|Oracle® Text Application Developer's Guide
10g Release 2 (10.2)
|PDF · Mobi · ePub|
This chapter provides an introduction to Oracle Text indexing. The following topics are covered:
The following sections discuss the different types of Oracle Text indexes, their structure, the indexing process, and limitations.
With Oracle Text, you can indexes of several types, using
INDEX. Table 3-1 describes each index type, its purpose, and what features it supports:
|Index Type||Description||Supported Preferences and Parameters||Query Operator||Notes|
||Use this index to build a text retrieval application when your text consists of large coherent documents. You can index documents of different formats such as MS Word, HTML or plain text.
With a context index, you can customize your index in a variety of ways.
This index type requires
These supported parameters include the index partition clause, and the format, charset, and language columns.
Grammar is called the
|Supports all documents services and query services.
Supports indexing of partitioned text tables.
||Use this index type for better mixed query performance. Typically, with this index type, you index small documents or text fragments. Other columns in the base table, such as item names, prices and descriptions can be included in the index to improve mixed query performance.
This index type is transactional, automatically updating itself after DML to base table. No
Format, charset, and language columns not supported.
Table and index partitioning not supported.
Grammar is called
Theme querying is supported.
|This index is larger and takes longer to build than a
The size of a
||See "CTXRULE Parameters and Limitations".||
||Single documents (plain text, HTML, or XML) can be classified using the
||Create this index when you need to speed up existsNode() queries on an XMLType column.||
||Use with existsNode()||Can only create this index on XMLType column.
Although this index type can be helpful for
See Also:Index Creation in this chapter
An Oracle Text index is an Oracle Database domain index.To build your query application, you can create an index of type
CONTEXT and query it with the
You create an index from a populated text table. In a query application, the table must contain the text or pointers to where the text is stored. Text is usually a collection of documents, but can also be small text fragments.
For better performance for mixed queries, you can create a
CTXCAT index. Use this index type when your application relies heavily on mixed queries to search small documents or descriptive text fragments based on related criteria such as dates or prices. You query this index with the
To build a document classification application using simple or rule-based classification, you create an index of type
CTXRULE. With such an index, you can classify plain text, HTML, or XML documents using the
MATCHES operator. You store your defining query set in the text table you index.
If you are working with XMLtype columns, you can create a
CTXXPATH index to speed up queries with existsNode.
You create a text index as a type of extensible index to Oracle Database using standard SQL. This means that an Oracle Text index operates like an Oracle Database index. It has a name by which it is referenced and can be manipulated with standard SQL statements.
The benefits of a creating an Oracle Text index include fast response time for text queries with the
MATCHES Oracle Text operators. These operators query the
CTXRULE index types respectively.
See Also:"Index Creation" in this chapter
Oracle Text indexes text by converting all words into tokens. The general structure of an Oracle Text
CONTEXT index is an inverted index where each token contains the list of documents (rows) that contain that token.
For example, after a single initial indexing operation, the word DOG might have an entry as follows:
|Word||Appears in Document|
|DOG||DOC1 DOC3 DOC5|
This means that the word DOG is contained in the rows that store documents one, three and five.
For more information, see optimizing the index in this chapter.
By default in English and French, Oracle Text indexes theme information with word information. You can query theme information with the
ABOUT operator. You can optionally enable and disable theme indexing.
See Also:"Creating Preferences" in this chapter to learn more about indexing theme information
This section describes the Oracle Text indexing process.You initiate the indexing process with the
INDEX statement. The goal is to create an Oracle Text index of tokens according to the parameters and preferences you specify.
Figure 3-1 shows the indexing process. This process is a data stream that is acted upon by the different indexing objects. Each object corresponds to an indexing preference type or section group you can specify in the parameter string of
INDEX. The sections that follow describe these objects.
The stream starts with the datastore reading in the documents as they are stored in the system according to your datastore preference. For example, if you have defined your datastore as
FILE_DATASTORE, the stream starts by reading the files from the operating system. You can also store your documents on the internet or in the Oracle Database. Wherever your files reside physically, you must always have a text table in the Oracle Database that points to the file.
The stream then passes through the filter. What happens here is determined by your
FILTER preference. The stream can be acted upon in one of the following ways:
No filtering takes place. This happens when you specify the
NULL_FILTER preference type or when the value of the format column is
IGNORE. Documents that are plain text, HTML, or XML need no filtering.
After being filtered, the marked-up text passes through the sectioner that separates the stream into text and section information. Section information includes where sections begin and end in the text stream. The type of sections extracted is determined by your section group type.
The section information is passed directly to the indexing engine which uses it later. The text is passed to the lexer.
The lexer breaks the text into tokens according to your language. These tokens are usually words. To extract tokens, the lexer uses the parameters as defined in your lexer preference. These parameters include the definitions for the characters that separate tokens such as whitespace, and whether to convert the text to all uppercase or to leave it in mixed case.
When theme indexing is enabled, the lexer analyses your text to create theme tokens for indexing.
The indexing engine creates the inverted index that maps tokens to the documents that contain them. In this phase, Oracle Text uses the stoplist you specify to exclude stopwords or stopthemes from the index. Oracle Text also uses the parameters defined in your
WORDLIST preference, which tell the system how to create a prefix index or substring index, if enabled.
You can create a partitioned
CONTEXT index on a partitioned text table. The table must be partitioned by range. Hash, composite and list partitions are not supported.
