13 Developing Applications

Fetch Reblocking

Oracle Database supports fetch reblocking with the HS_RPC_FETCH_REBLOCKING parameter.

When the value of this parameter is set to ON (the default), the array size for SELECT statements is determined by the HS_RPC_FETCH_SIZE value. The HS_RPC_FETCH_SIZE parameter defines the number of bytes sent with each buffer from the gateway to the Oracle Database 10g. The buffer may contain one or more qualified rows from the DRDA Server. This feature can provide significant performance enhancements, depending on your application design, installation type, and workload.

The array size between the client and the Oracle Database 10g is determined by the Oracle application. Refer to Chapter 11, "Configuring the Gateway", for more information.

Oracle Application and DRDA Server Stored Procedure Completion

If an Oracle Application attempts to invoke a stored procedure in a DB2/OS390 database. For an Oracle application to call a DB2 stored procedure, it is first necessary to create the DB2 stored procedure on the DB2 system by using the procedures documented in the IBM reference document for DB2 for OS/390 SQL.

After the stored procedure is defined in DB2, the gateway is able to access the data using a standard PL/SQL call. For example, an employee name, John Smythe, is passed to the DB2 stored procedure REVISE_SALARY. The DB2 stored procedure retrieves the salary value from the DB2 database in order to calculate a new yearly salary for John Smythe. The revised salary that is returned as result is used to update the EMP table of Oracle Database:

DECLARE
  INPUT VARCHAR2(15);
  RESULT NUMBER(8,2);
BEGIN
  INPUT := 'JOHN SMYTHE';
  REVISE_SALARY@DB2(INPUT, RESULT);
  UPDATE EMP SET SAL = RESULT WHERE ENAME = INPUT;
END;

When the gateway receives a call to run a stored procedure on the DRDA Server (for example DB2/OS390), it first does a lookup of the procedure name in the server catalog. The information that defines a stored procedure is stored in different forms on each DRDA Server. For example, DB2/OS390 V5.0 uses the table SYSIBM.SYSPROCEDURES, while DB2/OS390 V6.1 uses the table SYSIBM.SYSROUTINES and SYSIBM.SYSPARMS, and DB2/400 uses the table QSYS2.SYSPROCS and QSYS2.SYSPARMS. The gateway has a list of known catalogs to search, depending on the DRDA Server that is being accessed.

The search order of the catalogs is dependent on whether the catalogs support Location designators (such as LUNAME in SYSIBM.SYSPROCEDURES), and authorization or owner IDs (such as AUTHID in SYSIBM.SYSPROCEDURES or OWNER in SYSIBM.SYSROUTINES).

Some DRDA Servers allow blank or public authorization qualifiers. If the DRDA Server that is currently connected supports which form of qualification, then the gateway will apply those naming rules when searching for a procedure name in the catalog.

The matching rules will first search for a public definition, and then an owner qualified procedure name. For more detailed information, refer to the IBM reference document for DB2 for OS/390 SQL.

Procedural Feature Considerations with DB2

The following are special considerations for using the procedural feature with the gateway:

• DB2 stored procedures do not have the ability to coordinate, commit, and rollback activity on recoverable resources such as IMS or Customer Information Control System (CICS) transactions. Therefore, if the DB2 stored procedure calls a CICS or IMS transaction, then it is considered a separate unit of work and does not affect the completion of the stored procedure. This means that if you are running a DB2 stored procedure from an Oracle application, and if this procedure calls a CICS or IMS transaction, then the gateway cannot recover data from any activity that occurred within the CICS or IMS transaction.

  For example, the CICS transaction could roll back a unit of work, but this does not prevent the gateway from committing other DB2 work contained within the DB2 stored procedure.

  Likewise, if the DB2 stored procedure updated an irrecoverable resource such as a video surveillance and monitoring (VSAM) file, then the gateway would consider this activity separate from its own recoverable unit of work.

• PL/SQL records cannot be passed as parameters when invoking a DB2 stored procedure.

• The gateway supports the SIMPLE linkage convention of DB2 stored procedures.The SIMPLE linkage convention means that the parameters that are passed to and from the DB2 stored procedure cannot be null.

Compatible SQL Functions

Oracle Integrating Server automatically forwards compatible SQL functions to the DRDA database. Where SQL constructs with the same syntax and meaning are on both Oracle Database and the DRDA database. These SQL constructs are forwarded unmodified. All of the compatible functions are column functions. Functions that are not compatible are either translated to an equivalent DRDA SQL function or are compensated (post-processed) by Oracle Database after the data is returned from the DRDA database.

