| Oracle® Database Advanced Replication 10g Release 1 (10.1) Part Number B10732-01 |
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| Oracle® Database Advanced Replication 10g Release 1 (10.1) Part Number B10732-01 |
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View PDF |
This appendix contains information about replication support for column length semantics and Unicode.
This appendix contains these topics:
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 or higher 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));
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Note: A database must have a compatibility level of 9.0.1 or higher to have |
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:
The following sections describe column length semantics support for each table creation method.
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 or higher. If you specify BYTE length semantics, then these sites can be running a previous Oracle release.
This support is summarized in Table B-1.
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 or higher. 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.
When you create a materialized view, Oracle determines the length semantics of the columns in the materialized view in the following way:
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));
a column:
CREATE TABLE char_col (a VARCHAR2(10));
Materialized view creation fails if an Oracle9i or higher 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.
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 Oracle Database SQL Reference for more information about the |
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:
If, however, the length semantics do not match and the master is Oracle9i or higher, 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.
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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. |
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_REPOBJECTCREATE_MASTER_REPOBJECTEXECUTE_DDL
| See Also:
"Column Length Semantics for Replication Sites and Table Columns" for more information about specifying |
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 or higher, 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.
In an Advanced Replication environment involving Oracle Database 10g sites and sites running Oracle releases prior to release 9.1 that use NCHAR and NVARCHAR2 datatypes, an Oracle patch must be installed at the sites running Oracle releases prior to release 9.1. Contact Oracle Support Services to obtain the appropriate NLS patch as recommended in ALERT 140014.1, "Oracle8/8i to Oracle9i using New AL16UTF16 National Character Set" available on the Oracle MetaLink Web site.
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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. |
NCLOB datatype columns are always fixed width. Therefore, replication of NCLOB datatype columns is supported without restrictions.
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