You might create a partitioned text table to partition your data by date. For example, if your application maintains a large library of dated news articles, you can partition your information by month or year. Partitioning simplifies the manageability of large databases since querying, DML, and backup and recovery can act on single partitions.
See Also:Oracle Database Concepts for more information about partitioning
To query a partitioned table, you use
CONTAINS in the
WHERE clause of a
SELECT statement as you query a regular table. You can query the entire table or a single partition. However, if you are using the
SCORE clause, Oracle recommends that you query single partitions unless you include a range predicate that limits the query to a single partition.
When it is not practical to lock up your base table for indexing because of ongoing updates, you can create your index online with the
ONLINE parameter of
CREATE INDEX. This way an application with heavy DML need not stop updating the base table for indexing.
There are short periods, however, when the base table is locked at the beginning and end of the indexing process.
See Also:Oracle Text Reference to learn more about creating an index online
Oracle Text supports parallel indexing with
When you issue a parallel indexing command on a non-partitioned table, Oracle Text splits the base table into temporary partitions, spawns slave processes, and assigns a slave to a partition. Each slave indexes the rows in its partition. The method of slicing the base table into partitions is determined by Oracle Text and is not under your direct control. This is true as well for the number of slave processes actually spawned, which depends on machine capabilities, system load, your init.ora settings, and other factors. The actual parallel degree may not match the degree of parallelism requested.
Since indexing is an I/O intensive operation, parallel indexing is most effective in decreasing your indexing time when you have distributed disk access and multiple CPUs. Parallel indexing can only affect the performance of an initial index with
INDEX. It does not affect DML performance with
INDEX, and has minimal impact on query performance.
Since parallel indexing decreases the initial indexing time, it is useful for
Data staging, when your product includes an Oracle Text index
Rapid initial startup of applications based on large data collections
Application testing, when you need to test different index parameters and schemas while developing your application
See Also:"Frequently Asked Questions About Indexing Performance" in Chapter 7, "Tuning Oracle Text" to learn more about creating an index in parallel
Oracle SQL standards do not support creating indexes on views. If you need to index documents whose contents are in different tables, you can create a data storage preference using the
USER_DATASTORE object. With this object, you can define a procedure that synthesizes documents from different tables at index time.
See Also:Oracle Text Reference to learn more about
Oracle Text does support the creation of
CTXXPATH indexes on materialized views (
You use the
INDEX statement to create an Oracle Text index. When you create an index and specify no parameter string, an index is created with default parameters. You can create either a
You can also override the defaults and customize your index to suit your query application. The parameters and preference types you use to customize your index with
INDEX fall into the following general categories.
The basic prerequisite for an Oracle Text query application is to have a populated text table. The text table is where you store information about your document collection and is required for indexing.
When you create a
CONTEXT index, you can populate rows in your text table with one of the following elements:
Text information (can be documents or text fragments)
Path names of documents in your file system
URLs that specify World Wide Web documents
Figure 3-2 illustrates these different methods.
When creating a
CTXRULE index, only the first method shown is supported.
By default, the indexing operation expects your document text to be directly loaded in your text table, which is the first method shown previously.
However, when you create a
CONTEXT index, you can specify the other ways of identifying your documents such as with filenames or with URLs by using the corresponding data storage indexing preference.
With Oracle Text, you can create a
CONTEXT index with columns of type
Note:The column types
This section discusses how you can store text in directly in your table with the different indexes.
You can store documents in your text table in different ways.
You can store documents in one column using the
DIRECT_DATASTORE data storage type or over a number of columns using the
MULTI_COLUMN_DATASTORE type. When your text is stored over a number of columns, Oracle Text concatenates the columns into a virtual document for indexing.
You can also create master-detail relationships for your documents, where one document can be stored across a number of rows. To create master-detail index, use the
DETAIL_DATASTORE data storage type.
Oracle Text supports the indexing of the
XMLType datatype, which you use to store XML documents.
In your text table, you can store path names to files stored in your file system. When you do so, use the
FILE_DATASTORE preference type during indexing. This method of data storage is supported for
CONTEXT indexes only.
You can store URL names to index Web sites. When you do so, use the
URL_DATASTORE preference type during indexing. This method of data storage is supported for
CONTEXT indexes only.
In your text table, you can create additional columns to store structured information that your query application might need, such as primary key, date, description, or author.
If your documents are of mixed formats or of mixed character sets, you can create the following additional columns:
A format column to record the format (
BINARY) to help filtering during indexing. You can also use the format column to ignore rows for indexing by setting the format column to
IGNORE. This is useful for bypassing rows that contain data incompatible with text indexing such as images.
A character set column to record the document character set for each row.
When you create your index, you must specify the name of the format or character set column in the parameter clause of
For all rows containing the keywords
AUTOMATIC in character set or language columns, Oracle Text will apply statistical techniques to determine the character set and language respectively of the documents and modify document indexing appropriately.
Because the system can index most document formats including HTML, PDF, Microsoft Word, and plain text, you can load any supported type into the text column.
When you have mixed formats in your text column, you can optionally include a format column to help filtering during indexing. With the format column you can specify whether a document is binary (formatted) or text (non-formatted such as HTML).
See Also:Oracle Text Reference for more information about the supported document formats
When you index with
CREATE INDEX, you specify the location using the datastore preference. Use the appropriate datastore according to your application.
