Compare PostgreSQL, MySQL, and SQL Server — what are the meaningful differences?
Quick Answer
PostgreSQL is open-source, standards-compliant, and known for its extensibility (custom types, extensions like PostGIS/pgvector) and advanced features (transactional DDL, sophisticated indexing options). MySQL is open-source, historically simpler and very widely deployed (especially in web/LAMP-stack contexts), with InnoDB as its default storage engine. SQL Server is Microsoft's commercial offering, deeply integrated with the Windows/.NET ecosystem and enterprise tooling, with strong built-in BI/reporting/analytics features. All three are mature, fully capable relational databases for the vast majority of workloads. The differences that matter most in practice are ecosystem fit, licensing cost, and specific advanced features, not raw core SQL capability.
Detailed Answer
PostgreSQL
- Licensing: fully open-source (PostgreSQL License, a permissive license), no per-core/per-instance licensing cost.
- Standards compliance and features: widely regarded as closely following the SQL standard, with advanced capabilities like transactional DDL (schema changes can be rolled back — see the DDL/DML question), rich indexing options (GiST, GIN, BRIN, partial and expression indexes), native JSON/JSONB support with indexing, and a strong extension ecosystem (PostGIS for geospatial data, pgvector for vector similarity search, TimescaleDB for time-series).
- Concurrency model: MVCC (see that question), generally praised for read-heavy workload performance and its handling of complex queries.
- Where it's chosen: teams wanting maximum standards compliance, extensibility, and avoiding licensing costs. Increasingly the default choice for new greenfield projects across many industries, including where advanced data types (geospatial, vector embeddings for AI workloads) matter.
MySQL
- Licensing: open-source (GPL), with a commercial/enterprise edition (MySQL Enterprise) offering additional support and features; owned by Oracle.
- History and ecosystem: extremely widely deployed, historically the "M" in the LAMP stack, with enormous community familiarity, hosting-provider support, and tooling maturity for typical web application workloads.
- Storage engines: pluggable storage engine architecture, with InnoDB (ACID-compliant, row-level locking, the modern default) as the standard choice. The older MyISAM engine (table-level locking, no transactions) still exists but is rarely the right choice for new work.
- Where it's chosen: teams prioritizing broad ecosystem familiarity/tooling, particularly common in traditional web application stacks (WordPress, many PHP/web frameworks default to MySQL-compatible databases).
SQL Server
- Licensing: commercial, with per-core or per-server licensing costs (a free Express edition exists with resource limits); owned by Microsoft.
- Ecosystem integration: deep integration with the Windows Server and .NET ecosystems, Active Directory authentication, and Microsoft's broader enterprise/BI tooling (SQL Server Reporting Services, SQL Server Integration Services, Power BI).
- Tooling: SQL Server Management Studio (SSMS) is a mature, widely-used GUI management tool, with strong built-in support for complex analytical/BI workloads out of the box.
- Where it's chosen: organizations already standardized on Microsoft/.NET infrastructure, or with existing enterprise licensing agreements and BI tooling investments that make the ecosystem fit decisive.
What's mostly similar across all three
Core relational SQL capability — joins, transactions, indexing fundamentals, window functions (all three now support them), stored procedures — is broadly comparable across all three for the overwhelming majority of application workloads. The meaningful differentiators for a real decision are usually: licensing cost model, existing team/organizational familiarity, specific advanced feature needs (PostgreSQL's extensibility, SQL Server's BI integration), and hosting/cloud-provider ecosystem fit, not "which one is fundamentally more capable at SQL," since that gap has narrowed significantly over the past decade.