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Oracle® Transparent Gateway for Informix Administrator's Guide
10g Release 1 (10.1) for HP-UX

Part Number B10528-01
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3
Informix Gateway Features and Restrictions

After the gateway is installed and configured, you can use the gateway to access Informix data, pass Informix commands from applications to the Informix database, perform distributed queries, and copy data.

This chapter contains the following sections:

Using the Pass-Through Feature

The gateway can pass Informix commands or statements from the application to the Informix database using the DBMS_HS_PASSTHROUGH package.

Use the DBMS_HS_PASSTHROUGH package in a PL/SQL block to specify the statement to be passed to the Informix database, as follows:

DECLARE
    num_rows INTEGER;
BEGIN
    num_rows := DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE@IFMX('command');
END;
/

Where command cannot be one of the following:

The DBMS_HS_PASSTHROUGH package supports passing bind values and executing SELECT statements.

See Also:

PL/SQL Packages and Types Reference for more information about the DBMS_HS_PASSTHROUGH package and the different functions supported by this package.

Database Compatibility Issues for Informix

Informix and Oracle databases function differently in some areas, causing compatibility problems. The following compatibility issues are described in this section:

ANSI SQL Standard

The American National Standards Institute (ANSI) has established a set of industry standards for SQL. The gateway supports only Informix databases that comply with the ANSI standard. For more information about how to create or start up an ANSI-compliant Informix database, refer to your Informix documentation.

Naming Rules

Naming rule issues include the following:

Rules for Naming Objects

Oracle and Informix use different database object naming rules. For example, the maximum number of characters allowed for each object name can be different. Also, the use of single and double quotation marks, case sensitivity, and the use of alphanumeric characters can all be different.

See Also:

Oracle Database Reference and Informix documentation.

Object Names

Names of Informix database objects are limited to a maximum of 18 characters. An object name can be composed of these characters:

Case Sensitivity

Informix handles letter case differently from Oracle. Informix uses these rules:

The Oracle database server defaults to uppercase unless you surround identifiers with double quote characters. For example, to refer to the Informix table called emp, enter the name with double quote characters, as follows:

SQL> SELECT * FROM "emp"@IFMX;

However, to refer to the Informix table called emp owned by Scott from an Oracle application, enter the following:

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

If the Informix table called emp is owned by SCOTT, a table owner name in uppercase letters, you can enter the owner name without double quote characters, as follows:

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

or

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

Oracle Corporation recommends that you surround all Informix object names with double quote characters and use the exact letter case for the object names as they appear in the Informix data dictionary. This convention is not required when referring to the supported Oracle data dictionary tables or views listed in Appendix C, "Data Dictionary".

If existing applications cannot be changed according to these conventions, create views in Oracle to associate Informix names to the correct letter case. For example, to refer to the Informix table emp from an existing Oracle application by using only uppercase names, define the following view:

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

With this view, the application can issue statements such as the following:

SQL> SELECT EMPNO, ENAME FROM EMP;

Using views is a workaround solution that duplicates data dictionary information originating in the Informix data dictionary. You must be prepared to update the Oracle view definitions whenever the data definitions for the corresponding tables are changed in the Informix database.


Note:

The DELIMIDENT Informix parameter must be set in the Informix client in order for Informix to accept double quotes around object names.


Data Types

Data type issues include the following:

Binary, Byte and Text Literal Notation

Oracle SQL uses hexadecimal digits surrounded by single quotes to express literal values being compared or inserted into columns defined as data type RAW.

This notation is not converted to syntax compatible with Informix BINARY, BYTE and TEXT data types (a 0x followed by hexadecimal digits, surrounded by single quotes).

For example, the following statement is not supported:

SQL> INSERT INTO BYTE_TAB@IFMX VALUES ('Oxff');

where BYTE_TAB contains a column of data type BINARY, BYTE or TEXT. Use bind variables when inserting into or updating BINARY, BYTE or TEXT data types.

Data Type Conversion

Informix does not support implicit date conversions. Such conversions must be explicit.

For example, the gateway issues an error for the following SELECT statement:

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

To avoid problems with implicit conversions, add explicit conversions, as in the following:

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

See Also:

Appendix A, "Data Type Conversion" for more information about restrictions on data types.

Queries

Query issues include the following:

Row Selection

Informix evaluates a query condition for all selected rows before returning any of the rows. If there is an error in the evaluation process for one or more rows, no rows are returned even though the remaining rows satisfy the condition.

Oracle evaluates the query condition row-by-row and returns a row when the evaluation is successful. Rows are returned until a row fails the evaluation.

Empty Strings

The gateway passes the empty string to the Informix database without any conversion.

The gateway returns an empty string to the Oracle database server as a NULL value. This applies only to columns defined with a VARCHAR data type.

Empty Bind Variables

The gateway passes empty bind variables to the Informix database as a NULL value. This applies only to columns defined with a VARCHAR data type.

Locking

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

Known Restrictions

If you encounter incompatibility problems not listed in this section or in "Known Problems", please contact Oracle Support Services. The following section describes the known restrictions and includes suggestions for dealing with them when possible:

Transactional Integrity

The gateway cannot guarantee transactional integrity in the following cases:

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 in this transaction mode it is always the commit point site when the Informix database is updated by the transaction.