Table 3-2 summarizes the different ways you can store your text with the datastore preference type.
|Datastore Type||Use When|
||Data is stored internally in a text column. Each row is indexed as a single document.
Your text column can be
||Data is stored in a text table in more than one column. Columns are concatenated to create a virtual document, one document for each row.|
||Data is stored internally in a text column. Document consists of one or more rows stored in a text column in a detail table, with header information stored in a master table.|
||Data is stored externally in operating system files. Filenames are stored in the text column, one for each row.|
||Data is stored in a nested table.|
||Data is stored externally in files located on an intranet or the Internet. Uniform Resource Locators (URLs) are stored in the text column.|
||Documents are synthesized at index time by a user-defined stored procedure.|
Indexing time and document retrieval time will be increased for indexing URLs since the system must retrieve the document from the network.
See Also:Datastore Examples in this chapter
Formatted documents such as Microsoft Word and PDF must be filtered to text to be indexed. The type of filtering the system uses is determined by the
FILTER preference type. By default the system uses the
AUTO_FILTER filter type, which automatically detects the format of your documents and filters them to text.
Oracle Text can index most formats. Oracle Text can also index columns that contain documents with mixed formats.
If you are indexing HTML or plain text files, do not use the
AUTO_FILTER type. For best results, use the
NULL_FILTER preference type.
See Also:NULL_FILTER Example: Indexing HTML Documents in this chapter
If you have a mixed-format column such as one that contains Microsoft Word, plain text, and HTML documents, you can bypass filtering for plain text or HTML by including a format column in your text table. In the format column, you tag each row
BINARY. Rows that are tagged
TEXT are not filtered.
For example, you can tag the HTML and plain text rows as
TEXT and the Microsoft Word rows as
BINARY. You specify the format column in the
CREATE INDEX parameter clause.
A third format column type,
IGNORE, is provided for when you do not want a document to be indexed at all. This is useful, for example, when you have a mixed-format table that includes plain-text documents in both Japanese and English, but you only want to process the English documents; another example might be that of a mixed-format table that includes both plain-text documents and images. Because
IGNORE is implemented at the datastore level, it can be used with all filters.
You can create your own custom filter to filter documents for indexing. You can create either an external filter that is executed from the file system or an internal filter as a PL/SQL or Java stored procedure.
See Also:PROCEDURE_FILTER Example
You can bypass rows in your text table that are not to be indexed, such as rows that contain image data. To do so, create a format column in your table and set it to
IGNORE. You name the format column in the parameter clause of
The indexing engine expects filtered text to be in the database character set. When you use the
AUTO_FILTER filter type, formatted documents are converted to text in the database character set.
If your document set contains documents with different character sets, such as JA16EUC and JA16SJIS, you can index the documents provided you create a charset column. You populate this column with the name of the document character set for each row. You name the column in the parameter clause of the
CREATE INDEX statement.
Oracle Text can index most languages. By default, Oracle Text assumes the language of text to index is the language you specify in your database setup.
You use the
BASIC_LEXER preference type to index whitespace-delimited languages such as English, French, German, and Spanish. For some of these languages you can enable alternate spelling, composite word indexing, and base letter conversion.
You can also index Japanese, Chinese, and Korean.
See Also:Oracle Text Reference to learn more about indexing these languages
BASIC_LEXER, Japanese, Chinese and Korean lexers, Oracle Text provides a lexing solution for most languages. For other languages such as Thai and Arabic, you can create your own lexing solution using the user-defined lexer interface. This interface enables you to create a PL/SQL or Java procedure to process your documents during indexing and querying.
You can also use the user-defined lexer to create your own theme lexing solution or linguistic processing engine.
See Also:Oracle Text Reference to learn more about this lexer
Oracle Text can index text columns that contain documents of different languages, such as a column that contains documents written in English, German, and Japanese. To index a multi-language column, you need a language column in your text table. Use the
MULTI_LEXER preference type.
You can also incorporate a multi-language stoplist when you index multi-language columns.
See Also:MULTI_LEXER Example: Indexing a Multi-Language Table in this chapter
When you use the
BASIC_LEXER preference type, you can specify how non-alphanumeric characters such as hyphens and periods are indexed in relation to the tokens that contain them. For example, you can specify that Oracle Text include or exclude hyphen character (-) when indexing a word such as web-site.
These characters fall into
BASIC_LEXER categories according to the behavior you require during indexing. The way the you set the lexer to behave for indexing is the way it behaves for query parsing.
Some of the special characters you can set are as follows:
Define a non-alphanumeric character as
printjoin when you want this character to be included in the token during indexing.
For example, if you want your index to include hyphens and underscore characters, define them as printjoins. This means that words such as web-site are indexed as web-site. A query on website does not find web-site.
See Also:BASIC_LEXER Example: Setting Printjoins Characters in this chapter
Define a non-alphanumeric character as a
skipjoin when you do not want this character to be indexed with the token that contains it.
For example, with the hyphen (-) character defined as a skipjoin, the word web-site is indexed as website. A query on web-site finds documents containing website and web-site.
Other characters can be specified to control other tokenization behavior such as token separation (startjoins, endjoins, whitespace), punctuation identification (punctuations), number tokenization (numjoins), and word continuation after line-breaks (continuation). These categories of characters have defaults, which you can modify.