Translated SQL Functions

Translated functions have the same meaning but different names between the Oracle Integrating Server and the DRDA database. But all applications must use the Oracle function name. These SQL constructs that are supported with different syntax ( different function names) by the DRDA database, are automatically translated by the Oracle Database and then forwarded to the DRDA database. Oracle Integrating Server changes the function name before sending it to the DRDA database, in a manner that is transparent to your application.

Compensated SQL Functions

Some advanced SQL constructs that are supported by Oracle Database may not be supported in the same manner, if at all, by the DRDA database. Compensated functions are those SQL functions that are not recognized by the DRDA Server. If a SELECT statement containing one of these functions is passed from the Oracle Integrating Server to the gateway, then the gateway removes the function before passing the SQL statement to the DRDA Server. The gateway passes the selected DRDA database rows to Oracle Integrating Server. Oracle Integrating Server applies the function.

SELECT COS(X_COOR) FROM TABLE_X;

SELECT X_COOR FROM TABLE_X;

Native Semantic SQL Functions

Some SQL functions that are normally compensated may also be overridden, through the Native Semantics facility. If a SQL function has been enabled for Native Semantics, then the function may be passed on to the DRDA database for processing, instead of being compensated (post-processed). If a SQL function is enabled for Native Semantics and is therefore passed on the DRDA database for processing, then the SQL function is processed natively in the DRDA database. Refer to "Native Semantics" for more information.

DB2/OS390 SQL Compatibility

Table 13-1 describes how Oracle Database and the gateway handles SQL functions for a DB2/OS390

DB2/Universal Database SQL Compatibility

The ways that Oracle Database and gateway handle SQL functions for a DB2/UDB database are shown in the following table:

DB2/400 SQL Compatibility

The ways that Oracle Database and gateway handle SQL functions for a DB2/400 database are shown in the following table:

DB2/VM SQL Compatibility

The ways that Oracle Database and gateway handle SQL functions for a DB2/VM database are shown in the following table:

SQL Functions That Can Be Enabled

The following list contains SQL functions that are disabled (OFF) by default. They can be enabled (turned ON) as an option:

SQL Functions That Can Be Disabled

The following SQL functions are enabled (ON) by default. :

• GROUPBY

• HAVING

• ORDERBY

• WHERE

ORDERBY controls sort order, which may differ at various sort locations. For example, with ORDERBY ON, a DB2 sort would be based on Extended Binary Coded Decimal Interchange Code (EBCDIC) sorting order, whereas with ORDERBY OFF, an Oracle Database sort would be based on ASCII sorting order.

The other three functions, GROUPBY, HAVING, and WHERE, can take additional processing time. If you need to minimize the use of expensive resources, then you should choose the settings of these functions so that the processing is performed with cheaper resource. The above listed functions can also be disabled.

SQL Set Operators and Clauses

The WHERE and HAVING clauses are compatible for all versions of the DRDA Server. This means that these clauses are passed unchanged to the DRDA Server for processing. Whether clauses GROUP BY and ORDER BY are passed to the DRDA Server, or compensated by Oracle Database, is determined by the Native Semantics Parameters (see the previous section).

The set operators UNION and UNION ALL are compatible for all versions of the DRDA Server, meaning that they are passed unchanged to the DRDA Server for processing. The set operators INTERSECT and MINUS are compensated on all versions of the DRDA Server except DB2/UDB. For DB2/UDB, INTERSECT is compatible and MINUS is translated to EXCEPT.

Performing Character String Operations

The gateway performs all character string comparisons, concatenations, and sorts using the data type of the referenced columns, and determines the validity of character string values passed by applications using the gateway. The gateway automatically converts character strings from one data type to another and converts between character strings and dates when needed.

Frequently, DRDA databases are designed to hold non-character binary data in character columns. Applications executed on DRDA systems can generally store and retrieve data as though it contained character data. However, when an application accessing this data runs in an environment that uses a different character set, inaccurate data may be returned.

With the gateway running on the host, character data retrieved from a DB2/400, DB2/OS390, or DB2/VM host is translated from EBCDIC to ASCII. When character data is sent to DB2/400, DB2/OS390, or DB2/VM from the host, ASCII data is translated to EBCDIC. When the characters are binary data in a character column, this translation causes the application to receive incorrect information or errors. To resolve these errors, character columns on DB2/400, DB2/OS390, or DB2/VM that hold non-character data must be created with the FOR BIT DATA option. In the application, the character columns holding non-character data should be processed using the Oracle data types RAW and LONG RAW. The DESCRIBE information for a character column defined with FOR BIT DATA on the host always indicates RAW or LONG RAW.

Converting Character String Data types

The gateway binds character string data values from host variables as fixed-length character strings. The bind length is the length of the character string data value. The gateway performs this conversion on every bind.

The DRDA VARCHAR data type can be from 1 to 32740 bytes in length. This data type is converted to an Oracle VARCHAR2 data type if it is between 1 and 2000 characters in length. If it is between 2000 and 32740 characters in length, then it is converted to an Oracle LONG data type.

