PostgreSQL vs MySQL

Two mature open-source relational databases. Postgres is richer (types, constraints, query power, extensibility); MySQL is lighter, ubiquitous, and very fast for simple read-heavy web workloads. Both are single-primary and need sharding at scale.

Dimension	PostgreSQL	MySQL
Query & type richness	JSONB, arrays, ranges, partial/expression indexes, PostGIS, rich constraints.	Solid SQL, JSON support, but fewer advanced types and constraints.
Storage model	Heap + separate indexes; secondary indexes store row pointers.	Clustered (index-organized) tables; secondary indexes store primary keys.
Concurrency	MVCC with snapshot isolation; readers never block writers.	InnoDB MVCC; clustered index means secondary lookups cost two seeks.
Ecosystem & hosting	Strong, broad; managed options (RDS, Aurora, Neon).	Huge, ubiquitous; managed options (RDS, Aurora) mature.
Best fit	Complex queries, correctness, mixed relational/JSON.	Simple read-heavy web OLTP, broad tooling.

Query & type richness

PostgreSQL: JSONB, arrays, ranges, partial/expression indexes, PostGIS, rich constraints.
MySQL: Solid SQL, JSON support, but fewer advanced types and constraints.

Storage model

PostgreSQL: Heap + separate indexes; secondary indexes store row pointers.
MySQL: Clustered (index-organized) tables; secondary indexes store primary keys.

Concurrency

PostgreSQL: MVCC with snapshot isolation; readers never block writers.
MySQL: InnoDB MVCC; clustered index means secondary lookups cost two seeks.

Ecosystem & hosting

PostgreSQL: Strong, broad; managed options (RDS, Aurora, Neon).
MySQL: Huge, ubiquitous; managed options (RDS, Aurora) mature.

Best fit

PostgreSQL: Complex queries, correctness, mixed relational/JSON.
MySQL: Simple read-heavy web OLTP, broad tooling.

When PostgreSQL wins

You need advanced types, constraints, or query complexity (JSONB, GIS, CTEs).
Correctness and strict semantics matter more than raw read speed.

When MySQL wins

Read-heavy web workloads where MySQL's clustered index and ecosystem shine.
Your team and tooling are already MySQL-invested.

Verdict

Choose Postgres for richness and correctness; MySQL for simplicity, ubiquity, and read-heavy web defaults. Both need deliberate sharding to scale writes.