By default, all text tokens are converted to uppercase and then indexed. This results in case-insensitive queries. For example, separate queries on each of the three words cat, CAT, and Cat all return the same documents.
You can change the default and have the index record tokens as they appear in the text. When you create a case-sensitive index, you must specify your queries with exact case to match documents. For example, if a document contains Cat, you must specify your query as Cat to match this document. Specifying cat or CAT does not return the document.
To enable or disable case-sensitive indexing, use the mixed_case attribute of the
You can enable the following language specific features at index time:
For English and French, you can index document theme information. A document themes is a concept that is sufficiently developed in the document. Themes can be queried with the
You can index theme information in other languages provided you have loaded and compiled a knowledge base for the language.
By default themes are indexed in English and French. You can enable and disable theme indexing with the index_themes attribute of the
BASIC_LEXER preference type.
Some languages contain characters with diacritical marks such as tildes, umlauts, and accents. When your indexing operation converts words containing diacritical marks to their base letter form, queries need not contain diacritical marks to score matches. For example in Spanish with a base-letter index, a query of energía matches energía and energia in the index.
However, with base-letter indexing disabled, a query of energía matches only energía.
You can enable and disable base-letter indexing for your language with the base_letter attribute of the
BASIC_LEXER preference type.
Languages such as German, Danish, and Swedish contain words that have more than one accepted spelling. For instance, in German, ae can be substituted for ä. The ae character pair is known as the alternate form.
By default, Oracle Text indexes words in their alternate forms for these languages. Query terms are also converted to their alternate forms. The result is that these words can be queried with either spelling.
You can enable and disable alternate spelling for your language using the alternate_spelling attribute in the
BASIC_LEXER preference type.
German and Dutch text contain composite words. By default, Oracle Text creates composite indexes for these languages. The result is that a query on a term returns words that contain the term as a sub-composite.
For example, in German, a query on the term Bahnhof (train station) returns documents that contain Bahnhof or any word containing Bahnhof as a sub-composite, such as Hauptbahnhof, Nordbahnhof, or Ostbahnhof.
You can enable and disable the creation of composite indexes with the composite attribute of the
You index these languages with specific lexers:
These lexers have their own sets of attributes to control indexing.
See Also:Oracle Text Reference to learn more about these lexers
Fuzzy matching enables you to match similarly spelled words in queries.
Stemming enables you to match words with the same linguistic root. For example a query on $speak, expands to search for all documents that contain speak, speaks, spoke, and spoken.
Fuzzy matching and stemming are automatically enabled in your index if Oracle Text supports this feature for your language.
Fuzzy matching is enabled with default parameters for its similarity score lower limit and for its maximum number of expanded terms. At index time you can change these default parameters.
To improve the performance of stem queries, you can create a stem index by enabling the index_stems attribute of the
See Also:Oracle Text Reference
Wildcard queries enable you to issue left-truncated, right-truncated and doubly truncated queries, such as %ing, cos%, or %benz%. With normal indexing, these queries can sometimes expand into large word lists, degrading your query performance.
Wildcard queries have better response time when token prefixes and substrings are recorded in the index.
By default, token prefixes and substrings are not recorded in the Oracle Text index. If your query application makes heavy use of wildcard queries, consider indexing token prefixes and substrings. To do so, use the wordlist preference type. The trade-off is a bigger index for improved wildcard searching.
See Also:BASIC_WORDLIST Example: Enabling Substring and Prefix Indexing in this chapter
For documents that have internal structure such as HTML and XML, you can define and index document sections. Indexing document sections enables you to narrow the scope of your queries to within pre-defined sections. For example, you can specify a query to find all documents that contain the term dog within a section you define as Headings.
Sections must be defined prior to indexing and specified with the section group preference.
Oracle Text provides section groups with system-defined section definitions for HTML and XML. You can also specify that the system automatically create sections from XML documents during indexing.
A stopword is a word that is not to be indexed. Usually stopwords are low information words in a given language such as this and that in English.
By default, Oracle Text provides a list of stopwords called a stoplist for indexing a given language. You can modify this list or create your own with the
CTX_DDL package. You specify the stoplist in the parameter string of
A stoptheme is a word that is prevented from being theme-indexed or prevented from contributing to a theme. You can add stopthemes with the
You can search document themes with the
ABOUT operator. You can retrieve document themes programatically with the
CTX_DOC PL/SQL package.
You can also create multi-language stoplists to hold language-specific stopwords. A multi-language stoplist is useful when you use the
MULTI_LEXER to index a table that contains documents in different languages, such as English, German, and Japanese.
At indexing time, the language column of each document is examined, and only the stopwords for that language are eliminated. At query time, the session language setting determines the active stopwords, like it determines the active lexer when using the multi-lexer.
There are factors that influence indexing performance including memory allocation, document format, degree of parallelism, and partitioned tables.
If your table contains LOB structured columns that are frequently accessed in queries but rarely updated, you can improve query performance by storing these columns out of line.
You can create four types of indexes with Oracle Text:
By default, the system expects your documents to be stored in a text column. Once this requirement is satisfied, you can create a text index using the
CREATE INDEX SQL command as an extensible index of type
CONTEXT, without explicitly specifying any preferences. The system automatically detects your language, the datatype of the text column, format of documents, and sets indexing preferences accordingly.
