B
Column Length Semantics and Unicode

This appendix contains information about replication support for column length semantics and Unicode. This appendix contains the following topics:

• Column Length Semantics for Replication Sites and Table Columns
• Multimaster Support for Column Length Semantics
• Materialized View Support for Column Length Semantics
• DDL Propagation and Column Length Semantics
• Replication Support for Unicode

  See Also: The following documents contain more information about length semantics and Unicode: Oracle9i Database Globalization Support Guide Oracle9i SQL Reference

Column Length Semantics for Replication Sites and Table Columns

Column length semantics determine whether the length of a column is specified in bytes or in characters. You use BYTE to specify that the length is in bytes, and you use CHAR to specify that the length is in characters. CHAR length semantics is also known as codepoint length semantics.

Because some character sets require more than one byte for each character, a specification of 10 BYTE for a column might actually store less than 10 characters for certain character sets, but a 10 CHAR specification ensures that the column can store 10 characters, regardless of the character set. Only Oracle9i databases can specify CHAR length semantics.

You set the length semantics for an Oracle database using the NLS_LENGTH_SEMANTICS initialization parameter, and all VARCHAR2 and CHAR columns use the setting specified for this initialization parameter as the default. If this initialization parameter is not set, then the default setting is BYTE.

An individual column can override the length semantics for the database. For example, if the length semantics for a site is CHAR, then you can still specify BYTE for the length semantics of an individual column using the CREATE TABLE or ALTER TABLE statement.

The following statement creates a table and specifies the column length in bytes:

CREATE TABLE byte_col (a VARCHAR2(10 BYTE));

The following statement creates a table and specifies the column length in characters:

CREATE TABLE char_col (a VARCHAR2(10 CHAR));

Note: A database must have a compatibility level of 9.0.1 or higher to have CHAR length semantics. The compatibility level is controlled by the COMPATIBLE initialization parameter.

Multimaster Support for Column Length Semantics

All master sites in a master group must have the same length semantics, and the individual columns of a master table must have the same length semantics at all master sites. When you have a table in a master group at a master definition site and you want to replicate that table to a new master site, you can create the table at the new site in one of the following ways:

• Specify that Advanced Replication generate the table at the new master site when adding the new master site to the master group.
• Manually precreate the table at the new master site before adding the master site to the master group.

The following sections describe column length semantics support for each table creation method.

Column Length Semantics Support for Tables Generated by Advanced Replication

When you specify that Advanced Replication generate the table at the new master site, and you are using CHAR length semantics, then both the master definition site and the new master site must be running Oracle9i. If you specify BYTE length semantics, then these sites can be running a previous Oracle release.

This support is summarized in Table B-1.

Table B-1 Column Length Semantics Support for Generated Tables

Master Definition Site Release	Master Definition Site Column Semantics	New Master Site Release	Resulting Column Semantics at New Master Site
9.0 or higher	CHAR	9.0 or higher	CHAR
9.0 or higher	CHAR	Prior to 9.0	Not supported
Any release	BYTE	Any release	BYTE

Column Length Semantics Support for Precreated Tables

When you precreate the table at the new master site, and you are using CHAR length semantics, then both the master definition site and the new master site must be running Oracle9i. If you specify BYTE length semantics, then these sites can be running a previous Oracle release.

Also, because you precreated the table manually, it is possible that you specified a different length semantics for a column in the new master table than was specified for the column in the table at the master definition site. If so, Oracle raises an error because a column in a master table must be using the same length semantics at each master site.

This support is summarized in Table B-2.

Table B-2 Column Length Semantics Support for Precreated Tables

Master Definition Site Release	Master Definition Site Column Semantics	New Master Site Release	New Master Site Column Semantics	Supported?
9.0 or higher	CHAR	9.0 or higher	CHAR	Yes
9.0 or higher	CHAR	9.0 or higher	BYTE	No
9.0 or higher	BYTE	9.0 or higher	CHAR	No
9.0 or higher (Multibyte character set)	CHAR	Prior to 9.0	BYTE	No
9.0 or higher (Single-byte character set)	CHAR	Prior to 9.0	BYTE	Yes
Prior to 9.0	BYTE	9.0 or higher (Multibyte character set)	CHAR	No
Prior to 9.0	BYTE	9.0 or higher (Single-byte character set)	CHAR	Yes
Any release	BYTE	Any release	BYTE	Yes

Materialized View Support for Column Length Semantics

When you create a materialized view, Oracle determines the length semantics of the columns in the materialized view in the following way:

• If the master column is explicitly specified as either BYTE or CHAR, then the column in the materialized view retains that specification. In the following example, CHAR length semantics is explicitly specified for the a column:

  CREATE TABLE char_col (a VARCHAR2(10 CHAR));
  
  

• If the master column is not explicitly specified, then the column in the materialized view uses the default length semantics of the materialized view site. In the following example, length semantics is not explicitly specified for the a column:

  CREATE TABLE char_col (a VARCHAR2(10));
  
  

Materialized view creation fails if an Oracle9i master has a column with an explicit CHAR specification and a materialized view site running a release prior to Oracle9i attempts to create a materialized view based on this master.

