(XML, tech_doc)'
   

9.3.2 Augmenting Knowledge Base with Custom Thesaurus

You can add your custom thesaurus to a branch in the existing knowledge base. The knowledge base is a hierarchical tree of concepts used for theme indexing, ABOUT queries, and deriving themes for document services.

When you augment the existing knowledge base with your new thesaurus, you query with the ABOUT operator which implicitly expands to synonyms and narrower terms. You do not query with the thesaurus operators.

To augment the existing knowledge base with your custom thesaurus, follow these steps:

1. Create your custom thesaurus, linking new terms to existing knowledge base terms. See "Defining Thesaural Terms" and "Linking New Terms to Existing Terms".

2. Load thesaurus with ctxload. See "Loading a Thesaurus with ctxload".

3. Compile the loaded thesaurus with ctxkbtc compiler. "Compiling a Loaded Thesaurus" later in this section.

4. Index your documents. By default the system creates a theme component to your index.

5. Use ABOUT operator to query. For example, to find all documents that are related to the term politics including any synonyms or narrower terms as defined in the knowledge base, issue the query:

   'about(politics)'
   

health and medicine
 NT Anesthesia and Analgesia
 NT Anti-Allergic and Respiratory System Agents
 NT Anti-Inflamammatory Agents, Antirheumatic Agents, and Inflamation Mediators
 NT Antineoplastic and Immunosuppressive Agents

ctxload -thes -thescase y -name medthes -file med.thes -user ctxsys/ctxsys

ctxkbtc -user ctxsys/ctxsys -name medthes 

9.4.1 Adding a Language-Specific Knowledge Base

You can extend theme functionality to languages other than English or French by loading your own knowledge base for any single-byte whitespace delimited language, including Spanish.

Theme functionality includes theme indexing, ABOUT queries, theme highlighting, and the generation of themes, gists, and theme summaries with CTX_DOC.

You extend theme functionality by adding a user-defined knowledge base. For example, you can create a Spanish knowledge base from a Spanish thesaurus.

To load your language-specific knowledge base, follow these steps:

1. Load your custom thesaurus using ctxload.

2. Set NLS_LANG so that the language portion is the target language. The charset portion must be a single-byte character set.

3. Compile the loaded thesaurus using ctxkbtc:

ctxkbtc -user ctxsys/ctxsys -name my_lang_thes

This command compiles your language-specific knowledge base from the loaded thesaurus. To use this knowledge base for theme analysis during indexing and ABOUT queries, specify the NLS_LANG language as the THEME_LANGUAGE attribute value for the BASIC_LEXER preference.