3 Teradata Gateway Features and Restrictions

3.2.1 Schema Considerations

The Oracle concept of a schema does not exist in Teradata. An owner included in a query is recognized by Teradata as a database name.

When querying data dictionary tables, the following results are returned:

• ALL_* data dictionary tables, data for every Teradata database is returned. The Teradata database name is returned in the owner column.

• ALL_USERS data dictionary table, the name of every Teradata database is returned.

• USER_* data dictionary tables, data for the default Teradata database is returned. If a default Teradata database is not defined, the DBC Teradata system database is used.

  Note:

  You can specify a default database in Teradata in the DSN. For details refer to Teradata documentation.

3.2.2 Naming Rules

Naming rule issues include the following:

• Rules for Naming Objects

• Case Sensitivity

SQL> SELECT * FROM "emp"@TERA;

SQL> SELECT * FROM "Scott"."emp"@TERA;

SQL> SELECT * FROM SCOTT."emp"@TERA;

SQL> SELECT * FROM scott."emp"@TERA;

SQL> CREATE VIEW EMP (EMPNO, ENAME, SAL, HIREDATE)      AS SELECT "empno", "ename", "sal", "hiredate"      FROM "emp"@TERA;

SQL> SELECT EMPNO, ENAME FROM EMP;

3.2.3 Data Types

Data type issues include the following:

• Binary Literal Notation

• Date Format Support

• Data Type Conversion

SQL> INSERT INTO BINARY_TAB@TERA VALUES ('ff'xb)

SELECT DATE_COL FROM TEST@TERA WHERE DATE_COL = "1-JAN-2001";

SELECT DATE_COL FROM TEST@TERA WHERE DATE_COL = TO_DATE("1-JAN-2001")

3.2.4 Queries

Query issues include the following:

• Row Selection

• Empty Bind Variables

3.2.5 Locking

The locking model for an Teradata database differs significantly from the Oracle model. The gateway depends on the underlying Teradata behavior, so Oracle applications that access Teradata through the gateway can be affected by the following possible scenarios:

• Read access might block write access

• Write access might block read access

• Statement-level read consistency is not guaranteed

  See Also:

  Teradata documentation for information about the Teradata locking model.

3.3.1 Transactional Integrity

The gateway cannot guarantee transactional integrity in the following cases:

• When a statement that is processed by the gateway causes an implicit commit in the target database

• When the target database is configured to work in autocommit mode

  Note:

  Oracle corporation strongly recommends the following:

  • If you know that executing a particular statement causes an implicit commit in the target database, then ensure that this statement is executed in its own transaction.

  • Do not configure the target database to work in autocommit mode.

3.3.2 Transaction Capability

The gateway does not support savepoints. If a distributed update transaction is under way involving the gateway and a user attempts to create a savepoint, the following error occurs:

ORA-02070: database dblink does not support savepoint in this context

By default, the gateway is configured as COMMIT_CONFIRM and it is always the commit point site when the Teradata database is updated by the transaction.

3.3.3 COMMIT or ROLLBACK in PL/SQL Cursor Loops Closes Open Cursors

Any COMMIT or ROLLBACK issued in a PL/SQL cursor loop closes all open cursors, which can result in the following error:

ORA-1002:  fetch out of sequence 

To prevent this error, move the COMMIT or ROLLBACK statement outside the cursor loop.

3.3.4 Pass-Through Feature

Oracle Corporation recommends that you place a DDL statement in its own transaction when executing such a statement with the pass-through feature. An explicit COMMIT must be issued after the DDL statement.

If the SQL statements being passed through the gateway result in an implicit commit at the Teradata database, the Oracle transaction manager is unaware of the commit and an Oracle ROLLBACK command cannot be used to roll back the transaction.

3.3.5 Bind Variables for Date Columns

Due to limitations in NCR's ODBC driver for Teradata, you cannot compare columns of data type TIME or TIMESTAMP to a bind variable.

The following SQL statement causes an error message:

SQL> select time_column from time_table@tg4tera where time_column = :a;

The following error is issued:

ORA-28500: connection from ORACLE to a non-Oracle system returned this message:
[Transparent gateway for TERA]DRV_Execute: [NCR][ODBC Teradata][Teradata RDBMS]
Invalid operation on an ANSI Datetime or Interval value. (SQL State: 22005;SQL Code: -5407)
ORA-02063: preceding 2 lines from TERA

3.3.6 SQL Syntax

This section lists restrictions on the following SQL syntax:

• WHERE CURRENT OF Clause

• CONNECT BY Clause

• ROWID

• EXPLAIN PLAN Statement

  See Also:

  Appendix B, "Supported SQL Syntax and Functions" for more information about restrictions on SQL syntax.

3.3.7 SQL*Plus

You need to use double quotes to wrap around lower case table names.

For example:

copy from tkhouser/tkhouser@inst1 insert loc_tkhodept using select * from "tkhodept"@holink2;

3.3.8 Database Links

The gateway is not multithreaded and cannot support shared database links. Each gateway session spawns a separate gateway process and connections cannot be shared.

3.3.9 Stored Procedures

The gateway does not support the procedure feature that allows the execution of stored procedures in a non-Oracle database.

3.4.1 Encrypted Format Login

Oracle database server no longer supports the initialization parameter DBLINK_ENCRYPT_LOGIN. Up to version 7.3, this parameter's default TRUE value prevented the password for the login user ID from being sent over the network (in the clear). Later versions automatically encrypt the password.

If this parameter is set to TRUE in the initialization parameter file used by the Oracle9i database server, you must change the setting to FALSE, the default setting, to allow Oracle9i to communicate with the gateway.

In the current release, Oracle Database 10g, Release 10.2, the DBLINK_ENCRYPT_LOGIN initialization parameter is obsolete, so you need not check it.

3.4.2 Teradata LONG VARCHAR Data Type

The following restrictions apply when using LONG VARCHAR data types:

• An unsupported SQL function cannot be used in a SQL statement that accesses a column defined as Teradata data type LONG VARCHAR.

• You cannot use SQL*Plus to select data from a column defined as Teradata data type LONG VARCHAR when the data is greater than 80 characters in length. Oracle Corporation recommends using Pro*C or Oracle Call Interface to access such data in a Teradata database.

• LONG VARCHAR data types must be NULLABLE for INSERT or UPDATE to work.

• A table including a LONG VARCHAR column must have a unique index defined on the table or the table must have a separate column that serves as a primary key.

• LONG VARCHAR data cannot be read through pass-through queries.

The gateway does not support the PL/SQL function COLUMN_VALUE_LONG of the DBMS_SQL package.

3.4.3 Schema Names and PL/SQL

If you do not prefix a Teradata database object with its schema name in a SQL statement within a PL/SQL block, the following error message occurs:

ORA-6550 PLS-201 Identifier table_name must be declared.

Change the SQL statement to include the schema name of the object.

3.4.4 Data Dictionary Views and PL/SQL

You cannot refer to data dictionary views in SQL statements that are inside a PL/SQL block.