6 Classifying Documents in Oracle Text

6.1.1 Classification Applications

Oracle Text enables you to build document classification applications. A document classification application performs some action based on document content. Actions include assigning category IDs to a document for future lookup or sending a document to a user. The result is a set or stream of categorized documents. Figure 6-1 illustrates how the classification process works.

Oracle Text enables you to create document classification applications in different ways. This chapter defines a typical classification scenario and shows how you can use Oracle Text to build a solution.

Figure 6-1 Overview of a Document Classification Application

Description of "Figure 6-1 Overview of a Document Classification Application"

6.3.1 Rule-based Classification Example

In this example, we gather news articles on different subjects and then classify them.

Once our rules are created, we can index them and then use the MATCHES statement to classify documents. The steps are as follows:

Step 1   Create schema

We create the tables to store the data. The news_table stores the documents to be classified. The news_categories table stores the categories and rules that define our categories. The news_id_cat table stores the document ids and their associated categories after classification.

create table news_table (
       tk number primary key not null,
       title varchar2(1000),
       text clob);

create table news_categories (
        queryid  number primary key not null,
        category varchar2(100),
        query    varchar2(2000));

create table news_id_cat (
        tk number, 
        category_id number);

Step 2   Load Documents with SQLLDR

In this step, we load the HTML news articles into the news_table using the SQLLDR program. The filenames and titles are read from loader.dat.

LOAD DATA
     INFILE 'loader.dat'
     INTO TABLE news_table
     REPLACE
     FIELDS TERMINATED BY ';'
     (tk         INTEGER EXTERNAL,
      title      CHAR,
      text_file  FILLER CHAR,
      text       LOBFILE(text_file) TERMINATED BY EOF)

Step 3   Create Categories

In this step, we define our categories and write the rules for each of our categories.

Defined Categories:
United States	Europe	Middle East
Asia	Africa	Conflicts
Finance	Technology	Consumer Electronics
Latin America	World Politics	U.S. Politics
Astronomy	Paleontology	Health
Natural Disasters	Law	Music News

A rule is a query that selects documents for the category. For example, the category 'Asia' has a rule of 'China or Pakistan or India or Japan'. We insert our rules in the news_categories table as follows:

insert into news_categories values
  (1,'United States','Washington or George Bush or Colin Powell');

insert into news_categories values
  (2,'Europe','England or Britain or Germany');

insert into news_categories values
  (3,'Middle East','Israel or Iran or Palestine');

insert into news_categories values(4,'Asia','China or Pakistan or India or Japan');

insert into news_categories values(5,'Africa','Egypt or Kenya or Nigeria');

insert into news_categories values
  (6,'Conflicts','war or soliders or military or troops');

insert into news_categories values(7,'Finance','profit or loss or wall street');
insert into news_categories values
  (8,'Technology','software or computer or Oracle 
   or Intel or IBM or Microsoft');

insert into news_categories values
  (9,'Consumer electronics','HDTV or electronics');

insert into news_categories values
  (10,'Latin America','Venezuela or Colombia 
   or Argentina or Brazil or Chile');

insert into news_categories values
  (11,'World Politics','Hugo Chavez or George Bush 
   or Tony Blair or Saddam Hussein or United Nations');

insert into news_categories values
  (12,'US Politics','George Bush or Democrats or Republicans 
   or civil rights or Senate or White House');

insert into news_categories values
  (13,'Astronomy','Jupiter or Earth or star or planet or Orion 
   or Venus or Mercury or Mars or Milky Way 
   or Telescope or astronomer 
   or NASA or astronaut');

insert into news_categories values
  (14,'Paleontology','fossils or scientist 
   or paleontologist or dinosaur or Nature');

insert into news_categories values
  (15,'Health','stem cells or embryo or health or medical
   or medicine or World Health Organization or AIDS or HIV 
   or virus or centers for disease control or vaccination');

insert into news_categories values
  (16,'Natural Disasters','earthquake or hurricane or tornado');

insert into news_categories values
  (17,'Law','abortion or Supreme Court or illegal 
   or legal or legislation');

insert into news_categories values
  (18,'Music News','piracy or anti-piracy 
   or Recording Industry Association of America 
   or copyright or copy-protection or CDs 
   or music or artist or song');

commit;

Step 4   Create the CTXRULE index

In this step, we create a CTXRULE index on our news_categories query column.

create index news_cat_idx on news_categories(query)
indextype is ctxsys.ctxrule;

Step 5   Classify Documents

To classify the documents, we use the CLASSIFIER.THIS PL/SQL procedure (a simple procedure designed for this example), which scrolls through the news_table, matches each document to a category, and writes the categorized results into the news_id_cat table.

create or replace package classifier asprocedure this;end;/

show errors

create or replace package body classifier as

 procedure this
 is
  v_document    clob;
  v_item        number;
  v_doc         number;
 begin

  for doc in (select tk, text from news_table)
     loop
        v_document := doc.text;
        v_item := 0;
        v_doc  := doc.tk;
        for c in (select queryid, category from news_categories
             where matches(query, v_document) > 0 )
          loop
            v_item := v_item + 1;
            insert into news_id_cat values (doc.tk,c.queryid);
          end loop;
   end loop;

 end this;

end;
/
show errors
exec classifier.this

6.3.2 CTXRULE Parameters and Limitations

The following considerations apply to indexing a CTXRULE index.

• If the SVM_CLASSIFIER classifier is used, then you may use the BASIC_LEXER, CHINESE_LEXER, JAPANESE_LEXER, or KOREAN_MORPH_LEXER lexers. If SVM_CLASSIFIER is not used, only the BASIC_LEXER lexer type may be used for indexing your query set. (See Oracle Text Reference for more on lexer and classifier preferences.)