The DRDA VARCHAR data type can be no longer than 32740 bytes, which is much shorter than the maximum size for the Oracle LONG data type. If you define an Oracle LONG data type larger than 32740 bytes in length, then you receive an error message when it is mapped to the DRDA VARCHAR data type.

Performing Graphic String Operations

DB2 GRAPHIC data types store only double-byte string data. Sizes for DB2 GRAPHIC data types typically have maximum sizes that are half that of their Character counterparts. For example, the maximum size of a CHAR may be 255 characters, whereas the maximum size of a GRAPHIC may be 127 characters.

Oracle Database does not have a direct matching data type, and the gateway therefore converts between Oracle character data types to DB2 Graphic data types. Oracle Database character data types may contain single, mixed, or double-byte character data. The gateway converts the string data into appropriate double-byte-only format depending upon whether the target DB2 column is a Graphic type and whether Gateway Initialization parameters are set to perform this conversion. For more configuration information, refer to Appendix C, "DRDA-Specific Parameters" and Appendix D, "National Language Support".

Performing Date and Time Operations

The implementation of date and time data differs significantly in IBM DRDA databases and Oracle Database. Oracle Database has a single date data type, DATE, that can contain both calendar date and time of day information.

IBM DRDA databases support the following three distinct date and time data types:

DATE is the calendar date only.

TIME is the time of day only.

TIMESTAMP is a numerical value combining calendar date and time of day with microsecond resolution in the internal format of the IBM DRDA database.

SELECT * FROM EMP WHERE HIREDATE = '03-MAR-81'

SELECT * FROM EMP WHERE HIREDATE = TO_DATE('03-MAR-81')

SELECT * FROM EMP WHERE HIREDATE = TO_DATE(:1)

SELECT * FROM EMP WHERE HIREDATE = '1981-03-03'

date + number 
number + date 
date - number 
date1 - date2 

Dates

Date handling has two categories: 

• Two-digit year dates, which are treated as occurring 50 years before or 50 years after the year 2000.

• Four-digit year dates, which are not ambiguous with regard to the year 2000.

Oracle recommends that you set the Oracle Database 10g server and gateway default HS_NLS_DATE_FORMAT parameter to a format including a four-digit year.

Use one of the following methods to enter twenty-first century dates:

• The TO_DATE function

  Use any date format including a four character year field. Refer to the Oracle Database SQL Reference for the available date format string options.

  For example, TO_DATE('2008-07-23', 'YYYY-MM-DD') can be used in any SELECT, INSERT, UPDATE, or DELETE statement.

• The HS_NLS_DATE_FORMAT parameter

  The HS_NLS_DATE_FORMAT parameter defines a default format for Oracle Database explicit TO_DATE functions without a pattern and for implicit string to date conversions.

  For example, with HS_NLS_DATE_FORMAT defined as 'YYYY-MM-DD', '2008-07-23' can be used in any SELECT, INSERT, UPDATE, or DELETE statement.

HS_NLS_DATE_FORMAT Support

The following table lists the four patterns that can be used for the HS_NLS_DATE_FORMAT.

The Oracle Database default format of 'DD-MON-YY' is not permitted with DB2.

The following example demonstrates how to enter and select date values in the twenty-first century:

ALTER SESSION SET HS_NLS_DATE_FORMAT = 'YYYY-MM-DD';
INSERT INTO EMP (HIREDATE) VALUES ('2008-07-23');
SELECT * FROM EMP WHERE HIREDATE = '2008-07-23';
UPDATE EMP SET HIREDATE = '2008-07-24'
   WHERE HIREDATE = '2008-07-23';
DELETE FROM EMP WHERE HIREDATE = '2008-07-24';

Oracle TO_DATE Function

The Oracle TO_DATE function is preprocessed in SQL INSERT, UPDATE, DELETE, and SELECT WHERE clauses. TO_DATE functions in SELECT result lists are not preprocessed.

The TO_DATE function is often needed to provide values to update or compare with date columns. Therefore, the gateway replaces the information included in the TO_DATE clause with an acceptable value before the SQL statement is sent to DB2.

Except for the SELECT result list, all TO_DATE functions are preprocessed and turned into values that are the result of the TO_DATE function. Only TO_DATE(literal) or TO_DATE(:bind_variable) is permitted. Except in SELECT result lists, the TO_DATE(column_name) function format is not supported.

The preprocessing of the Oracle TO_DATE functions into simple values is useful in an INSERT VALUES clause because DB2 does not allow functions in the VALUES clause. In this case, DB2 receives a simple value in the VALUES list. All forms of the TO_DATE function (with one, two, or three operands) are supported.

