Tips on Migrating PL/SQL Code to PostgreSQL

April 03 07:49 2023 by Sheri Gill Print This Article

Migrating PL/SQL source code from Oracle to PostgreSQL requires attention to syntax and semantic differences between the two database management systems (DBMS). While Oracle uses packages to organize functions into semantic groups, PostgreSQL uses schemas for the same purpose. Additionally, PostgreSQL does not have package-level variables, but they can be emulated as data of a temporary service table. To ensure a successful migration, 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.

Before commencing the migration process, having a thorough understanding of both Oracle and PostgreSQL is essential. 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 vital to ensure a smooth transition.

To assist with the migration, some differences between Oracle and PostgreSQL are worth noting.

1. PostgreSQL requires all subselects to have aliases, whereas in Oracle, it does not need a name.
2. Oracle does not distinguish NULL and an empty string, which must be carefully considered to preserve proper semantics in the migrated code in PostgreSQL.
3. Oracle may use special syntax for outer join using the operator (+), which must be translated into SQL standard syntax when migrating to PostgreSQL.
4. Oracle and PostgreSQL treat sequences differently, and the Oracle method to produce the next value mysequence.NEXTVAL must be converted into the PostgreSQL equivalent nextval(‘mysequence’).
5. Oracle supports the specific statement DECODE, which must be replaced by CASE/WHEN in PostgreSQL code.
6. PostgreSQL treats the body of stored procedures and functions as a string, so it must be enclosed in dollar-quotes $$. Additionally, the DBMS requires a ‘LANGUAGE’ specification at the end of the body.
7. Finally, create statements of Oracle triggers include source code right in the CREATE TRIGGER declaration. 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 cautiously. This product 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, and 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 a deep understanding of both DBMSs and attention to detail. While some differences between the two systems can be addressed with automated tools such as the Oracle to PostgreSQL Code Converter, semantic differences must be carefully considered to ensure a successful migration.