Same SQL, Different Results: A Subtle Oracle vs. PostgreSQL Migration Bug
3 days ago
- #implicit-casting
- #database-migration
- #operator-precedence
- Database migration from Oracle to PostgreSQL can lead to subtle bugs due to differences in operator precedence and implicit casting.
- Oracle uses implicit type conversion and may rewrite expressions internally, while PostgreSQL follows strict operator precedence and requires explicit casting.
- A real-world example shows how the same SQL logic produces different results in Oracle and PostgreSQL due to these differences.
- Oracle's execution plan reveals implicit conversions that are not obvious from the source code.
- The bug is hard to catch because it doesn't throw errors, looks correct, and often only surfaces with edge cases.
- The fix involves making the intent explicit by properly grouping and casting expressions to ensure consistent behavior across databases.
- Key takeaways include the importance of understanding execution plans, being wary of mixed operators, and recognizing that migration is about semantics, not just syntax.