3 Indexing with Oracle Text

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, then the stream starts by reading the files from the operating system. You can also store your documents on the Internet or in Oracle Database. Wherever your files reside physically, you must always have a text table in 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 AUTO_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

You create a lexer preference using one of the Oracle Text lexer types to specify the language of the text to be indexed. 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 analyzes 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 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 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). If you mix HTML and XML documents in 1 index, you might not be able to configure your index to your needs; you cannot prevent stylesheet information from being added to the index.

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.

Table 3-2 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, because 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 AUTO_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.

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.

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 Character Set Detection

When the CHARSET column is set to AUTO, the AUTO_FILTER filter detects the character set of the document and converts it from the detected character set to the database character set, if there is a difference.

3.2.4.2 Language Detection

The lexer preference AUTO_LEXER performs language detection and any necessary transformations. At index time, the lexer automatically detects the language of the document and tokenizes and stems the document appropriately. At query time, the language of the query is inherited from the query template, or if no language is specified through the query template, then the language of the query is inherited from the session language.

3.2.4.3 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 for each row. You name the column in the parameter clause of the CREATE INDEX statement.

3.2.5.1 Language 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 Printjoin Characters

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 Skipjoin Characters

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 Automatic Language Detection with AUTO_LEXER

The AUTO_LEXER lexer type automatically detects the language of the document. At index creation, AUTO_LEXER automatically detects the language of the document and tokenizes and stems the document appropriately.

3.2.8.2 Indexing Themes

For English and French, you can index document theme information. A document theme 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.

3.2.8.3 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.

3.2.8.4 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.

3.2.8.5 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.

3.2.8.6 Korean, Japanese, and Chinese Indexing

Index these languages with specific lexers:

These lexers have their own sets of attributes to control indexing.

3.2.12.1 Automatic Language Detection and Stoplists

At index time, the lexer preference AUTO_LEXER automatically detects the language of the document and tokenizes and stems the document appropriately. At query time, the language of the query is inherited from the query template, or from the session language (if no language is specified through the query template).

3.2.12.2 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 index creation, 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 Printjoin 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, enter 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');

Because 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 statements

3.3.5.1 CONTEXT Index and DML

A CONTEXT index is not transactional. When a record is deleted, the index change is immediate. That is, your own session will no longer find the record from the moment you make the change, and other users will not find the record as soon as you commit. For inserts and updates, the new information will not be visible to text searches until an index synchronization has occurred. Therefore, when you perform inserts or updates on the base table, you must explicitly synchronize the index with CTX_DDL.SYNC_INDEX.

3.3.5.2 Default CONTEXT Index Example

The following statement 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 actions by default for all languages:

• 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 AUTO_FILTER filter.

  To learn more about configuring your environment to use the AUTO_FILTER 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.

3.3.5.3 Incrementally Creating an Index with ALTER INDEX and CREATE INDEX

The ALTER INDEX and CREATE INDEX statements support incrementally creating a global CONTEXT index.

• For creating a global index, CREATE INDEX supports the NOPOPULATE keyword of the REBUILD clause. Using the NOPOPULATE keyword in the REPLACE parameter, you can create indexes incrementally. This is valuable for creating text indexes in large installations that cannot afford to have the indexing process running continuously.

• For creating a local index partition, ALTER INDEX ... REBUILD partition ... parameters ('REPLACE ...') parameter string is modified to support the NOPOPULATE keyword.

• For creating a partition on a local index, CREATE INDEX ... LOCAL ... (partition ... parameters ('NOPOPULATE')) is supported. The partition-level POPULATE or NOPOPULATE keywords override any POPULATE or NOPOPULATE specified at the index level.

3.3.5.4 Creating a CONTEXT Index Incrementally with POPULATE_PENDING

For large installations that cannot afford to have the indexing process run continuously, you can use the CTX_DDL.POPULATE_PENDING procedure. This also provides finer control over creating the indexes. The preferred method is to create an empty index, place all the rowids into the pending queue, and build the index through CTX_DDL.SYNC_INDEX.

This procedure populates the pending queue with every rowid in the base table or table partition.

3.3.5.5 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'
);

3.3.5.6 CONTEXT Index Example: Query Processing with FILTER BY and ORDER BY

To enable more efficient query processing and better response time for mixed queries, you can use FILTER BY and ORDER BY clauses as shown in the following example.

CREATE INDEX myindex on docs(text) INDEXTYPE is CTXSYS.CONTEXT
FILTER BY category, publisher, pub_date
ORDER BY pub_date desc;

In this example, by specifying the clause FILTER BY category, publisher, pub_date at query time, Oracle Text will also consider pushing any relational predicate on any of these columns into the Text index row source for more efficient query processing.

Also, when the query has matching ORDER BY criteria, by specifying ORDER BY pub_date desc, Oracle Text will determine whether to push the SORT into the Text index row source for better response time.

3.3.6.1 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.

3.3.6.2 About CTXCAT Sub-Indexes and Their Costs

A CTXCAT index comprises 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 the following factors:

• 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, because 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 that each component index gives your application before adding it to your index set.