See Also:Default CONTEXT Index Example in this chapter
To create an Oracle Text index, do the following:
Optionally, determine your custom indexing preferences, section groups, or stoplists if not using defaults. The following table describes these indexing classes:
|Datastore||How are your documents stored?|
|Filter||How can the documents be converted to plaintext?|
|Lexer||What language is being indexed?|
|Wordlist||How should stem and fuzzy queries be expanded?|
|Storage||How should the index data be stored?|
|Stop List||What words or themes are not to be indexed?|
|Section Group||How are documents sections defined?|
You can optionally create your own custom index preferences to override the defaults. Use the preferences to specify index information such as where your files are stored and how to filter your documents. You create the preferences then set the attributes.
The following sections give examples for setting direct, multi-column, URL, and file datastores.
See Also:Oracle Text Reference for more information about data storage
The following example creates a table with a
CLOB column to store text data. It then populates two rows with text data and indexes the table using the system-defined preference
CTXSYS.DEFAULT_DATASTORE which uses the
DIRECT_DATASTORE preference type.
create table mytable(id number primary key, docs clob); insert into mytable values(111555,'this text will be indexed'); insert into mytable values(111556,'this is a default datastore example'); commit; create index myindex on mytable(docs) indextype is ctxsys.context parameters ('DATASTORE CTXSYS.DEFAULT_DATASTORE');
The following example creates a multi-column datastore preference called
my_multi on the three text columns to be concatenated and indexed:
begin ctx_ddl.create_preference('my_multi', 'MULTI_COLUMN_DATASTORE'); ctx_ddl.set_attribute('my_multi', 'columns', 'column1, column2, column3'); end;
This example creates a
URL_DATASTORE preference called my_url to which the http_proxy, no_proxy, and timeout attributes are set. The timeout attribute is set to 300 seconds. The defaults are used for the attributes that are not set.
begin ctx_ddl.create_preference('my_url','URL_DATASTORE'); ctx_ddl.set_attribute('my_url','HTTP_PROXY','www-proxy.us.oracle.com'); ctx_ddl.set_attribute('my_url','NO_PROXY','us.oracle.com'); ctx_ddl.set_attribute('my_url','Timeout','300'); end;
The following example creates a data storage preference using the
FILE_DATASTORE. This tells the system that the files to be indexed are stored in the operating system. The example uses
CTX_DDL.SET_ATTRIBUTE to set the PATH attribute of to the directory
begin ctx_ddl.create_preference('mypref', 'FILE_DATASTORE'); ctx_ddl.set_attribute('mypref', 'PATH', '/docs'); end;
If your document set is entirely HTML, Oracle recommends that you use the
NULL_FILTER in your filter preference, which does no filtering.
For example, to index an HTML document set, you can specify the system-defined preferences for
HTML_SECTION_GROUP as follows:
create index myindex on docs(htmlfile) indextype is ctxsys.context parameters('filter ctxsys.null_filter section group ctxsys.html_section_group');
Consider a filter procedure
CTXSYS.NORMALIZE that you define with the following signature:
PROCEDURE NORMALIZE(id IN ROWID, charset IN VARCHAR2, input IN CLOB, output IN OUT NOCOPY VARCHAR2);
To use this procedure as your filter, you set up your filter preference as follows:
ctx_ddl.create_preference('myfilt', 'procedure_filter'); ctx_ddl.set_attribute('myfilt', 'procedure', 'normalize'); ctx_ddl.set_attribute('myfilt', 'input_type', 'clob'); ctx_ddl.set_attribute('myfilt', 'output_type', 'varchar2'); ctx_ddl.set_attribute('myfilt', 'rowid_parameter', 'TRUE'); ctx_ddl.set_attribute('myfilt', 'charset_parameter', 'TRUE');
Printjoin characters are non-alphanumeric characters that are to be included in index tokens, so that words such as web-site are indexed as web-site.
begin ctx_ddl.create_preference('mylex', 'BASIC_LEXER'); ctx_ddl.set_attribute('mylex', 'printjoins', '_-'); end;
To create the index with printjoins characters set as previously shown, issue the following statement:
create index myindex on mytable ( docs ) indextype is ctxsys.context parameters ( 'LEXER mylex' );
You use the
MULTI_LEXER preference type to index a column containing documents in different languages. For example, you can use this preference type when your text column stores documents in English, German, and French.
The first step is to create the multi-language table with a primary key, a text column, and a language column as follows:
create table globaldoc ( doc_id number primary key, lang varchar2(3), text clob );
Assume that the table holds mostly English documents, with some German and Japanese documents. To handle the three languages, you must create three sub-lexers, one for English, one for German, and one for Japanese:
ctx_ddl.create_preference('english_lexer','basic_lexer'); ctx_ddl.set_attribute('english_lexer','index_themes','yes'); ctx_ddl.set_attribute('english_lexer','theme_language','english'); ctx_ddl.create_preference('german_lexer','basic_lexer'); ctx_ddl.set_attribute('german_lexer','composite','german'); ctx_ddl.set_attribute('german_lexer','mixed_case','yes'); ctx_ddl.set_attribute('german_lexer','alternate_spelling','german'); ctx_ddl.create_preference('japanese_lexer','japanese_vgram_lexer');
Create the multi-lexer preference:
Since the stored documents are mostly English, make the English lexer the default using
Now add the German and Japanese lexers in their respective languages with
CTX_DDL.ADD_SUB_LEXER procedure. Also assume that the language column is expressed in the standard ISO 639-2 language codes, so add those as alternate values.