Informix version 7.23 has a bug which prevents configuring the gateway as TWO_PHASE_COMMIT.

See Also:

Appendix D, "Heterogeneous Services Initialization Parameters" and the Oracle Database Heterogeneous Connectivity Administrator's Guide for more information about customizing the initialization parameter file.

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.

Pass-Through Feature

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

Informix SMALLFLOAT and REAL Data Types

Informix SMALLFLOAT and REAL data types have a precision of 6.

Informix NCHAR and NVARCHAR Data Types

The gateway cannot select a column defined with an Informix NCHAR or NVARCHAR data type.

SQL Syntax

This section lists restrictions on the following SQL syntax:

WHERE CURRENT OF Clause

UPDATE and DELETE statements with the WHERE CURRENT OF clause are not supported by the gateway because they rely on the Oracle ROWID implementation. To update or delete a specific row through the gateway, a condition style WHERE clause must be used.

CONNECT BY Clause

The gateway does not support the CONNECT BY clause in a SELECT statement.

Use of NULL Keyword in SELECT Statement

The NULL keyword cannot be used in the select list of a SELECT statement because that syntax is not ANSI SQL.

For example, the following statement cannot be used:

SQL> SELECT NULL FROM ...

Subqueries in UPDATE Statement

Subqueries in the SET clause of an UPDATE statement are not supported.

Whenever the use of a condition is allowed in an UPDATE statement, the gateway allows the condition to contain a subquery. Each subquery, however, must reference an Informix table. For example, using the table GTW_DEPT, the following statement results in a 10% salary increase for all employees working in the RESEARCH department:

SQL> UPDATE "gtw_emp"@IFMX SET "sal"="sal" * 1.1
2 WHERE "deptno"=(SELECT "deptno" FROM "gtw_dept"@IFMX
3 WHERE "dname"='RESEARCH');

If "gtw_dept"@IFMX is replaced by "dept" in the subquery, where dept is the same table but located in the Oracle database, the following error results after the statement is issued:

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

Subqueries in INSERT Statement

Subqueries of INSERT statements cannot use multiple aliases for the same table. For example, the following statement is not supported:

SQL> INSERT INTO "emp_target"@IFMX
         SELECT a."empno" FROM "emp_source"@IFMX a,
            "emp_source"@IFMX b WHERE b."empno"=9999

Subqueries in DELETE, INSERT, and UPDATE Statements

SQL statements in subqueries of DELETE, INSERT, and UPDATE statements cannot refer to the same table as specified in the outer query. This is because of the locking mechanism in Informix.

ROWID

The Oracle ROWID implementation is not supported.

EXPLAIN PLAN Statement

The EXPLAIN PLAN statement is not supported.

SQL*Plus

In SQL*Plus, the gateway does not support using a SELECT statement to retrieve data from an Informix column defined as data type BYTE.

The gateway does not support the SQL*Plus COPY command for lowercase table names.

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.

Gateway Data Dictionary Views

Only the first 64 characters of the view definition are returned when querying ALL_VIEWS and USER_VIEWS in the gateway data dictionary.

Stored Procedures

Known Problems

This section describes known problems and includes suggestions for correcting them when possible. If you have any questions or concerns about the problems, contact Oracle Support Services. A current list of problems is available online. Contact your local Oracle Corporation office for information about accessing the list.

The following known problems are described in this section:

Encrypted Format Login

Oracle database server no longer supports the initialization parameter DBLINK_ENCRYPT_LOGIN. In databases up through 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.

Aggregate Function with CREATE TABLE or CREATE VIEW Statement

The Oracle database server does not send the gateway a SELECT statement containing an aggregate function that is part of a CREATE TABLE or CREATE VIEW statement. For example, it does not send the following statement:

CREATE TABLE sum_calls_table AS 
SELECT SUM(calls_abandoned), SUM(calls_completed),
SUM(calls_failed) FROM monthly_calls@IFMX;

Instead, Oracle interprets what the SQL statement requests and sends the gateway a statement or statements to retrieve the data required for the request. After the data is retrieved, Oracle performs the aggregate function originally requested and passes the results to the application.

A solution to this problem is to use a different series of SQL statements. For example, instead of using the CREATE TABLE statement in the preceding example, use the following statements:

DROP TABLE sum_calls_table;
CREATE TABLE sum_calls_table (x1sum NUMBER, x2sum NUMBER,
x3sum NUMBER);
DECLARE
x1 NUMBER;
x2 NUMBER;
x3 NUMBER;
BEGIN
SELECT SUM(calls_abandoned), SUM(calls_completed),
SUM(calls_failed) INTO x1, x2, x3 FROM monthly_calls@IFMX;
INSERT INTO sum_calls_table VALUES (x1, x2, x3);
END;
/

Informix BYTE and TEXT Data Types

The following restrictions apply when using BYTE and TEXT data types:

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

See Also:

Appendix B, "Supported SQL Syntax and Functions".

Schema Names and PL/SQL

If you do not prefix a Informix 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.

Data Dictionary Views and PL/SQL

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


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