Performing Numeric data type Operations

IBM versions of the DRDA Server perform automatic conversions to the numeric data type of the destination column (such as integer, double-precision floating point, or decimal). The user has no control over the data type conversion, and this conversion can be independent of the data type of the destination column in the database.

For example, if PRICE is an integer column of the PRODUCT table in an IBM DRDA database, then the update shown in the following example inaccurately sets the price of an ice cream cone to $1.00 because the IBM DRDA Server automatically converts a floating point to an integer:

UPDATE PRODUCT 
SET PRICE = 1.50 
WHERE PRODUCT_NAME = 'ICE CREAM CONE    '; 

Because PRICE is an integer, the IBM DRDA Server automatically converts the decimal data value of 1.50 to 1.

Mapping the COUNT Function

Oracle Database supports the following four operands for the COUNT function:

• COUNT(*)

• COUNT(DISTINCT colname)

• COUNT(ALL colname)

• COUNT(colname)

Some DRDA servers do not support all forms of COUNT, specifically COUNT(colname) and COUNT(ALL colname). In those cases the COUNT function and its arguments are translated into COUNT(*). This may not yield the desired results, especially if the column being counted contains NULLs.

For those DRDA servers that do not support the above forms, it may be possible to achieve equivalent functionality by adding a WHERE clause. For example,

SELECT COUNT(colname) FROM table@dblink WHERE colname IS NOT NULL 

or

SELECT COUNT(ALL colname) FROM table@dblink WHERE colname IS NOT NULL

Refer to Chapter 2, "SQL Limitations" for known DRDA servers which do not support all forms of COUNT.

Performing Zoned Decimal Operations

A zoned decimal field is described as packed decimal on Oracle Database. However, an Oracle application such as a Pro*C program can insert into a zoned decimal column using any supported Oracle numeric data type. The gateway converts this number into the most suitable data type. Data can be fetched from a DRDA database into any Oracle data type, provided that it does not result in a loss of information.

Using DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE

To run a passthrough SQL statement using DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE, use the following syntax:

number_of_rows = DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE@dblink ('native_DRDA_sql'); 

where:

number_of_rows is a variable that is assigned the number of rows affected by the passthrough SQL completion. For DDL statements, a zero is returned for the number of rows affected.

dblink is the name of the database link used to access the gateway.

native_DRDA_sql is a valid nonquery SQL statement (except CONNECT, COMMIT, and ROLLBACK). The statement cannot contain bind variables. Native SQL statements that cannot be dynamically prepared are rejected by the DRDA Server. The SQL statement passed by the DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE function must be a character string. For more information regarding valid SQL statements, refer to the SQL Reference for the particular DRDA Server.

Retrieving Results Sets Through Passthrough

Oracle Transparent Gateway for DRDA provides a facility to retrieve results sets from a SELECT SQL statement entered through passthrough. Refer to Oracle Database Heterogeneous Connectivity Administrator's Guide for additional information.

DECLARE
  CRS binary_integer;
  RET binary_integer;
  VAL VARCHAR2(10)
BEGIN
  CRS:=DBMS_HS_PASSTHROUGH.OPEN_CURSOR@gtwlink;
  DBMS_HS_PASSTHROUGH.PARSE@gtwlink(CRS,'SELECT NAME FROM PT_TABLE');
BEGIN
  RET:=0;
  WHILE (TRUE)
  LOOP
    RET:=DBMS_HS_PASSTHROUGH.FETCH_ROW@gtwlink(CRS,FALSE);
    DBMS_HS_PASSTHROUGH.GET_VALUE@gtwlink(CRS,1,VAL);
    INSERT INTO PT_TABLE_LOCAL VALUES(VAL);
  END LOOP;
  EXCEPTION
    WHEN NO_DATA_FOUND THEN
      BEGIN
        DBMS_OUTPUT.PUT_LINE('END OF FETCH');
        DBMS_HS_PASSTHROUGH.CLOSE_CURSOR@gtwlink(CRS);
      END;
    END;  
END;
/ 

Using the Gateway Data Dictionary

The gateway data dictionary views provide users with an Oracle-like interface to the contents and use of the DRDA database. Some of these views are required by Oracle products. The gateway supports the DB2/OS390, DB2/400, and DB2/UDB catalog views. DB2/VM catalog views are not available.

You can query the gateway data dictionary views to see the objects in the DRDA database and to determine the authorized users of the DRDA database. Many Oracle catalog views are supported by the Oracle Transparent Gateway for DRDA. Refer to Appendix A for descriptions of Oracle DB2 catalog views. These views are completely compatible with the gateway.

Using the DRDA Catalog

Each DRDA database has its own catalog tables and views, which you might find useful. Refer to the appropriate IBM documentation for descriptions of these catalogs.