Now create the index
globalx, specifying the multi-lexer preference and the language column in the parameter clause as follows:
create index globalx on globaldoc(text) indextype is ctxsys.context parameters ('lexer global_lexer language column lang');
The following example sets the wordlist preference for prefix and substring indexing. Having a prefix and sub-string component to your index improves performance for wildcard queries.
For prefix indexing, the example specifies that Oracle Text create token prefixes between three and four characters long:
ctx_ddl.create_preference('mywordlist', 'BASIC_WORDLIST'); ctx_ddl.set_attribute('mywordlist','PREFIX_INDEX','TRUE'); ctx_ddl.set_attribute('mywordlist','PREFIX_MIN_LENGTH', '3'); ctx_ddl.set_attribute('mywordlist','PREFIX_MAX_LENGTH', '4'); ctx_ddl.set_attribute('mywordlist','SUBSTRING_INDEX', 'YES');
When documents have internal structure such as in HTML and XML, you can define document sections using embedded tags before you index. This enables you to query within the sections using the
WITHIN operator. You define sections as part of a section group.
The following code defines a section group called
htmgroup of type
HTML_SECTION_GROUP. It then creates a zone section in
heading identified by the <H1> tag:
begin ctx_ddl.create_section_group('htmgroup', 'HTML_SECTION_GROUP'); ctx_ddl.add_zone_section('htmgroup', 'heading', 'H1'); end;
A stopword is a word that is not to be indexed. A stopword is usually a low information word such as this or that in English.
The system supplies a list of stopwords called a stoplist for every language. By default during indexing, the system uses the Oracle Text default stoplist for your language.
You can edit the default stoplist
CTXSYS.DEFAULT_STOPLIST or create your own with the following PL/SQL procedures:
You specify your custom stoplists in the parameter clause of
You can also dynamically add stopwords after indexing with the
ALTER INDEX statement.
You can create multi-language stoplists to hold language-specific stopwords. A multi-language stoplist is useful when you use the
MULTI_LEXER to index a table that contains documents in different languages, such as English, German, and Japanese.
To create a multi-language stoplist, use the
CTX_DLL.CREATE_STOPLIST procedure and specify a stoplist type of
MULTI_STOPLIST. You add language specific stopwords with
You can also specify that numbers are not to be indexed. A class of alphanumeric characters such a numbers that is not to be indexed is a stopclass.
You record your own stopwords, stopthemes, stopclasses by creating a single stoplist, to which you add the stopwords, stopthemes, and stopclasses. You specify the stoplist in the paramstring for
You use the following procedures to manage stoplists, stopwords, stopthemes, and stopclasses:
See Also:Oracle Text Reference to learn more about using these commands
You create an Oracle Text index as an extensible index using the
CREATE INDEX SQL command.
You can create four types of indexes:
The context index type is well-suited for indexing large coherent documents such as MS Word, HTML or plain text. With a context index, you can also customize your index in a variety of ways.
The documents must be loaded in a text table.
CONTEXT index is not transactional. When you perform inserts, updates, or deletes on the base table, you must explicitly synchronize the index with
See Also:"Synchronizing the Index" in this chapter
The following command creates a default
context index called
myindex on the
text column in the
CREATE INDEX myindex ON docs(text) INDEXTYPE IS CTXSYS.CONTEXT;
When you use
CREATE INDEX without explicitly specifying parameters, the system does the following for all languages by default:
Detects the column type and uses filtering for the binary column types of BLOB and BFILE. Most document formats are supported for filtering. If your column is plain text, the system does not use filtering.
Note:For document filtering to work correctly in your system, you must ensure that your environment is set up correctly to support the
To learn more about configuring your environment to use the
You can always change the default indexing behavior by creating your own preferences and specifying these custom preferences in the parameter string of
To index an HTML document set located by URLs, you can specify the system-defined preference for the
NULL_FILTER in the
You can also specify your section group
htmgroup that uses
HTML_SECTION_GROUP and datastore
my_url that uses
URL_DATASTORE as follows:
begin ctx_ddl.create_preference('my_url','URL_DATASTORE'); ctx_ddl.set_attribute('my_url','HTTP_PROXY','www-proxy.us.oracle.com'); ctx_ddl.set_attribute('my_url','NO_PROXY','us.oracle.com'); ctx_ddl.set_attribute('my_url','Timeout','300'); end; begin ctx_ddl.create_section_group('htmgroup', 'HTML_SECTION_GROUP'); ctx_ddl.add_zone_section('htmgroup', 'heading', 'H1'); end;
You can then index your documents as follows:
create index myindex on docs(htmlfile) indextype is ctxsys.context parameters('datastore my_url filter ctxsys.null_filter section group htmgroup');
See Also:"Creating Preferences" in this chapter for more examples on creating a custom
CTXCAT indextype is well-suited for indexing small text fragments and related information. If created correctly, this type of index can give better structured query performance over a
CTXCAT index is transactional. When you perform DML (inserts, updates, and deletes) on the base table, Oracle Text automatically synchronizes the index. Unlike a
CONTEXT index, no
CTX_DDL.SYNC_INDEX is necessary.
