advancedTesting the Persistence Layer — @DataJpaTest & Testcontainers

Why can a repository test pass against H2 but the same code fail against production PostgreSQL?

H2 in PostgreSQL compatibility mode is not the same actual database engine — it doesn't support every PostgreSQL-specific feature (certain functions, JSONB specifics, constraint behaviors), and subtle SQL interpretation differences mean it can silently behave differently than real PostgreSQL for exactly the kind of edge case a test is meant to catch.

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