12 Developing Applications

Fetch Reblocking

The gateway 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 server. The buffer might 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 server is still determined by the Oracle application.

Refer to Chapter 10, "Configuring the Gateway" for more information.

Oracle Application and DRDA Server Stored Procedure Completion

As an example, suppose an Oracle application attempts to call a stored procedure in a DB2/OS390 database. In order for an Oracle application to call a DB2 stored procedure, the DB2 stored procedure must first be created 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 to DB2, the gateway will be 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 to calculate a new yearly salary for JOHN SMYTHE. The revised salary returned in RESULT is used to update the EMP table of an Oracle database server:

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 tables SYSIBM.SYSROUTINES and SYSIBM.SYSPARMS, and DB2/400 uses the tables QSYS2.SYSPROCS and QSYS2.SYSPARMS. The gateway has a list of known catalogs to search, depending on the DRDA Server 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 enable blank or public Authorization qualifiers. If the currently connected DRDA Server supports this 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 for the underlying database of the DRDA Server.

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 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 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 VSAM file, then the gateway considers this activity separate from its own recoverable unit of work.

• PL/SQL records cannot be passed as parameters when calling 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

The Oracle integrating server automatically forwards to the DRDA database compatible SQL functions, that is, SQL constructs with the same syntax and meaning on both the Oracle server and 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 by the Oracle server 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 (such as different function names) by the DRDA database, are automatically translated by the Oracle server and then forwarded to the DRDA database. The Oracle integrating server, transparent to your application, changes the function name before sending it to the DRDA database.

Compensated SQL Functions

Some advanced SQL constructs that are supported by the Oracle server 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 the Oracle integrating server. The Oracle integrating server then applies the function.

The Oracle server can compensate for the missing or incompatible function by automatically excluding the incompatible SQL construct from the SQL request that is forwarded to the DRDA database. The Oracle server then retrieves the necessary data from the DRDA database and applies the function. This activity is known as postprocessing.

The gateway attempts to pass all SQL functions to DRDA databases. But when a DRDA database does not support a function represented in the computation, then the gateway changes that function. For example, if a program requests:

SELECT COS(X_COOR) FROM TABLE_X;

from a DB2/OS390 database, which does not support the meaning of COS, then the gateway changes the SELECT statement to:

SELECT X_COOR FROM TABLE_X;

All data in the X_COOR column of TABLE_X is passed from the DB2/OS390 database to the Oracle integrating server. After the data is moved to the Oracle integrating server, the COS function is performed.

If you are performing operations on large amounts of data stored in a DRDA database, then keep in mind that some functions require postprocessing.

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). Refer to Native Semantics.

DB2/OS390 SQL Compatibility

The ways that the Oracle Database server and gateway handle SQL functions for a DB2/OS390 database are shown in Table 12-1.

DB2/Universal Database SQL Compatibility

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

DB2/400 SQL Compatibility

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

DB2/VM SQL Compatibility

The ways that the Oracle Database server 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 not enabled by default. They can be enabled as an option:

SQL Functions That Can Be Disabled

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

• COUNTCOL, to control SQL COUNT(column) function

• GROUPBY, to control SQL GROUP BY clause

• HAVING, to control SQL HAVING clause

• ORDERBY, to control SQL ORDER BY clause

• WHERE, to control SQL WHERE clause

ORDERBY controls sort order, which may differ at various sort locations. For example, with ORDERBY ON, a DB2 sort would be based on EBCDIC sorting order, whereas with ORDERBY OFF, an Oracle sort would be based on ASCII sorting order.

Three other 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 on the cheaper resource.

SQL Set Operators and Clauses

The clauses WHERE and HAVING are compatible for all versions of the DRDA Server, meaning that they 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 the Oracle server is determined by the Native Semantics parameters (refer to 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 noncharacter binary data in character columns. Applications run 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 might 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 noncharacter data must be created with the FOR BIT DATA option. In the application, the character columns holding noncharacter 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 will 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 which are half that of their character counterparts. For example, the maximum size of a CHAR data type may be 255 characters, whereas the maximum size of a GRAPHIC data type may be 127 characters.

Oracle does not have a direct matching data type, and the gateway therefore converts between Oracle character data types and DB2 Graphic data types. Oracle character data types may contain single, mixed, or double-byte character data. The gateway converts the string data into suitable 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 in the Oracle server. The Oracle server 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 data and time of day.

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'

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, and 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 the Oracle database server 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 patterns can be used for the HS_NLS_DATE_FORMAT:

The Oracle default format of 'DD-MON-YY' is not permitted with DB2. As a result, the gateway local date exit is provided to change the Oracle default date format of 'DD-MON-YY' or 'DD-MON-RR' to the DB2 ISO format of 'YYYY-MM-DD' before passing the date to DB2.

The following example demonstrates the most efficient way 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 permit 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

The Oracle Database server 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 NULL values.

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

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 an Oracle server. 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 loss of information.

Processing DDL Statements through Passthrough

As noted above, SQL statements which are processed through the DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE function are not interpreted by the Oracle database server. As a result, the Oracle database server will not know if such statements are making any modifications to the DRDA Server. This means that unless you keep the Oracle database's cached information up to date after changes to the DRDA Server, the database may continue to rely upon inaccurate or outdated information in subsequent queries within the same session.

An example of this occurs when you alter the structure of a table by either adding or removing a column. When an application references a table through the gateway (for example, when you perform a query on it), the Oracle database server caches the table definition. Now, suppose that (within the same session) the application subsequently alters the table's form, by using DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE to add a column. Then, the next reference to the table (by the application) will return the old column definitions of the table and will ignore the table's new column. This is because the Oracle database server did not process the statement and, so, has no knowledge of the alteration. Because the database does not know of the alteration, it has no reason to requery the table form, and, so, it will use the already-cached form to handle any new queries.

In order for the Oracle database server to acquire the new form of the table, the existing session with the gateway must be closed and a new session must be opened. This can be accomplished in either of two ways:

• By ending the application session with the Oracle database server and starting a new session after modifications have been made to the DRDA Server; or

• By running the ALTER SESSION CLOSE DATABASE LINK command after making any modifications to the DRDA Server.

Either of the above actions will void the cached table definitions and will force the Oracle database server to acquire new definitions on the next reference.

Using the DBMS_HS_PASSTHROUGH.EXECUTE_IMMEDIATE Function

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 that are 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 that is 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 that is 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 Result Sets Through Passthrough

Oracle Transparent Gateway for DRDA provides a facility to retrieve results sets from a SELECT SQL statement entered through passthrough.

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');
  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 IBM documentation for descriptions of these catalogs.