Note:Applications that insert without invoking triggers such as SQL*Loader will not result in automatic index synchronization as described previously.
CTXCAT index is comprised of sub-indexes that you define as part of your index set. You create a sub-index on one or more columns to improve mixed query performance.
However, adding sub-indexes to the index set has its costs. The time Oracle Text takes to create a
CTXCAT index depends on its total size, and the total size of a
CTXCAT index is directly related to
Total text to be indexed
Number of sub-indexes in the index set
Number of columns in the base table that make up the sub-indexes
Having many component indexes in your index set also degrades DML performance since more indexes must be updated.
Because of the added index time and disk space costs for creating a
CTXCAT index, carefully consider the query performance benefit each component index gives your application before adding it to your index set.
An online auction site that must store item descriptions, prices and bid-close dates for ordered look-up provides a good example for creating a
Figure 3-3 shows a table called
AUCTION with the following schema:
create table auction(
item_id number, title varchar2(100), category_id number, price number, bid_close date);
To create your sub-indexes, create an index set to contain them:
In our example, this means all queries include a clause for the
title column which is the text column.
Assume that the structured clauses fall into the following categories:
|Structured Clauses||Sub-index Definition to Serve Query||Category|
|'price < 200'
'price = 150'
'order by price'
|'price = 100 order by bid_close'
'order by price, bid_close'
The structured query clause contains an expression for only the price column as follows:
SELECT FROM auction WHERE CATSEARCH(title, 'camera', 'price < 200')> 0; SELECT FROM auction WHERE CATSEARCH(title, 'camera', 'price = 150')> 0; SELECT FROM auction WHERE CATSEARCH(title, 'camera', 'order by price')> 0;
These queries can be served using sub-index B, but for efficiency you can also create a sub-index only on
price, which we call sub-index A:
ctx_ddl.add_index('auction_iset','price'); /* sub-index A */
The structured query clause includes an equivalence expression for
price ordered by
bid_close, and an expression for ordering by price and
bid_close in that order:
SELECT FROM auction WHERE CATSEARCH(title, 'camera','price = 100 order by bid_close')> 0; SELECT FROM auction WHERE CATSEARCH(title, 'camera','order by price, bid_close')> 0;
These queries can be served with a sub-index defined as follows:
ctx_ddl.add_index('auction_iset','price, bid_close'); /* sub-index B */
Like a combined b-tree index, the column order you specify with
CTX_DDL.ADD_INDEX affects the efficiency and viability of the index scan Oracle Text uses to serve specific queries. For example, if two structured columns
q have a b-tree index specified as
'p,q', Oracle Text cannot scan this index to sort '
order by q,p'.
The following example combines the previous examples and creates the index set preference with the two sub-indexes:
ctx_ddl.create_index_set('auction_iset'); ctx_ddl.add_index('auction_iset','price'); /* sub-index A */ ctx_ddl.add_index('auction_iset','price, bid_close'); /* sub-index B */
Figure 3-3 shows how the sub-indexes A and B are created from the auction table. Each sub-index is a b-tree index on the text column and the named structured columns. For example, sub-index A is an index on the
title column and the
You create the combined catalog index with
INDEX as follows:
CREATE INDEX auction_titlex ON AUCTION(title) INDEXTYPE IS CTXSYS.CTXCAT PARAMETERS ('index set auction_iset');
See Also:Oracle Text Reference to learn more about creating a
You use the
CTXRULE index to build a document classification application. In such an application, a stream of incoming documents is classified based on their content.
See Also:Chapter 6, "Classifying Documents in Oracle Text" for more information on document classification and the
Document routing is achieved by creating a
CTXRULE index on a table or queries. The queries define your categories. You can use the
MATCHES operator to classify single documents.
The first step is to create a table of queries that define your classifications. We create a table
myqueries to hold the category name and query text:
CREATE TABLE myqueries (
queryid NUMBER PRIMARY KEY, category VARCHAR2(30), query VARCHAR2(2000)
Populate the table with the classifications and the queries that define each. For example, consider a classification for the subjects US Politics, Music, and Soccer.:
INSERT INTO myqueries VALUES(1, 'US Politics', 'democrat or republican'); INSERT INTO myqueries VALUES(2, 'Music', 'ABOUT(music)'); INSERT INTO myqueries VALUES(3, 'Soccer', 'ABOUT(soccer)');
You can also generate a table of rules (queries) with the
CTX_CLS.TRAIN procedure, which takes as input a document training set.
See Also:Oracle Text Reference for more information on
CREATE INDEX to create the
CTXRULE index. You can specify lexer, storage, section group, and wordlist parameters if needed:
CREATE INDEX ON myqueries(query) INDEXTYPE IS CTXRULE PARAMETERS ('lexer lexer_pref storage storage_pref section group section_pref wordlist wordlist_pref');
Assume that incoming documents are stored in the table
CREATE TABLE news (
newsid NUMBER, author VARCHAR2(30), source VARCHAR2(30), article CLOB);
You can create a before insert trigger with
MATCHES to route each document to another table
news_route based on its classification:
BEGIN -- find matching queries FOR c1 IN (select category from myqueries where MATCHES(query, :new.article)>0) LOOP INSERT INTO news_route(newsid, category) VALUES (:new.newsid, c1.category); END LOOP; END;
This section describes maintaining your index in the event of an error or indexing failure.
