PostgreSQL vs MongoDB: The definitive guide for 2024

Introduction

The debate is over. Both databases have matured to the point where they can handle 90% of use cases. But the remaining 10% -- the edge cases at scale -- are where your choice matters.

Architecture Comparison

Storage Engine

PostgreSQL uses a heap-based storage engine with WAL (Write Ahead Log). MongoDB uses WiredTiger, a B-Tree based engine with checkpointing.

graph TD
    subgraph PG [PostgreSQL Storage]
        T[Table Heap] --> P[Page 8kb]
        P --> T1["Tuple (Fixed Columns)"]
        P --> T2["Tuple (JSONB Binary)"]
    end
    
    subgraph Mongo [MongoDB Storage]
        C[Collection] --> D["Document (BSON)"]
        D --> F["Field (Flexible)"]
        D --> A["Array (Embedded)"]
    end

Consistency Models

• PostgreSQL: Strong consistency (ACID) by default.
• MongoDB: Tunable consistency. Use readConcern: majority and writeConcern: majority for strong consistency.

Performance Benchmarks

For complex joins, Postgres wins. For inserting massive amounts of unstructured data, Mongo wins.

When to Choose What?

Choose PostgreSQL if:

• You need complex relational queries (JOINs).
• Data integrity is paramount (ACID transactions are simpler).
• You have structured data with a stable schema.

Choose MongoDB if:

• You need flexible schema for rapid iteration.
• You need horizontal scaling out of the box (Sharding is easier).
• You are storing documents/JSON native data.