Migrating Oracle PL/SQL source code of stored procedures, functions, triggers to PostgreSQL requires manual translation of a skilled developer to ensure the migrated code functions correctly. To guarantee a successful migration, it is extremely important to carefully investigate the best practices for translating all database logic entries specified above from Oracle to PostgreSQL while preserving the semantics of every unit in terms of solving a particular routine, instead of simply converting the syntax.
One of the significant differences between Oracle and PostgreSQL is the way they organize functions into semantic groups. Oracle uses packages, while PostgreSQL uses schemas. In Oracle, package-level variables are used, which do not exist in PostgreSQL. Instead, they can be emulated as data of a temporary service table. Understanding these differences is essential to ensure a smooth transition.
Before commencing the migration process, it is vital to have a thorough understanding of both Oracle and PostgreSQL. If the application uses Oracle’s proprietary features, those parts of the application that utilize Oracle need to be rewritten. Addressing any syntax and functionality differences between the two systems is also crucial to ensure a successful migration.
There are some differences between Oracle and PostgreSQL worth noting to assist with the migration process. PostgreSQL requires all subselects to have aliases, whereas in Oracle, it does not need a name. Oracle does not distinguish NULL and an empty string, which must be carefully considered to preserve proper semantics in the migrated code in PostgreSQL. Additionally, Oracle may use special syntax for outer join using the operator (+), which must be translated into SQL standard syntax when migrating to PostgreSQL:
SELECT … FROM tbl1,tbl2 WHERE tbl1.id=tbl2.id(+)
Into
SELECT … FROM tbl1 LEFT JOIN tbl2 ON tbl1.id = tbl2.id
Oracle and PostgreSQL treat sequences differently. The Oracle method to produce the next value mysequence.NEXTVAL must be converted into the PostgreSQL equivalent nextval(‘mysequence’). Another challenge of the migration is that Oracle supports the specific statement DECODE, which must be replaced by CASE/WHEN in PostgreSQL code:
DECODE($exp, $when, $then, …)
into
CASE $exp WHEN $when THEN $then … END
PostgreSQL treats the body of stored procedures and functions as a string, so it must be enclosed in dollar-quotes $$, and the DBMS requires a ‘LANGUAGE’ specification at the end of the body. Finally, PostgreSQL requires that the source code of the trigger be arranged as a standalone function with a reference from the CREATE TRIGGER declaration.
While the migration of stored procedures, functions, triggers, and views from Oracle to PostgreSQL can be partially automated via special tools such as the Oracle to PostgreSQL Code Converter, it is essential to use these tools with caution. The tool can migrate Oracle stored procedures, functions, triggers, and views into PostgreSQL equivalents, and predefined Oracle types and built-in functions are mapped into PostgreSQL equivalents. All reserved words and identifiers are handled intelligently. However, the tool should not be relied upon to provide a complete migration solution, as the semantics of the migrated code still require attention to detail.
In conclusion, migrating PL/SQL source code from Oracle to PostgreSQL requires attention to syntax and semantic differences between the two DBMSs. While some differences can be addressed with automated tools such as the Oracle to PostgreSQL Code Converter, semantic differences require manual translation to ensure the migrated code functions correctly. It is crucial to research the best practices for translating stored procedures, functions, triggers, and views from Oracle to PostgreSQL while preserving the semantics of every unit in terms of solving a particular routine, instead of simply converting the syntax. With careful planning and attention to detail, a successful migration from Oracle to PostgreSQL can be achieved.
