2 Teradata Gateway Features and Restrictions

2.6.1 Schema Considerations

The Oracle concept of a schema does not exist in Teradata. A schema name included in a query is recognized by Teradata as a database name. In the Oracle database, the schema of an object is identical to the owner of that object. However, when one retrieves or references an OWNER field in a Data Dictionary view such as ALL_TABLES, the OWNER field really is referencing a Teradata 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, INDEX_OWNER, or TABLE_OWNER column depending on the data dictionary table being referenced.

• ALL_USERS data dictionary table, each Teradata database name is returned in the USERNAME column.

• USER_* data dictionary tables, data for the default Teradata database is returned for the OWNER or TABLE_OWNER column depending on the data dictionary table being referenced. If a default Teradata database is not defined, the DBC Teradata system database is used.

2.6.2 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;

2.6.3 Data Types

Data type issues include the following:

• Binary Literal Notation

• 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")

2.6.4 Queries

Query issues include the following:

• Row Selection

• Empty Bind Variables

2.6.5 Locking

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

• 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.

2.7.1 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 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.

2.7.2 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.

2.7.3 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.

2.7.4 Pass-Through Feature

Oracle 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.

2.7.5 Bind Variables for Date Columns

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@TERA where time_column = :a;

The following error is issued:

Invalid operation on an ANSI Datetime or Interval value. 

2.7.6 SQL Syntax

This section lists restrictions on the following SQL syntax:

• WHERE CURRENT OF Clause

• CONNECT BY Clause

• ROWID

• EXPLAIN PLAN Statement

• Callback Support

2.7.7 SQL*Plus

You need to use double quotes to wrap around lowercase table names.

For example:

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

2.7.8 Database Links

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

2.7.9 Stored Procedures

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

2.7.10 CALLBACK links

Oracle Database Gateway for Teradata does not support CALLBACK links. Attempting to use a CALLBACK link with the gateway will return the following error message:

ORA-02025: All tables in the SQL statement must be at the remote database

2.8.1 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 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.

2.8.2 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.

2.8.3 Data Dictionary Views and PL/SQL

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