Postgres vs MySQL vs MongoDB: 2026 Decision Tree
Database choice rarely comes down to one feature. The decision tree below catches the common forks.
Postgres: the default
Postgres: feature-rich; strong on JSON; rich ecosystem; the modern default for most new SaaS.
Best when you want one database that handles structured + semi-structured well.
MySQL: the simple-and-fast
- MySQL: fast for read-heavy workloads; battle-tested; simpler operations than Postgres at scale.
- Best when you are scale-pushed and want predictable performance.
MongoDB: the document model
MongoDB: document-first; flexible schema; horizontally scalable.
Best when your data is genuinely document-shaped and JOINs are rare.
The four-question tree
Q1: Is the data document-shaped? Yes → Mongo. No → relational.
Q2: Need JSON-rich queries on relational data? Yes → Postgres.
Q3: Massive read scale + simple schema? → MySQL.
Q4: Mixed? → Postgres is the safest default.
Antipatterns
- MongoDB for relational data. JOIN pain compounds.
- Postgres for true document workloads. JSONB does not match Mongo’s native model.
- MySQL for advanced JSON/extensions. Postgres is better there.
What to do this week
Three moves. (1) Run a 30-day trial of the candidate against your real workload. (2) Compare TCO + workflow fit, not just feature checklists. (3) Decide and commit; running both in parallel is the most expensive option.