3 Indexing

3.1.2.1 Merged Word and Theme Index

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.

3.1.3.1 Datastore Object

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.

3.1.3.2 Filter Object

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.

• Formatted documents (binary) are filtered to marked-up text. This happens when you specify the INSO_FILTER preference type or when the value of the format column is BINARY.

• Text is converted from a non-database character set to the database character set. This happens when you specify CHARSET_FILTER preference type.

3.1.3.3 Sectioner Object

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.

3.1.3.4 Lexer Object

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.

3.1.3.5 Indexing Engine

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.

3.1.4.1 Querying Partitioned Tables

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 ORDER BY SCORE clause, Oracle recommends that you query single partitions unless you include a range predicate that limits the query to a single partition.

3.2.1.1 Supported Column Types

With Oracle Text, you can create a CONTEXT index with columns of type VARCHAR2, CLOB, BLOB, CHAR, BFILE, XMLType, and URIType.

3.2.1.2 Storing Text in the Text Table

This section discusses how you can store text in directly in your table with the different indexes.

3.2.1.3 Storing File Path Names

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.

3.2.1.4 Storing URLs

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.

3.2.1.5 Storing Associated Document Information

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.

3.2.1.6 Format and Character Set Columns

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 (TEXT or 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 on a per-row basis.

When you create your index, you must specify the name of the format or character set column in the parameter clause of CREATE INDEX.

For all rows containing the keywords 'AUTO' or '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.

3.2.1.7 Supported Document Formats

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).

3.2.1.8 Summary of DATASTORE Types

When you index with CREATE INDEX, you specify the location using the datastore preference. Use the appropriate datastore according to your application.

The following table summarizes the different ways you can store your text with the datastore preference type.

Indexing time and document retrieval time will be increased for indexing URLs since the system must retrieve the document from the network.

3.2.2.1 No Filtering for HTML

If you are indexing HTML or plain text files, do not use the INSO_FILTER type. For best results, use the NULL_FILTER preference type.

3.2.2.2 Filtering Mixed-Format Columns

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 TEXT or 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.

3.2.2.3 Custom Filtering

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.

For external custom filtering, use the USER_FILTER filter preference type.

For internal filtering, use the PROCEDURE_FILTER filter type.

3.2.4.1 Mixed Character Set Columns

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 on a per-row basis. You name the column in the parameter clause of the CREATE INDEX statement.

3.2.5.1 Languages Features Outside BASIC_LEXER

With the 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.

3.2.5.2 Indexing Multi-language Columns

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.

3.2.6.1 Printjoins Character

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.

3.2.6.2 Skipjoins Character

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.

3.2.6.3 Other Characters

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.

3.2.8.1 Indexing Themes

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 ABOUT operator.

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.

3.2.8.2 Base-Letter Conversion for Characters with Diacritical Marks

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.

3.2.8.3 Alternate Spelling

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.

3.2.8.4 Composite Words

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 BASIC_LEXER preference.

3.2.8.5 Korean, Japanese, and Chinese Indexing

You index these languages with specific lexers:

The KOREAN_MORPH_LEXER has its own set of attributes to control indexing. Features include composite word indexing.

3.2.12.1 Multi-Language Stoplists

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.

3.3.2.1 Datastore Examples

The following sections give examples for setting direct, multi-column, URL, and file datastores.

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');

begin
ctx_ddl.create_preference('my_multi', 'MULTI_COLUMN_DATASTORE');
ctx_ddl.set_attribute('my_multi', 'columns', 'column1, column2, column3');
end;

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_preference('mypref', 'FILE_DATASTORE');
ctx_ddl.set_attribute('mypref', 'PATH', '/docs'); 
end;

3.3.2.2 NULL_FILTER Example: Indexing HTML Documents

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 NULL_FILTER and 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');

3.3.2.3 PROCEDURE_FILTER Example

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:

begin

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');

end;

3.3.2.4 BASIC_LEXER Example: Setting Printjoins Characters

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.

The following example sets printjoin characters to be the hyphen and underscore with the BASIC_LEXER:

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' ); 

3.3.2.5 MULTI_LEXER Example: Indexing a Multi-Language Table

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:

ctx_ddl.create_preference('global_lexer', 'multi_lexer');

Since the stored documents are mostly English, make the English lexer the default using CTX_DDL.ADD_SUB_LEXER:

ctx_ddl.add_sub_lexer('global_lexer','default','english_lexer');

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.

ctx_ddl.add_sub_lexer('global_lexer','german','german_lexer','ger');
ctx_ddl.add_sub_lexer('global_lexer','japanese','japanese_lexer','jpn');

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');

3.3.2.6 BASIC_WORDLIST Example: Enabling Substring and Prefix Indexing

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:

begin 

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');

end;

3.3.3.1 Example: Creating HTML Sections

The following code defines a section group called htmgroup of type HTML_SECTION_GROUP. It then creates a zone section in htmgroup called 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;

3.3.4.1 Multi-Language Stoplists

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 CTX_DDL.ADD_STOPWORD.