3.3.6.3 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 "Figure 3-3 Auction Table Schema and CTXCAT Index"

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 enter. 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;

3.3.6.4 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')
;

3.3.7.1 Step One: 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)');

3.3.7.2 Step Two: 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');

3.3.7.3 Step Three: Classify a Document

With a CTXRULE index created on a 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;

3.4.4.1 Re-creating a Global Index

Oracle Text provides RECREATE_INDEX_ONLINE to re-create a CONTEXT index with new preferences, while preserving the base table DML and query capability during the re-create process. You can use RECREATE_INDEX_ONLINE in a one step procedure to re-create a CONTEXT index online for global indexes. Because the new index is created alongside the existing index, this operation requires storage roughly equal to the size of the existing index. Also, because the RECREATE_INDEX_ONLINE operation is performed online, you may issue DML on the base table during the operation. All DML that occurs during recreation is logged into an online pending queue.

• After the re-create index operation is complete, any new information resulting from DML during the recreation process may not be immediately reflected. As with creating an index online, the index should be synchronized after the re-create index operation is complete in order to bring it fully up-to-date.

• Synchronizations issued against the index during the recreation are processed against the old, existing data. Synchronizations are blocked during this time when queries return errors.

• Optimize commands issued against the index during the recreation return immediately without error and without processing.

• During RECREATE_INDEX_ONLINE, the index can be queried normally most of the time. Queries return results based on the existing index and policy until after the final swap. Also, if you issue DML statements and synchronize them, then you will be able to see the new rows when you query on the existing index.

Re-creating a Local Partitioned Index

If the index is locally partitioned, you cannot re-create the index in one step. You must first create a shadow policy, and then run the RECREATE_INDEX_ONLINE procedure for every partition. You can specify SWAP or NOSWAP, which indicates whether re-creating the index for the partition will swap the index partition data and index partition metadata.

This procedure can also be used to update the metadata (for example the storage preference) of each partition when you specify NOPOPULATE in the parameter string. This is useful for incremental building of a shadow index through time-limited synchronization. If NOPOPULATE is specified, then NOSWAP is silently enforced.

• When all of the partitions use NOSWAP, the storage requirement is approximately equal to the size of the existing index. During the recreation of the index partition, since no swapping is performed, queries on the partition are processed normally. Queries spanning multiple partitions return consistent results across partitions until the swapping stage is reached.

• When the partitions are rebuilt with SWAP, the storage requirement for the operation is equal to the size of the existing index partition. Since index partition data and metadata are swapped after recreation, queries spanning multiple partitions will not return consistent results from partition to partition, but will always be correct with respect to each index partition.

• If SWAP is specified, then DML and synchronization on the partition are blocked during the swap process.

Re-creating a Global Index with Time Limit for Synch

You can control index recreation to set a time limit for SYNC_INDEX during non-business hours and incrementally re-create the index. You use the CREATE_SHADOW_INDEX procedure with POPULATE_PENDING and maxtime.

Re-creating a Global Index with Scheduled Swap

With CTX_DDL.EXCHANGE_SHADOW_INDEX you can perform index re-creation during non-business hours when query failures and DML blocking can be tolerated.

Re-creating a Local Index with All-at-Once Swap

You can re-create a local partitioned index online to create or change preferences. The swapping of the index and partition metadata occurs at the end of the process. Queries spanning multiple partitions return consistent results across partitions when re-create is in process, except at the end when EXCHANGE_SHADOW_INDEX is running.

Scheduling Local Index Re-creation with All-at-Once Swap

With RECREATE_INDEX_ONLINE of the CTX.DDL package, you can incrementally re-create a local partitioned index, where partitions are all swapped at the end.

Re-creating a Local Index with Per-Partition Swap

Instead of swapping all partitions at once, you can re-create the index online with new preferences, with each partition being swapped as it is completed. Queries across all partitions may return inconsistent results during this process. This procedure uses CREATE_SHADOW_INDEX with RECREATE_INDEX_ONLINE.

3.5.2.1 Example

The following example synchronizes the index with 2 megabytes of memory:

begin

ctx_ddl.sync_index('myindex', '2M');

end;

3.5.2.2 Maxtime Parameter for SYNC_INDEX

The sync_index procedure includes a maxtime parameter that, like optimize_index, indicates a suggested time limit in minutes for the operation. The sync_index will process as many documents in the queue as possible within the given time limit.

• NULL maxtime is equivalent to CTX_DDL.MAXTIME_UNLIMITED.

• The time limit is approximate. The actual time taken may be somewhat less than, or greater than what you specify.

• There is no change to the ALTER INDEX... sync command, which is deprecated.

• The maxtime parameter is ignored when sync_index is invoked without an index name

• The maxtime parameter cannot be communicated for automatic synchronizations (for example, sync on commit or sync every).

3.5.2.3 Locking Parameter for SYNC_INDEX

The locking parameter of sync_index enables you to configure how the synchronization deals with the scenario where another sync is already running on the index.

• The locking parameter is ignored when sync_index is invoked without an index name.

• The locking parameter cannot be communicated for automatic syncs (i.e. sync on commit/sync every)

• When locking mode is LOCK_WAIT, in the event of not being able to get a lock, it will wait forever and ignore the maxtime setting.

The options are as follows:

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 (for example, 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 as follows:

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 mode.

Less frequent batch processing results in longer document lists, reducing the number of rows in the index and thus 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, 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, a list of most fragmented tokens, and the amount of garbage data in your index. Although this report might take a long time 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.