6 Supporting Multilingual Databases with Unicode

Supplementary Characters

The first version of Unicode was a 16-bit, fixed-width encoding that used two bytes to encode each character. This allowed 65,536 characters to be represented. However, more characters need to be supported, especially additional CJK ideographs that are important for the Chinese, Japanese, and Korean markets.

Unicode 3.2 defines supplementary characters to meet this need. It uses two 16-bit code points (also known as supplementary characters) to represent a single character. This enables an additional 1,048,576 characters to be defined. The Unicode 3.2 standard defines 45,960 supplementary characters.

Adding supplementary characters increases the complexity of Unicode, but it is less complex than managing several different encodings in the same configuration.

Unicode Encodings

Unicode 3.2 encodes characters in different ways: UTF-8, UCS-2, and UTF-16. Conversion between different Unicode encodings is a simple bit-wise operation that is defined in the Unicode standard.

This section contains the following topics:

• UTF-8 Encoding

• UCS-2 Encoding

• UTF-16 Encoding

• Examples: UTF-16, UTF-8, and UCS-2 Encoding

Examples: UTF-16, UTF-8, and UCS-2 Encoding

Figure 6-1 shows some characters and their character codes in UTF-16, UTF-8, and UCS-2 encoding. The last character is a treble clef (a music symbol), a supplementary character that has been added to the Unicode 3.2 standard.

Figure 6-1 UTF-16, UTF-8, and UCS-2 Encoding Examples

Description of the illustration nlspg031.gif

Oracle's Support for Unicode

Oracle Corporation started supporting Unicode as a database character set in version 7. Table 6-1 summarizes the Unicode character sets supported by the Oracle database server.

Enabling Multilingual Support with Unicode Databases

The database character set specifies the encoding to be used in the SQL CHAR datatypes as well as the metadata such as table names, column names, and SQL statements. A Unicode database is a database with a UTF-8 character set as the database character set. There are three Oracle character sets that implement the UTF-8 encoding. The first two are designed for ASCII-based platforms while the third one should be used on EBCDIC platforms.

• AL32UTF8

  The AL32UTF8 character set supports the latest version of the Unicode standard. It encodes characters in one, two, or three bytes. Supplementary characters require four bytes. It is for ASCII-based platforms.

• UTF8

  The UTF8 character set encodes characters in one, two, or three bytes. It is for ASCII-based platforms.

  The UTF8 character set has supported Unicode 3.0 since Oracle8i release 8.1.7 and will continue to support Unicode 3.0 in future releases of the Oracle database server. Although specific supplementary characters were not assigned code points in Unicode until version 3.1, the code point range was allocated for supplementary characters in Unicode 3.0. If supplementary characters are inserted into a UTF8 database, then it does not corrupt the data in the database. The supplementary characters are treated as two separate, user-defined characters that occupy 6 bytes. Oracle Corporation recommends that you switch to AL32UTF8 for full support of supplementary characters in the database character set.

• UTFE

  The UTFE character set is for EBCDIC platforms. It is similar to UTF8 on ASCII platforms, but it encodes characters in one, two, three, and four bytes. Supplementary characters are converted as two 4-byte characters.

CREATE DATABASE sample 

CONTROLFILE REUSE 
LOGFILE 

GROUP 1 ('diskx:log1.log', 'disky:log1.log') SIZE 50K, 
GROUP 2 ('diskx:log2.log', 'disky:log2.log') SIZE 50K 

MAXLOGFILES 5 
MAXLOGHISTORY 100 
MAXDATAFILES 10 
MAXINSTANCES 2 
ARCHIVELOG
CHARACTER SET AL32UTF8
NATIONAL CHARACTER SET AL16UTF16 
DATAFILE 

'disk1:df1.dbf' AUTOEXTEND ON, 
'disk2:df2.dbf' AUTOEXTEND ON NEXT 10M MAXSIZE UNLIMITED 

DEFAULT TEMPORARY TABLESPACE temp_ts 
UNDO TABLESPACE undo_ts 
SET TIME_ZONE = '+02:00';

Enabling Multilingual Support with Unicode Datatypes

An alternative to storing Unicode data in the database is to use the SQL NCHAR datatypes (NCHAR, NVARCHAR, NCLOB). You can store Unicode characters into columns of these datatypes regardless of how the database character set has been defined. The NCHAR datatype is a Unicode datatype exclusively. In other words, it stores data encoded as Unicode.

In releases before Oracle9i, the NCHAR datatype supported fixed-width Asian character sets that were designed to provide higher performance. Examples of fixed-width character sets are JA16SJISFIXED and ZHT32EUCFIXED. No Unicode character set was supported as the national character set before Oracle9i.