• Filter, memory, datastore, and [no]populate parameters are not applicable to index type CTXRULE.

• The CREATE INDEX storage clause is supported for creating the index on the queries.

• Wordlists are supported for stemming operations on your query set.

• Queries for CTXRULE are similar to those of CONTAINS queries. Basic phrasing ("dog house") is supported, as are the following CONTAINS operators: ABOUT, AND, NEAR, NOT, OR, STEM, WITHIN, and THESAURUS. Section groups are supported for using the MATCHES operator to classify documents. Field sections are also supported; however, CTXRULE does not directly support field queries, so you must use a query rewrite on a CONTEXT query.

6.4.1 Decision Tree Supervised Classification

To use Decision Tree classification, you set the preference argument to CTX_CLS.TRAIN to RULE_CLASSIFIER.

This form of classification uses a decision tree algorithm for creating rules. Generally speaking, a decision tree is a method of deciding between two (or more, but usually two) choices. In document classification, the choices are "the document matches the training set" or "the document does not match the training set."

A decision tree has a set of attributes that can be tested. In this case, these include:

• words from the document

• stems of words from the document (as an example, the stem of running is run)

• themes from the document (if themes are supported for the language in use)

The learning algorithm in Oracle Text builds one or more decision trees for each category provided in the training set. These decision trees are then coded into queries suitable for use by a CTXRULE index. As a trivial example, if one category is provided with a training document that consists of "Japanese beetle" and another category with a document reading "Japanese currency," the algorithm may create decision trees based on the words "Japanese," "beetle," and "currency," and classify documents accordingly.

The decision trees include the concept of confidence. Each rule that is generated is allocated a percentage value that represents the accuracy of the rule, given the current training set. In trivial examples, this accuracy is almost always 100%, but this merely represents the limitations of the training set. Similarly, the rules generated from a trivial training set may seem to be less than what you might expect, but these are sufficient to distinguish the different categories given the current training set.

The advantage of the Decision Tree method is that it can generate rules that are easily inspected and modified by a human. Using Decision Tree classification makes sense when you want to the computer to generate the bulk of the rules, but you want to fine tune them afterward by editing the rule sets.

6.4.2 SVM-Based Supervised Classification

The second method we can use for training purposes is known as Support Vector Machine (SVM) classification. SVM is a type of machine learning algorithm derived from statistical learning theory. A property of SVM classification is the ability to learn from a very small sample set.

Using the SVM classifier is much the same as using the Decision Tree classifier, with the following differences.

• The preference used in the call to CTX_CLS.TRAIN should be of type SVM_CLASSIFIER instead of RULE_CLASSIFIER. (If you do not want to modify any attributes, you can use the predefined preference CTXSYS.SVM_CLASSIFIER.)

• The CONTEXT index on the table does not have to be populated; that is, you can use the NOPOPULATE keyword. The classifier uses it only to find the source of the text, by means of datastore and filter preferences, and to determine how to process the text, through lexer and sectioner preferences.

• The table for the generated rules must have (as a minimum) these columns:

  cat_id      number,
  type        number,
  rule        blob );
  

As you can see, the generated rule is written into a BLOB column. It is therefore opaque to the user, and unlike Decision Tree classification rules, it cannot be edited or modified. The trade-off here is that you often get considerably better accuracy with SVM than with Decision Tree classification.

With SVM classification, allocated memory has to be large enough to load the SVM model; otherwise, the application built on SVM will incur an out-of-memory error. Here is how to calculate the memory allocation:

Minimum memory request (in bytes) = number of unique categories x number of features 
                                    example: (value of MAX_FEATURES attributes) x 8

If necessary to meet the minimum memory requirements, either:

• increase SGA memory (if in shared server mode)

• increase PGA memory (if in dedicated server mode)

select cat_id, match_score(1) from restab 
       where matches(rule, '4 5 6',1)>50;

select cat_id, match_score(1) from restab 
       where matches(rule, 'f h j',1)>50;

drop table doc;
drop table testcategory;
drop table restab;
exec ctx_ddl.drop_preference('my_classifier');
exec ctx_ddl.drop_preference('my_filter');

6.5.1 Clustering Example

The following SQL example creates a small collection of documents in the collection table and creates a CONTEXT index. It then creates a document assignment and cluster description table, which are populated with a call to the CLUSTERING procedure. The output would then be viewed with a select statement:

set serverout on

/* collect document into a table */
create table collection (id number primary key, text varchar2(4000));
insert into collection values (1, 'Oracle Text can index any document or textual content.');
insert into collection values (2, 'Ultra Search uses a crawler to access documents.');
insert into collection values (3, 'XML is a tag-based markup language.');
insert into collection values (4, 'Oracle Database 11g XML DB treats XML 
as a native datatype in the database.');
insert into collection values (5, 'There are three Text index types to cover 
all text search needs.');
insert into collection values (6, 'Ultra Search also provides API 
for content management solutions.');

create index collectionx on collection(text) 
   indextype is ctxsys.context parameters('nopopulate');

/* prepare result tables, if you omit this step, procedure will create table automatically */
create table restab (       
       docid NUMBER,
       clusterid NUMBER,
       score NUMBER);

create table clusters (
       clusterid NUMBER,
       descript varchar2(4000),
       label varchar2(200),
       sze   number,
       quality_score number,
       parent number);

/* set the preference */
exec ctx_ddl.drop_preference('my_cluster');
exec ctx_ddl.create_preference('my_cluster','KMEAN_CLUSTERING');
exec ctx_ddl.set_attribute('my_cluster','CLUSTER_NUM','3');

/* do the clustering */
exec ctx_output.start_log('my_log');
exec ctx_cls.clustering('collectionx','id','restab','clusters','my_cluster');
exec ctx_output.end_log;