The PostgreSQL Compatibility Standard: Building the 'New Linux' for Enterprise Data

As PostgreSQL cements its position as the enterprise database of choice, the community faces a critical question: what does it truly mean to be "PostgreSQL compatible"? This challenge has sparked a comprehensive initiative at PGConf.EU 2025, where developers, contributors, and community members are collaborating to establish a practical framework for PostgreSQL compatibility.

From Binary Certification to Granular Compatibility

The evolution of this standard represents a significant shift in thinking. Early discussions centered around a simple "Pass/Fail" certification model, but the Riga Consensus has pivoted toward a more nuanced approach. Rather than blunt verdicts that could stifle innovation, the new framework employs a weighted checklist that distinguishes between Core and Optional features.

This granular approach acknowledges that many derivatives achieve 95% compatibility while differing on specific features. A product that implements all core functionality but innovates in certain areas shouldn't be automatically deemed incompatible. The compatibility matrix allows for this spectrum of compliance, recognizing that the PostgreSQL ecosystem thrives on both standardization and innovation.

Managed Environments and the Reality of Cloud Services

One of the most pragmatic aspects of the new standard is its accommodation for "Managed" PostgreSQL environments. Cloud providers and managed service offerings often require specific exceptions, such as restricted superuser access or limitations on file-system operations. Rather than forcing these services to fail compatibility tests, the matrix allows for documented exceptions without invalidating the entire suite.

This approach reflects the reality that PostgreSQL's success is partly due to its adaptability across different deployment models. A standard that couldn't accommodate managed services would be impractical and potentially harmful to PostgreSQL's continued growth.

The Non-Negotiable: No Silent Failures

At the heart of the compatibility standard lies a fundamental principle: compatibility requires that SQL commands perform their expected functions. This might seem obvious, but it addresses a critical concern in the ecosystem. A command like CREATE INDEX must actually build the index, not merely return "success" without taking action.

This principle extends throughout the standard, ensuring that compatibility isn't just about syntax but about actual behavior and results. It's a commitment to functional parity that protects users and applications relying on PostgreSQL's predictable behavior.

Core Behavioral Requirements

The standard defines several critical behavioral areas that must be preserved:

Core SQL Compliance encompasses everything documented in PostgreSQL's "Part II: SQL Commands." Even rarely used rules must be supported, as compatibility means supporting the complete SQL implementation, not just the popular features.

Implicit Behaviors represent a fascinating challenge. Users often rely on undocumented behaviors that become part of the expected experience. For instance, INSERT ... SELECT ... ORDER BY must respect physical insertion order—a detail critical for clustering and performance optimizations that applications may depend upon.

Transaction Isolation requires support for standard levels (Read Committed, Repeatable Read, Serializable) with identical behavior regarding race conditions and automatic deadlock detection. This ensures that applications can rely on consistent transactional semantics across different PostgreSQL implementations.

Error Codes must return identical SQLSTATE error codes for common errors. This consistency is crucial for application-level error handling, allowing code written for one PostgreSQL implementation to work reliably on another.

System Catalogs and Connectivity

The PostgreSQL ecosystem relies heavily on system catalogs and standard tooling. The pg_catalog must be present, and standard monitoring tools must be able to connect and query these catalogs without crashing. Even if an engine doesn't populate every data point, the structural compatibility must exist.

Connectivity requirements ensure that psql reports a standard version structure to prevent client warnings or errors. Standard drivers like JDBC and psycopg must work without modification, and pg_dump must be able to export schema from Community Postgres and import to the derivative.

Hard Limits and Functional Dependencies

The standard establishes that derivatives must generally match PostgreSQL's hard limits, such as maximum columns. Significant reductions in these limits may deem a product incompatible, as applications often design around these boundaries.

Feature Dependencies reveal the interconnected nature of PostgreSQL functionality. The standard recognizes that triggers imply PL/pgSQL support—while triggers can be written in C or SQL, the ecosystem overwhelmingly relies on PL/pgSQL. Therefore, trigger compatibility implies full PL/pgSQL language support.

Execution Plans, Indexing, and Data Types

Interestingly, the standard distinguishes between execution plans and results. Identical execution plans aren't required, as innovation in storage and planning is allowed. However, functional results must match. If partitioning is supported, partition pruning logic must effectively reduce the scan, even if the specific plan node differs.

Indexing commands must perform actual work—CREATE INDEX cannot be a no-op success. This ensures that performance optimizations relying on indexes remain functional across compatible implementations.

Data Types support for specific types like ARRAY, BYTEA, and JSONB is required for core compatibility, reflecting their fundamental importance to PostgreSQL's value proposition.

Backups, Replication, and the Test Harness

The standard addresses critical operational concerns:

Logical Replication must be bi-directional, allowing creation of subscriptions and publications between different implementations. It must be monitorable via standard replication catalogs, and vendor-specific extensions must not break connections to vanilla Postgres nodes.

Physical Replication must allow physical copying of binary files or equivalent export of the cluster's canonical state. Hybrid clusters should support mixing vanilla and vendor nodes, such as promoting a vanilla master over vendor replicas.

Tools Compatibility with standard tools like Patroni, pgbasebackup, and RepMgr is a key metric for the "Backup" compatibility tier.

Recovery requires Point-In-Time Recovery (PITR) and access to WAL data, ensuring that disaster recovery remains consistent across implementations.

The Test Harness Architecture

The compatibility test suite sits outside the PostgreSQL codebase, with test suite versions corresponding numerically to Postgres versions but remaining independent entities. This architecture allows for parallel development and testing without coupling to the core codebase.

Vendor Responsibility requires vendors to provide compatible build targets (container/environment) for testing. This ensures that the testing process is practical and that vendors can demonstrate compliance.

Bug-for-Bug testing focuses on feature compliance rather than bugs. If a derivative fixes a known PostgreSQL bug, it shouldn't fail the compatibility test—the standard measures intended functionality, not implementation flaws.

Future Work and Open Questions

The standard acknowledges several areas for future development, including support for the v3 client wire protocol, SSL/TLS compatibility, configuration file standards, additional procedural languages, specific indexing method behaviors, time functions, extension compatibility, Foreign Data Wrapper support, and ABI stability.

Perhaps most philosophically significant is the question of Brand Identity: if a product satisfies all compatibility criteria but isn't the vanilla C implementation, is it "Postgres" or "Postgres-compatible"? This question touches on the broader issue of what constitutes the PostgreSQL brand and how the community wants to position compatible implementations.

The Significance of This Standard

This compatibility initiative represents PostgreSQL's maturation as a platform. Just as Linux established standards that allowed distributions to flourish while maintaining compatibility, PostgreSQL is creating a framework that enables innovation while preserving the core value proposition that attracted users in the first place.

The standard acknowledges that PostgreSQL's strength lies not just in its code but in its ecosystem—the tools, drivers, extensions, and operational practices that have developed around it. By defining compatibility in terms of this complete ecosystem rather than just SQL syntax, the standard protects the investments organizations have made in PostgreSQL expertise and tooling.

As PostgreSQL becomes "the new Linux" for enterprise data, this compatibility standard ensures that the platform can support the diverse needs of modern organizations while maintaining the consistency and reliability that made it successful in the first place. It's a delicate balance between innovation and standardization, and the framework emerging from PGConf.EU 2025 represents a thoughtful approach to navigating this challenge.