You can create a table using the NVARCHAR2 and NCHAR datatypes. The column length specified for the NCHAR and NVARCHAR2 columns is always the number of characters instead of the number of bytes:

CREATE TABLE product_information 
    ( product_id          NUMBER(6) 
    , product_name        NVARCHAR2(100) 
    , product_description VARCHAR2(1000));

The encoding used in the SQL NCHAR datatypes is the national character set specified for the database. You can specify one of the following Oracle character sets as the national character set:

• AL16UTF16

  This is the default character set for SQL NCHAR datatypes. The character set encodes Unicode data in the UTF-16 encoding. It supports supplementary characters, which are stored as four bytes.

• UTF8

  When UTF8 is specified for SQL NCHAR datatypes, the data stored in the SQL datatypes is in UTF-8 encoding.

You can specify the national character set for the SQL NCHAR datatypes when you create a database using the CREATE DATABASE statement with the NATIONAL CHARACTER SET clause. The following statement creates a database with WE8ISO8859P1 as the database character set and AL16UTF16 as the national character set.

CREATE DATABASE sample 

CONTROLFILE REUSE 
LOGFILE 

GROUP 1 ('diskx:log1.log', 'disky:log1.log') SIZE 50K, 
GROUP 2 ('diskx:log2.log', 'disky:log2.log') SIZE 50K 

MAXLOGFILES 5 
MAXLOGHISTORY 100 
MAXDATAFILES 10 
MAXINSTANCES 2 
ARCHIVELOG
CHARACTER SET WE8ISO8859P1
NATIONAL CHARACTER SET AL16UTF16 
DATAFILE 

'disk1:df1.dbf' AUTOEXTEND ON, 
'disk2:df2.dbf' AUTOEXTEND ON NEXT 10M MAXSIZE UNLIMITED 

DEFAULT TEMPORARY TABLESPACE temp_ts 
UNDO TABLESPACE undo_ts 
SET TIME_ZONE = '+02:00';

How to Choose Between a Unicode Database and a Unicode Datatype Solution

To choose the right Unicode solution for your database, consider the following questions:

• Programming environment: What are the main programming languages used in your applications? How do they support Unicode?

• Ease of migration: How easily can your data and applications be migrated to take advantage of the Unicode solution?

• Performance: How much performance overhead are you willing to accept in order to use Unicode in the database?

• Type of data: Is your data mostly Asian or European? Do you need to store multilingual documents into LOB columns?

• Type of applications: What type of applications are you implementing: a packaged application or a customized end-user application?

This section describes some general guidelines for choosing a Unicode database or a Unicode datatype solution. The final decision largely depends on your exact environment and requirements. This section contains the following topics:

• When Should You Use a Unicode Database?

• When Should You Use Unicode Datatypes?

Comparing Unicode Character Sets for Database and Datatype Solutions

Oracle provides two solutions to store Unicode characters in the database: a Unicode database solution and a Unicode datatype solution. After you select the Unicode database solution, the Unicode datatype solution or a combination of both, determine the character set to be used in the Unicode database or the Unicode datatype.

Table 6-4 contains advantages and disadvantages of different character sets for a Unicode database solution. The Oracle character sets that can be Unicode database character sets are AL32UTF8, UTF8, and UTFE.

Table 6-5 contains advantages and disadvantages of different character sets for a Unicode datatype solution. The Oracle character sets that can be national character sets are AL16UTF16 and UTF8. The default is AL16UTF16.

Specifying Column Lengths for Multilingual Data

When you use NCHAR and NVARCHAR2 datatypes for storing multilingual data, the column size specified for a column is defined in number of characters. (The number of characters means the number of Unicode code units.) Table 6-6 shows the maximum size of the NCHAR and NVARCHAR2 datatypes for the AL16UTF16 and UTF8 national character sets.

When you use CHAR and VARCHAR2 datatypes for storing multilingual data, the maximum length specified for each column is, by default, in number of bytes. If the database needs to support Thai, Arabic, or multibyte languages such as Chinese and Japanese, then the maximum lengths of the CHAR, VARCHAR, and VARCHAR2 columns may need to be extended. This is because the number of bytes required to encode these languages in UTF8 or AL32UTF8 may be significantly larger than the number of bytes for encoding English and Western European languages. For example, one Thai character in the Thai character set requires 3 bytes in UTF8 or AL32UTF8. In addition, the maximum column lengths for CHAR, VARCHAR, and VARCHAR2 datatypes are 2000 bytes, 4000 bytes, and 4000 bytes respectively. If applications need to store more than 4000 bytes, then they should use the CLOB datatype.