Sometimes an indexing operation might fail or not complete successfully. When the system encounters an error indexing a row, it logs the error in an Oracle Text view.
For example to view the most recent errors on your indexes, you can issue:
SELECT err_timestamp, err_text FROM ctx_user_index_errors ORDER BY err_timestamp DESC;
To clear the view of errors, you can issue:
DELETE FROM ctx_user_index_errors;
This view is cleared automatically when you create a new index.
See Also:Oracle Text Reference to learn more about these views
You must drop an existing index before you can re-create it with
If you try to create an index with an invalid
PARAMETERS string, you still need to drop it before you can re-create it.
For example, to drop an index called
newsindex, issue the following SQL command:
DROP INDEX newsindex;
If Oracle Text cannot determine the state of the index, for example as a result of an indexing malfunction, you cannot drop the index as described previously. Instead use:
DROP INDEX newsindex FORCE;
See Also:Oracle Text Reference to learn more about this command
You can sometimes resume a failed index creation operation using the
INDEX command. You typically resume a failed index after you have investigated and corrected the index failure. Not all index failures can be resumed.
Index optimization commits at regular intervals. Therefore if an optimization operation fails, all optimization work up to the commit point has already been saved.
You can rebuild a valid index using
INDEX. You might rebuild an index when you want to index with a new preference.
Generally, there is no advantage in rebuilding an index over dropping it and re-creating it with
You might drop a custom index preference when you no longer need it for indexing.
You drop index preferences with the procedure
Dropping a preference does not affect the index created from the preference.
See Also:Oracle Text Reference to learn more about the syntax for the
DML operations to the base table refer to when documents are inserted, updated or deleted from the base table. This section describes how you can monitor, synchronize, and optimize the Oracle Text
CONTEXT index when DML operations occur.
When documents in the base table are inserted, updated, or deleted, their ROWIDs are held in a DML queue until you synchronize the index. You can view this queue with the
For example, to view pending DML on all your indexes, issue the following statement:
SELECT pnd_index_name, pnd_rowid, to_char(pnd_timestamp, 'dd-mon-yyyy hh24:mi:ss') timestamp FROM ctx_user_pending;
This statement gives output in the form:
PND_INDEX_NAME PND_ROWID TIMESTAMP ------------------------------ ------------------ -------------------- MYINDEX AAADXnAABAAAS3SAAC 06-oct-1999 15:56:50
See Also:Oracle Text Reference to learn more about this view
Synchronizing the index involves processing all pending updates, inserts, and deletes to the base table. You can do this in PL/SQL with the
The following example synchronizes the index with 2 megabytes of memory:
You can set
CTX_DDL.SYNC_INDEX to run automatically at regular intervals using the
SUBMIT procedure. Oracle Text includes a SQL script you can use to do this. The location of this script is:
To use this script, you must be the index owner and you must have execute privileges on the
CTX_DDL package. You must also set the
job_queue_processes parameter in your Oracle Database initialization file.
For example, to set the index synchronization to run every 360 minutes on myindex, you can issue the following in SQL*Plus:
SQL> @drjobdml myindex 360
See Also:Oracle Text Reference to learn more about the
Frequent index synchronization can fragment your
CONTEXT index. Index fragmentation can adversely affect query response time. You can optimize your
CONTEXT index to reduce fragmentation and index size and so improve query performance.
To understand index optimization, you must understand the structure of the index and what happens when it is synchronized.
CONTEXT index is an inverted index where each word contains the list of documents that contain that word. For example, after a single initial indexing operation, the word DOG might have an entry as follows:
DOG DOC1 DOC3 DOC5
When new documents are added to the base table, the index is synchronized by adding new rows. Thus if you add a new document (DOC 7) with the word dog to the base table and synchronize the index, you now have:
DOG DOC1 DOC3 DOC5 DOG DOC7
Subsequent DML will also create new rows:
DOG DOC1 DOC3 DOC5 DOG DOC7 DOG DOC9 DOG DOC11
Adding new documents and synchronizing the index causes index fragmentation. In particular, background DML which synchronizes the index frequently generally produces more fragmentation than synchronizing in batch.
Less frequent batch processing results in longer document lists, reducing the number of rows in the index and hence reducing fragmentation.
You can reduce index fragmentation by optimizing the index in either
FAST mode with
When documents are removed from the base table, Oracle Text marks the document as removed but does not immediately alter the index.
Because the old information takes up space and can cause extra overhead at query time, you must remove the old information from the index by optimizing it in
FULL mode. This is called garbage collection. Optimizing in
FULL mode for garbage collection is necessary when you have frequent updates or deletes to the base table.
In addition to optimizing the entire index, you can optimize single tokens. You can use token mode to optimize index tokens that are frequently searched, without spending time on optimizing tokens that are rarely referenced.
For example, you can specify that only the token DOG be optimized in the index, if you know that this token is updated and queried frequently.
An optimized token can improve query response time for the token.
To optimize an index in token mode, you can use
CTX_REPORT.INDEX_STATS procedure, you can create a statistical report on your index. The report includes information on optimal row fragmentation, list of most fragmented tokens, and the amount of garbage data in your index. Although this report might take long to run for large indexes, it can help you decide whether to optimize your index.
See Also:Oracle Text Reference to learn more about using this procedure
To optimize an index, Oracle recommends that you use
See Also:Oracle Text Reference for the