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:
The gateway can pass Informix commands or statements from the application to the Informix database using 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:
Informix tool commands
DBMS_HS_PASSTHROUGH package supports passing bind values and executing
Note:It is recommended that you
COMMITafter each DDL statement in the pass-through.
See Also:Oracle Database PL/SQL Packages and Types Reference and Chapter 3, Features of Oracle Database Gateways, of Oracle Database Heterogeneous Connectivity Administrator's Guide for more information about the
Informix and Oracle databases function differently in some areas, causing compatibility problems. The following compatibility issues are described in this section:
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 rule issues include the following:
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.
Names of Informix database objects are limited to a maximum of 18 characters. An object name can be composed of these characters:
Numbers 0 to 9
Lowercase letters a to z
Uppercase letters A to Z
Underscore character (_)
Informix handles letter case differently from Oracle. Informix uses these rules:
Table owner names default to uppercase letters, unless the name is surrounded by double quote characters
Column names, table names, view names, and so on, are always treated as lowercase letters
The Oracle database 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;
SQL> SELECT * FROM scott."emp"@IFMX;
Oracle 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.
Data type issues include the following:
This notation is not converted to syntax compatible with Informix
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');
BYTE_TAB contains a column of data type
TEXT. Use bind variables when inserting into or updating
TEXT data types.
Informix does not support implicit date conversions. Such conversions must be explicit.
For example, the gateway issues an error for the following
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.
Query issues include the following:
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.
Oracle processes an empty string in a SQL statement as a null value. Informix processes an empty string as an empty string.
Comparing to an empty string
The gateway passes literal empty strings to the Informix database without any conversion. If you intended an empty string to represent a null value, Informix does not process the statement that way; it uses the empty string.
You can avoid this problem by using
IS NULL in the SQL statement instead of the empty string syntax, as in the following example:
SELECT * from "emp"@IFMX where "ename" IS NULL;
Selecting an empty string
VARCHAR columns, the gateway returns an empty string to the Oracle database as
CHAR columns, the gateway returns the full size of the column with each character as empty space (' ').
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:
Read access might block write access
Write access might block read access
Statement-level read consistency is not guaranteed
See Also:Informix documentation for information about the Informix locking model.
If you encounter incompatibility problems not listed in this section or in "Known Problems", contact Oracle Support Services. The following section describes the known restrictions and includes suggestions for dealing with them when possible:
Note:If you have any questions or concerns about the restrictions, contact Oracle Support Services.
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.
Informix version 7.23 has a bug which prevents configuring the gateway as
See Also:Appendix D, "Initialization Parameters" and the Oracle Database Heterogeneous Connectivity Administrator's Guide for more information about customizing the initialization parameter file.
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
ROLLBACK statement outside the cursor loop.
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.
This section lists restrictions on the following SQL syntax:
See Also:Appendix B, "Supported SQL Syntax and Functions" for more information about restrictions on SQL syntax.
DELETE statements with the
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.
NULL keyword cannot be used in the select list of a
SELECT statement because that syntax is not
For example, the following statement cannot be used:
SQL> SELECT NULL FROM ...
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
SQL statements in subqueries of
UPDATE statements cannot refer to the same table as specified in the outer query. This is because of the locking mechanism in Informix.
SQL statements that require the gateway to callback to Oracle database would not be supported.
The following categories of SQL statements will result in a callback:
Any DML with a sub-select, which refers to a table in Oracle database. For example:
INSERT INTO emp@non_oracle SELECT * FROM oracle_emp;
UPDATE or "
SELECT... FOR UPDATE..." SQL statement containing SQL functions or statements that need to be executed at the originating Oracle database.
These SQL functions include
SYSDATE, and the SQL statements are in selects of data from the originating Oracle database. For example:
DELETE FROM emp@non_oracle WHERE hiredate > SYSDATE;
SELECT ename FROM tkhoemp@non_oracle WHERE hiredate IN (SELECT hiredate FROM tkhoemp) FOR UPDATE OF empno;
Any SQL statement that involves a table in Oracle database, and a
LOB column in a remote table. For example:
SELECT a.long1, b.empno FROM scott.table@non_oracle a, emp b WHERE a.id=b.empno;
SELECT a.long1, b.dummy FROM table_non@non_oracle a, dual b;
a.long1 is a
In SQL*Plus, the gateway does not support using a
SELECT statement to retrieve data from an Informix column defined as data type
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"@holink2;
The gateway is not multithreaded and cannot support shared database links. Each gateway session spawns a separate gateway process and connections cannot be shared.
Only the first 64 characters of the view definition are returned when querying
USER_VIEWS in the gateway data dictionary.
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 office for information about accessing the list.
The following known problems are described in this section:
Oracle database 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.
The following restrictions apply when using
TEXT data types:
An unsupported SQL function cannot be used in a SQL statement that accesses a column defined as Informix data type
You cannot use SQL*Plus to select data from a column defined as Informix data type
TEXT 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 Informix database.
TEXT data types must be
UPDATE to work.
A table including a
TEXT column must have a unique index defined on the table or the table must have a separate column that serves as a primary key.
TEXT data in a view cannot be accessed.
TEXT data cannot be read through pass-through queries.
Data less than 32,739 bytes can not be inserted into
TEXT columns using bind variables.
The gateway does not support the PL/SQL function
COLUMN_VALUE_LONG of the
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.