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Working With a Thesaurus

This chapter describes how to improve your query application with a thesaurus. The following topics are discussed in this chapter:

• Overview of Thesauri
• Defining Thesaural Terms
• Using a Thesaurus in a Query Application
• About the Supplied Knowledge Base

Overview of Thesauri

Users of your query application looking for information on a given topic might not know which words have been used in documents that refer to that topic.

Oracle Text enables you to create case-sensitive or case-insensitive thesauri which define synonym and hierarchical relationships between words and phrases. You can then retrieve documents that contain relevant text by expanding queries to include similar or related terms as defined in the thesaurus.

You can create a thesaurus and load it into the system.

Thesaurus Creation and Maintenance

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

CTX_THES Package

To maintain and browse your thesaurus programatically, you can use the PL/SQL package, CTX_THES. With this package, you can browse terms and hierarchical relationships, add and delete terms, and add and remove thesaurus relations.

Thesaurus Operators

You can also use the thesaurus operators in the CONTAINS clause to expand query terms according to your loaded thesaurus. For example, you can use the SYN operator to expand a term such as dog to its synonyms as follows:

'syn(dog)'

ctxload Utility

The ctxload utility can be used for loading thesauri from a plain-text file into the thesaurus tables, as well as dumping thesauri from the tables into output (dump) files.

The thesaurus dump files created by ctxload can be printed out or used as input for other applications. The dump files can also be used to load a thesaurus into the thesaurus tables. This can be useful for using an existing thesaurus as the basis for creating a new thesaurus.

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.

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.

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.

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.

Supplied Thesaurus Structure and Content

The supplied thesaurus is similar to a traditional thesaurus, such as Roget's Thesaurus, in that it provides a list of synonymous and semantically related terms.

The supplied thesaurus provides additional value by organizing the terms into a hierarchy that defines real-world, practical relationships between narrower terms and their broader terms.

Additionally, cross-references are established between terms in different areas of the hierarchy.

Supplied Thesaurus Location

The exact name and location of the thesaurus load file is operating system dependent; however, the file is generally named dr0thsus (with an appropriate extension for text files) and is generally located in the following directory structure:

<Oracle_home_directory>
    <interMedia_Text_directory>
       sample
           thes

Defining Thesaural Terms

You can create synonyms, related terms, and hierarchical relationships with a thesaurus. The following sections give examples.

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 allows 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'

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.

Using a Thesaurus in a Query Application

Defining a custom thesaurus allows you to process queries more intelligently. Since users of your application might not know which words represent a topic, you can define synonyms or narrower terms for likely query terms. You can use the thesaurus operators to expand your query into your thesaurus terms.

There are two ways to enhance your query application with a custom thesaurus so that you can process queries more intelligently:

• Load your custom thesaurus and issue queries with thesaurus operators
• Augment the knowledge base with your custom thesaurus (English only) and use the ABOUT operator to expand your query.

Each approach has its advantages and disadvantages.

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 
   

3. 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)'
   

Advantage

The advantage of using this method is that you can modify the thesaurus after indexing.

Limitations

This method requires you to use thesaurus expansion operators in your query. Long queries can cause extra overhead in the thesaurus expansion and slow your query down.

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)'
   

Advantage

Compiling your custom thesaurus with the existing knowledge base before indexing allows for faster and simpler queries with the ABOUT operator. Document services can also take full advantage of the customized information for creating theme summaries and Gists.

Limitations

Use of the ABOUT operator requires a theme component in the index, which requires slightly more disk space. You must also define the thesaurus before indexing your documents. If you make any change to the thesuarus, you must recompile your thesaurus and re-index your documents.

Linking New Terms to Existing Terms

When adding terms to the knowledge base, Oracle recommends that new terms be linked to one of the categories in the knowledge base for best results in theme proving.

If new terms are kept completely separate from existing categories, fewer themes from new terms will be proven. The result of this is poor precision and recall with ABOUT queries as well as poor quality of gists and theme highlighting.

You link new terms to existing terms by making an existing term the broader term for the new terms.

Example: Linking New Terms to Existing Terms

You purchase a medical thesaurus medthes containing a a hierarchy of medical terms. The four top terms in the thesaurus are the following:

• Anesthesia and Analgesia
• Anti-Allergic and Respiratory System Agents
• Anti-Inflammatory Agents, Antirheumatic Agents, and Inflammation Mediators
• Antineoplastic and Immunosuppressive Agents

To link these terms to the existing knowledge base, add the following entries to the medical thesaurus to map the new terms to the existing health and medicine branch:

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

Loading a Thesaurus with ctxload

Assuming the medical thesaurus is in a file called med.thes, you load the thesaurus as medthes with ctxload as follows:

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

Compiling a Loaded Thesaurus

To link the loaded thesaurus medthes to the knowledge base, use ctxkbtc as follows:

ctxkbtc -user ctxsys/ctxsys -name medthes 

About the Supplied Knowledge Base

Oracle Text supplies a knowledge base for English and French. The supplied knowledge contains the information used to perform theme analysis. Theme analysis includes theme indexing, ABOUT queries, and theme extraction with the CTX_DOC package.

The knowledge base is a hierarchical tree of concepts and categories. It has six main branches:

• science and technology
• business and economics
• government and military
• social environment
• geography
• abstract ideas and concepts

  See Also: Oracle Text Reference for the breakdown of the category hierarchy.

The supplied knowledge base is like a thesaurus in that it is hierarchical and contains broader term, narrower term, and related term information. As such, you can improve the accuracy of theme analysis by augmenting the knowledge base with your industry-specific thesaurus by linking new terms to existing terms.

You can also extend theme functionality to other languages by compiling a language-specific thesuarus into a knowledge base.

Knowledge Base Character Set

Knowledge bases can be in any single-byte character set. Supplied knowledge bases are in WE8ISO8859P1. You can store an extended knowledge base in another character set such as US7ASCII.

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 thesuarus.

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.

Limitations

The following limitations hold for adding knowledge bases:

• Oracle supplies knowledge bases in English and French only. You must provide your own thesaurus for any other language.
• You can only add knowledge bases for languages with single-byte character sets. You cannot create a knowledge base for languages which can be expressed only in multi-byte character sets. If the database is a multi-byte universal character set, such as UTF-8, the NLS_LANG parameter must still be set to a compatible single-byte character set when compiling the thesaurus.
• Adding a knowledge base works best for whitespace delimited languages.
• You can have at most one knowledge base per NLS language.

• Obtaining hierarchical query feedback information such as broader terms, narrower terms and related terms does not work in languages other than English and French. In other languages, the knowledge bases are derived entirely from your thesauri. In such cases, Oracle recommends that you obtain hierarchical information directly from your thesauri.

  See Also: Oracle Text Reference for more information about theme indexing, ABOUT queries, using the CTX_DOC package, and the supplied English knowledge base.