Choose MongoDB if your data changes shape often and you want to store it as flexible, JSON-like documents with built-in sharding. Choose PostgreSQL if your data is relational, you want SQL and a free open source licence, and you'd like JSON support on the side through its jsonb type. Both support ACID transactions, so the decision is about your data model and who will run the database, not whether your data is safe.
Developers argue about this one a lot, so I went back to MongoDB's manual and plans page and PostgreSQL's about, licence and JSON documentation to compare what each says about itself now.
MongoDB and PostgreSQL at a glance
| MongoDB | PostgreSQL | |
|---|---|---|
| Data model | JSON-like documents with a flexible schema | Object-relational tables using SQL |
| Transactions | Multi-document ACID transactions | ACID-compliant since 2001 |
| JSON | Native document format | json and jsonb data types |
| Scaling out | Sharding built in | Not covered on the pages I read |
| Licence | Community Edition and Enterprise Advanced | PostgreSQL License, similar to BSD or MIT |
| Managed cloud | MongoDB Atlas | Not sold by the PostgreSQL project |
| Free route | Atlas Free tier | Free and open source download |
MongoDB's two pages don't agree on free storage
If you're sizing a free MongoDB Atlas project, read the small print twice. MongoDB's plans page describes the Free tier as free forever for learning and exploring, with 512MB of storage and shared RAM and CPU, while its What is MongoDB page describes the free Atlas sandbox as up to 5GB of storage. The Flex option on the plans page is the one that lists 5GB, so plan around 512MB for the free tier and check the console before you build on it.
Where MongoDB fits
MongoDB's manual describes a document database that stores records as field and value pairs, much like JSON objects, so you can model data the way your code already uses it. The flexible schema lets you change your data model without downtime, and it handles full-text search, vector search, geospatial queries and time series from the same engine. Replication with automatic failover and sharding for horizontal scaling are part of the core design, and MongoDB 9.0 is the current release.
Where PostgreSQL fits
PostgreSQL's about page traces it back to the POSTGRES project at the University of California at Berkeley in 1986, with nearly 40 years of active development since. It runs on every major operating system, has been ACID-compliant since 2001 and is free under a liberal open source licence. It's also highly extensible: you can define your own data types and functions, add PostGIS for geospatial data, and store JSON in jsonb, a binary format that's faster to query than plain json because it isn't reparsed every time.
Deployment is the other big difference. As well as Atlas in the cloud, MongoDB offers a self-managed Community Edition and Enterprise Advanced for running it on your own infrastructure. PostgreSQL is self-managed by default: you download it free, run it on any major operating system and decide for yourself where it's hosted, so no single vendor controls it.
Questions that settle the choice
- Does your data have clear relationships, like customers, orders and invoices?
- Will the shape of your records change every few weeks?
- Do you need to scale writes across many servers soon?
- Who will run backups, upgrades and failover?
- Does your team already know SQL?
This guide on choosing the right database for your business covers the business side, and if you're adding AI search, read what a vector database does before you decide.
My advice on MongoDB or PostgreSQL
If your team knows SQL and your data has natural relationships, PostgreSQL is a safe, free default, and jsonb covers the occasional flexible field. If you're building fast, your records change shape often, or you expect to shard across servers, MongoDB's document model will feel more natural, and Atlas takes the operations work off your hands. Whichever you pick, prototype your three most common queries in it before you commit.
Do you make database tools? Tool makers can ask to be considered for a featured place in comparisons like this one.
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Frequently asked questions
Is PostgreSQL free?
Yes. PostgreSQL is free and open source under the PostgreSQL License, which its site describes as similar to the BSD or MIT licences.
Does MongoDB support transactions?
Yes. MongoDB's manual says it supports multi-document ACID transactions for operations that need data consistency.
Can PostgreSQL store JSON?
Yes. PostgreSQL has two JSON types, json and jsonb, plus a jsonpath type for queries. The docs say jsonb is stored in a decomposed binary format.
How much storage does MongoDB Atlas Free include?
MongoDB's plans page lists 512MB for the Free tier. Its What is MongoDB page mentions up to 5GB for the free sandbox, so check the console when you sign up.
Can I run MongoDB on my own servers?
Yes. MongoDB offers a self-managed Community Edition and Enterprise Advanced alongside its Atlas cloud service.
Which is older?
PostgreSQL. Its origins go back to 1986 at the University of California at Berkeley.
