PostgreSQL vs MySQL

PostgreSQL and MySQL are the two most popular open-source relational databases, but they differ significantly in architecture and philosophy.

Architecture Differences

PostgreSQL uses a process-per-connection model with a sophisticated multi-version concurrency control (MVCC) implementation. It stores row versions directly in the main table, which can lead to table bloat but provides excellent snapshot isolation. MySQL with InnoDB uses a thread-per-connection model and stores undo information in a separate rollback segment, making cleanup more straightforward.

PostgreSQL's query planner is notably more advanced, supporting hash joins, merge joins, and sophisticated cost-based optimization. MySQL's optimizer has improved significantly in recent versions but still lags behind for complex multi-table joins.

Performance Characteristics

For simple primary-key lookups and basic CRUD, MySQL often edges ahead due to lower per-query overhead. However, PostgreSQL excels at complex queries involving multiple joins, subqueries, window functions, and CTEs. If your workload is analytics-heavy or requires advanced SQL, PostgreSQL is the clear winner.

PostgreSQL's JSONB support also makes it a strong choice for semi-structured data patterns without sacrificing transactional guarantees.

Trade-offs

MySQL's simplicity is both a strength and a weakness. It is easier to get started with but may require workarounds for advanced use cases. PostgreSQL's extensibility through extensions like PostGIS, pg_vector, and TimescaleDB means you can add specialized capabilities without switching databases.

For system design interviews, understanding when to pick each database matters. MySQL shines in read-heavy web apps (think WordPress, Shopify), while PostgreSQL fits complex domains like financial systems or geospatial platforms.

Replication and Scaling

Both databases support streaming replication. MySQL's Group Replication and InnoDB Cluster provide semi-synchronous multi-primary setups. PostgreSQL offers logical replication for selective table replication and is the foundation for distributed systems like Citus and CockroachDB.

Real-World Usage

Instagram, Reddit, and Apple use PostgreSQL for its reliability and advanced features. MySQL powers Facebook (with heavy modifications), GitHub, and Airbnb. Both are proven at massive scale.

For a deeper dive into relational database internals, see our database fundamentals guide. Preparing for interviews? Check our database interview questions and explore pricing plans for full access.