Storing Data in Multiple Languages

The Unicode character set includes characters of most written languages around the world, but it does not contain information about the language to which a given character belongs. In other words, a character such as ä does not contain information about whether it is a French or German character. In order to provide information in the language a user desires, data stored in a Unicode database should accompany the language information to which the data belongs.

There are many ways for a database schema to relate data to a language. The following sections provide different approaches:

• Store Language Information with the Data

• Select Translated Data Using Fine-Grained Access Control

ALTER TABLE scott.product_information add (language_id VARCHAR2(50)):

CREATE OR REPLACE VIEW product AS 
   SELECT product_id, product_name 
   FROM   product_information 
   WHERE  language_id = sys_context('USERENV','LANG');

WHERE language_id = sys_context('userenv', 'LANG')

create function func1 ( sch varchar2 , obj varchar2 )
return varchar2(100);
begin
return 'language_id = sys_context(''userenv'', ''LANG'')';
end
/

DBMS_RLS.ADD_POLICY ('scott', 'product_information', 'lang_policy', 'scott', 'func1', 'select');

Storing Documents in Multiple Languages in LOB Datatypes

You can store documents in multiple languages in CLOB, NCLOB, or BLOB datatypes and set up Oracle Text to enable content search for the documents.

Data in CLOB columns is stored in a format that is compatible with UCS-2 when the database character set is multibyte, such as UTF8 or AL32UTF8. This means that the storage space required for an English document doubles when the data is converted. Storage for an Asian language document in a CLOB column requires less storage space than the same document in a LONG column using UTF8, typically around 30% less, depending on the contents of the document.

Documents in NCLOB format are also stored in a proprietary format that is compatible with UCS-2 regardless of the database character set or national character set. The storage space requirement is the same as for CLOB data. Document contents are converted to UTF-16 when they are inserted into a NCLOB column. If you want to store multilingual documents in a non-Unicode database, then choose NCLOB. However, content search on NCLOB is not yet supported.

Documents in BLOB format are stored as they are. No data conversion occurs during insertion and retrieval. However, SQL string manipulation functions (such as LENGTH or SUBSTR) and collation functions (such as NLS_SORT and ORDER BY) cannot be applied to the BLOB datatype.

Table 6-7 lists the advantages and disadvantages of the CLOB, NCLOB, and BLOB datatypes when storing documents:

Creating Indexes for Searching Multilingual Document Contents

Oracle Text enables you to build indexes for content search on multilingual documents stored in CLOB format and BLOB format. It uses a language-specific lexer to parse the CLOB or BLOB data and produces a list of searchable keywords.

Create a multilexer to search multilingual documents. The multilexer chooses a language-specific lexer for each row, based on a language column. This section describe the high level steps to create indexes for documents in multiple languages. It contains the following topics:

• Creating Multilexers

• Creating Indexes for Documents Stored in the CLOB Datatype

• Creating Indexes for Documents Stored in the BLOB Datatype

  
  

  See Also: Oracle Text Reference

ctx_ddl.create_preference('english_lexer', 'basic_lexer'); 
ctx_ddl.set_attribute('english_lexer','index_themes','yes'); 
ctx_ddl.create_preference('german_lexer', 'basic_lexer'); 
ctx_ddl.set_attribute('german_lexer','composite','german'); 
ctx_ddl.set_attribute('german_lexer','alternate_spelling','german'); 
ctx_ddl.set_attribute('german_lexer','mixed_case','yes'); 
ctx_ddl.create_preference('japanese_lexer', 'JAPANESE_VGRAM_LEXER'); 

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

ctx_ddl.add_sub_lexer('global_lexer', 'german', 'german_lexer'); 
ctx_ddl.add_sub_lexer('global_lexer', 'japanese', 'japanese_lexer'); 
ctx_ddl.add_sub_lexer('global_lexer', 'default','english_lexer'); 

CREATE TABLE globaldoc
  (doc_id    NUMBER       PRIMARY KEY, 
  language  VARCHAR2(30), 
  text      CLOB); 

CREATE INDEX globalx ON globaldoc(text) 
  indextype IS ctxsys.context 
  parameters ('lexer 
               global_lexer 
               language 
               column 
               language'); 

CREATE TABLE globaldoc ( 
   doc_id       NUMBER       PRIMARY KEY, 
   language     VARCHAR2(30), 
   characterset VARCHAR2(30), 
   format       VARCHAR2(10), 
   text         BLOB 
  ); 

CREATE INDEX globalx ON globaldoc(text) 
  indextype is ctxsys.context 
  parameters ('filter inso_filter 
                lexer global_lexer 
                language column language 
                format  column format 
                charset column characterset');