|Oracle Text Application Developer's Guide
Part Number A96517-01
This chapter describes how to improve your query application with a thesaurus. The following topics are discussed in this chapter:
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.
The Oracle Text thesauri formats and functionality are compliant with both the ISO-2788 and ANSI Z39.19 (1993) standards.
Thesauri and thesaurus entries can be created, modified, and deleted by all Oracle Text users with the
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.
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:
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.
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.
To take full advantage of query expansions that result from a case-sensitive thesaurus, your index must also be case-sensitive.
When loading a thesaurus, you can specify that the thesaurus be loaded case-sensitive using the -thescase parameter.
When creating a thesaurus with
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.
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
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.
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:
Oracle Text Reference to learn more about using
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 Text Reference to learn more about using
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.
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:
For more information about the directory structure for Oracle Text, see the Oracle9i installation documentation specific to your operating system.
You can create synonyms, related terms, and hierarchical relationships with a thesaurus. The following sections give examples.
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.
You can thus use the SYN operator to expand XML into its synonyms:
is expanded to:
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:
'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.
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:
ABOUToperator to expand your query.
Each approach has its advantages and disadvantages.
To build a custom thesaurus, follow these steps:
tech_docfrom an import file named
The advantage of using this method is that you can modify the thesaurus after indexing.
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.
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:
ctxload. See "Loading a Thesaurus with ctxload".
ctxkbtccompiler. "Compiling a Loaded Thesaurus" later in this section.
ABOUToperator 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:
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.
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.
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.
Oracle Text Reference for more information about the supplied English knowledge base.
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.
You purchase a medical thesaurus
medthes containing a a hierarchy of medical terms. The four top terms in the thesaurus are the following:
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
Assuming the medical thesaurus is in a file called
med.thes, you load the thesaurus as
ctxload as follows:
To link the loaded thesaurus
medthes to the knowledge base, use
ctxkbtc as follows:
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
The knowledge base is a hierarchical tree of concepts and categories. It has six main branches:
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.
"Augmenting Knowledge Base with Custom Thesaurus" in this chapter.
You can also extend theme functionality to other languages by compiling a language-specific thesuarus into a knowledge base.
"Adding a Language-Specific Knowledge Base" in this chapter.
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.
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
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:
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
The following limitations hold for adding knowledge bases:
NLS_LANGparameter must still be set to a compatible single-byte character set when compiling the thesaurus.
Oracle Text Reference for more information about theme indexing,