3.3.4.2 Stopthemes and Stopclasses

In addition to defining your own stopwords, you can define stopthemes, which are themes that are not to be indexed. This feature is available for English and French only.

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 CREATE INDEX.

3.3.4.3 PL/SQL Procedures for Managing Stoplists

You use the following procedures to manage stoplists, stopwords, stopthemes, and stopclasses:

• CTX_DDL.CREATE_STOPLIST

• CTX_DDL.ADD_STOPWORD

• CTX_DDL.ADD_STOPTHEME

• CTX_DDL.ADD_STOPCLASS

• CTX_DDL.REMOVE_STOPWORD

• CTX_DDL.REMOVE_STOPTHEME

• CTX_DDL.REMOVE_STOPCLASS

• CTX_DDL.DROP_STOPLIST

  
  

  See Also: Oracle Text Reference to learn more about using these commands.

  
  

3.3.6.1 CONTEXT Index and DML

A CONTEXT index is not transactional. When you perform inserts, updates, or deletes on the base table, you must explicitly synchronize the index with CTX_DDL.SYNC_INDEX.

3.3.6.2 Default CONTEXT Index Example

The following command creates a default context index called myindex on the text column in the docs table:

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:

• Assumes that the text to be indexed is stored directly in a text column. The text column can be of type CLOB, BLOB, BFILE, VARCHAR2, or CHAR.

• 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 Inso filter. To learn more about configuring your environment to use the Inso filter, see the Oracle Text Reference.

  
  

• Assumes the language of text to index is the language you specify in your database setup.

• Uses the default stoplist for the language you specify in your database setup. Stoplists identify the words that the system ignores during indexing.

• Enables fuzzy and stemming queries for your language, if this feature is available for your language.

You can always change the default indexing behavior by creating your own preferences and specifying these custom preferences in the parameter string of CREATE INDEX.

3.3.6.3 Custom CONTEXT Index Example: Indexing HTML Documents

To index an HTML document set located by URLs, you can specify the system-defined preference for the NULL_FILTER in the CREATE INDEX statement.

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');

3.3.7.1 CTXCAT Index and DML

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.

3.3.7.2 About CTXCAT Sub-Indexes and Their Costs

A 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.

3.3.7.3 Creating CTXCAT Sub-indexes

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 CTXCAT index.

Figure 3-3 Auction Table Schema and CTXCAT Index

Description of the illustration ccapp010.gif

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:

begin

ctx_ddl.create_index_set('auction_iset');

end;

Next, determine the structured queries your application is likely to issue. The CATSEARCH query operator takes a mandatory text clause and optional structured clause.

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:

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;

begin

ctx_ddl.add_index('auction_iset','price'); /* sub-index A */

end;

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;

begin

ctx_ddl.add_index('auction_iset','price, bid_close'); /* sub-index B */

end;

3.3.7.4 Creating CTXCAT Index

The following example combines the previous examples and creates the index set preference with the two sub-indexes:

begin

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 */

end;

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 bid_close column.

You create the combined catalog index with CREATE INDEX as follows:

CREATE INDEX auction_titlex ON AUCTION(title) INDEXTYPE IS CTXSYS.CTXCAT PARAMETERS ('index set auction_iset');

3.3.8.1 Create a Table of Queries

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)');

3.3.8.2 Create the CTXRULE Index

Use 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');

3.3.8.3 Classifying a Document

With a CTXRULE index created on query set, you can use the MATCHES operator to classify a document.

Assume that incoming documents are stored in the table news:

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;

3.4.3.1 Example: Resuming a Failed Index

The following command resumes the indexing operation on newsindex with 10 megabytes of memory:

ALTER INDEX newsindex REBUILD PARAMETERS('resume memory 10M');

3.4.4.1 Example: Rebuilding and Index

The following command rebuilds the index, replacing the lexer preference with my_lexer.

ALTER INDEX newsindex REBUILD PARAMETERS('replace lexer my_lexer');

3.4.5.1 Example

The following code drops the preference my_lexer.

begin
ctx_ddl.drop_preference('my_lexer');
end;

3.5.2.1 Setting Background DML

You can set CTX_DDL.SYNC_INDEX to run automatically at regular intervals using the DBMS_JOB.SUBMIT procedure. Oracle Text includes a SQL script you can use to do this. The location of this script is:

$ORACLE_HOME/ctx/sample/script/drjobdml.sql

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

3.5.3.1 CONTEXT Index Structure

The 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

3.5.3.2 Index Fragmentation

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 FULL or FAST mode with CTX_DDL.OPTIMIZE_INDEX.

3.5.3.3 Document Invalidation and Garbage Collection

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.

3.5.3.4 Single Token Optimization

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_DDL.OPTIMIZE_INDEX.

3.5.3.5 Viewing Index Fragmentation and Garbage Data

With the 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.

3.5.3.6 Examples: Optimizing the Index

To optimize an index, Oracle recommends that you use CTX_DDL.OPTIMIZE_INDEX.