"Drop-in Replacement": Defining Compatibility for Postgres and MySQL Derivatives

The success of open source databases like PostgreSQL and MySQL/MariaDB has created an ecosystem of derivatives claiming "drop-in compatibility." But as the distance between upstream and these derivatives grows, user confusion and brand dilution can follow.

To address this, we explore the challenge of compatibility with de facto standards from two distinct angles: a governance perspective on defining the compatibility criteria, and a systems engineering case study on implementing them.

1. The Standard: We present the findings from the "Establishing the PostgreSQL Standard" working group held at PGConf.EU 2025. This progress report details the community's consensus on the hard requirements needed to fix the "wild west" of marketing claims, including:
   • Core SQL: Defining the non-negotiable functions, types, and PL/pgSQL.
   • Protocol: Why wire compatibility is insufficient without consistent transactional and pg_catalog behaviour.
   • Ecosystem: The critical requirements for integration with logical replication and tools like Patroni.
2. The Implementation: Maintaining compatibility with MySQL/MariaDB in TiDB, a distributed database engine, is far more complex than matching syntax for an evolving SQL dialect:
   • We explore the architectural friction of making TiDB speak the MySQL wire protocol and support the MySQL syntax.
   • We cover compatibility with the MySQL binary log based replication.