Materialized Views with Prebuilt Container Tables

If you prebuild a container table at a materialized view site before you create the materialized view, then the length semantics of the columns in the container table must match the length semantics of the columns in the master. If the length semantics do not match, then an Oracle returns an ORA-12060 error during materialized view creation. You use the ON PREBUILT TABLE clause of the CREATE MATERIALIZED VIEW statement to prebuild a table for a materialized view.

See Also: The Oracle9i SQL Reference for more information about the ON PREBUILT TABLE clause in the CREATE MATERIALIZED VIEW statement

Column Length Semantics Support for Updatable Materialized Views

The following operations are always supported if the length semantics of the columns of an updatable materialized view matches the length semantics of the columns of the materialized view's master:

• Refreshing the updatable materialized view
• Pushing DML changes made at the materialized view to the master

If, however, the length semantics do not match and the master is Oracle9i, then Oracle raises an error when you try to add the materialized view to a materialized view group. To be updatable, a materialized view must belong to a materialized view group. If you use the replication management API, then you run the CREATE_MVIEW_REPOBJECT procedure in the DBMS_REPCAT package to add the materialized view to a materialized view group.

Table B-3 summarizes the length semantics support for updatable materialized views.

Table B-3 Column Length Semantics Support for Updatable Materialized Views

Master Site Release	Master Site Column Semantics	Materialized View Site Release	Materialized View Site Column Semantics	Updatable Materialized View Supported?
9.0 or higher	CHAR	9.0 or higher	CHAR	Yes
9.0 or higher	CHAR	9.0 or higher	BYTE	No
9.0 or higher	BYTE	9.0 or higher	CHAR	No
9.0 or higher (Multibyte character set)	CHAR	Prior to 9.0	BYTE	No
9.0 or higher (Single-byte character set)	CHAR	Prior to 9.0	BYTE	Yes
Prior to 9.0	BYTE	9.0 or higher	CHAR	Yes
Any release	BYTE	Any release	BYTE	Yes

Note: The master site in Table B-3 can be either a master site in a multimaster replication environment or a master materialized view site.

DDL Propagation and Column Length Semantics

You can use the DBMS_REPCAT package to propagate a data definition language (DDL) statement that creates a new replicated table or adds columns to an existing replicated table. If you want any of the new columns created by these DDL statements to use CHAR column length semantics, then make sure you specify CHAR column length semantics explicitly. Otherwise, the column always has BYTE length semantics, even if the replication site itself has CHAR column length semantics set as the default.

The following procedures in the DBMS_REPCAT package enable you to propagate DDL statements:

• ALTER_MASTER_REPOBJECT
• CREATE_MASTER_REPOBJECT
• EXECUTE_DDL

  See Also: "Column Length Semantics for Replication Sites and Table Columns" for more information about specifying CHAR column length semantics explicitly

Replication Support for Unicode

Unicode is a universal encoded character set that enables you to store information from any language using a single character set. Unicode provides a unique code value for every character, regardless of the platform, program, or language. Unicode is supported in both multimaster and materialized view replication environments. In Oracle9i, all columns specified as NCHAR or NVARCHAR2 datatype are stored in Unicode format.

For both master sites and materialized view sites, replication is possible in an environment with different releases of Oracle using an NCHAR or NVARCHAR2 datatype. However, replication is not recommended when one of the replication sites is a release prior to Oracle9i and uses a variable width character set because, in this case, there is a possibility of data loss.

Table B-4 summarizes when replication is recommended.

Table B-4 Replication Support for Globalization Support Character Sets

Release of Local Database with NCHAR or NVARCHAR2 Columns	Release of Remote Database with NCHAR and NVARCHAR2 Columns	Replication Recommended?
9.0 or higher (Stored in Unicode format)	9.0 or higher (Stored in Unicode format)	Yes
Prior to 9.0 (Fixed or variable width national character set format)	Prior to 9.0 (Fixed or variable width national character set format)	Yes
9.0 or higher (Stored in Unicode format)	Prior to 9.0 (Variable width national character set format)	Not Recommended
9.0 or higher (Stored in Unicode format)	Prior to 9.0 (Fixed width national character set format)	Yes
Prior to 9.0 (Variable width national character set format)	9.0 or higher (Stored in Unicode format)	Not Recommended
Prior to 9.0 (Fixed width national character set format)	9.0 or higher (Stored in Unicode format)	Yes

Caution: Where Table B-4 specifies that replication is not supported, Oracle does not detect an error when you set up replication between the two sites, but data loss may occur later. If data loss occurs, then an error is raised.

Replication of NCLOB Datatype Columns

NCLOB datatype columns are always fixed width. Therefore, replication of NCLOB datatype columns is supported without restrictions.