Here is an opinion that will save a lot of startups pain: default to Postgres, and choose MongoDB only when you have a specific reason. This is not because MongoDB is bad. It is a fine database for the right problems. It is because most startups do not have the problems MongoDB solves, and they do have the problem Postgres solves, which is storing related data you will want to query in ways you cannot yet predict. Picking the wrong database early is one of the more painful things to unwind later.
Key Takeaways
- For most startups, a relational database like Postgres is the sane default.
- Your data almost always has relationships, and you will query it in unforeseen ways.
- MongoDB fits specific needs, not as a general-purpose first choice.
- Postgres is also the most-used database, which helps with hiring (Stack Overflow 2025).
Why Relational Is the Default
Most application data is relational whether you admit it or not. Users have orders, orders have items, items belong to products, and you will constantly want to ask questions that cross those boundaries. Relational databases like Postgres are built exactly for this, with the ability to join related data and run queries you did not anticipate at design time (PostgreSQL). The flexibility of a document database can feel freeing early, when you are moving fast and your data shape is in flux, and then it can bite later, when you need to run a report that spans documents that were never designed to be joined.
Postgres has also grown far beyond plain tables. It handles JSON well, so you can store flexible, document-like data in a relational database when you need to, which removes one of the classic reasons people used to reach for MongoDB in the first place.
When MongoDB Actually Fits
None of this means MongoDB is wrong. It fits genuine cases: data that really is document-shaped and rarely joined, like content or event logs; workloads that need MongoDB's particular scaling model; or a team already fluent in it and productive. The mistake is not using MongoDB. The mistake is choosing it as a default because it felt easier to start with, and then discovering your data was relational all along.
| Situation | Sane default |
|---|---|
| Related data, evolving queries | Postgres |
| Genuinely document-shaped, rarely joined | MongoDB can fit |
| Need flexible JSON sometimes | Postgres (it does JSON) |
| Unsure | Postgres |
A Concrete Version
Picture a team that chose MongoDB early because storing flexible JSON documents felt fast and freeing. For six months it was. Then the business asked a normal question: show revenue by customer segment over time, joining orders, customers, and products. In a relational database this is a routine query. In their document store, the data was never modeled to be joined that way, so answering it meant awkward application-side stitching or a painful migration. Had they started on Postgres, the flexibility they wanted early was available through its JSON support, and the query they needed later would have been straightforward. That asymmetry is why Postgres is the safer default.
The Honest Counterpoint
Defaulting to Postgres can be taken too far into dogma. There are real products where a document model genuinely fits from day one, and forcing them into rigid relational tables would be its own mistake, adding friction for no benefit. A team deeply fluent in MongoDB will also outbuild a team fighting an unfamiliar Postgres, so existing expertise counts. The claim here is not that Postgres is always right. It is that it is the right default for the typical startup with relational data and unpredictable future queries, and that MongoDB should be a deliberate choice for a specific reason, not the path of least resistance.
What This Means for Hiring
Your database choice quietly shapes hiring, and Postgres helps here too. It consistently ranks as the most-used database in the developer survey, so the pool of engineers comfortable with it is deep and easy to hire from (Stack Overflow 2025). SQL fluency in general is a common, screenable skill, which matters because your data engineers and backend developers will live in the database (how to hire a data engineer). Choose the database your product actually needs, lean toward Postgres when it is a toss-up, and you also get an easier hiring market as a bonus. See available engineers.
Frequently Asked Questions
Should a startup use Postgres or MongoDB?
For most startups, Postgres is the sane default, because application data is usually relational and you will query it in unpredictable ways. Choose MongoDB for a specific reason, not as a general first pick.
Isn't MongoDB easier to start with?
It can feel that way early, when your data shape is in flux. The cost often appears later, when you need to query across documents that were never designed to be joined. Postgres also handles flexible JSON.
When does MongoDB make sense?
When your data is genuinely document-shaped and rarely joined, when you need its particular scaling model, or when your team is already fluent and productive in it.
Does the database affect hiring?
Yes. Postgres is the most-used database, so the pool of engineers comfortable with it is large. SQL skills in general are common and easy to screen for.
The Bottom Line
Default to Postgres. Most startup data is relational, your future queries are unpredictable, and a relational database with strong JSON support gives you both structure and flexibility. MongoDB is a fine tool for genuinely document-shaped problems, but it should be a deliberate choice, not the easy-feeling default that turns painful the first time you need to join your data. When in doubt, relational, and enjoy the deeper hiring pool while you are at it.
Roberto Espinoza is CEO of Ruzora, which helps US startups hire pre-vetted senior LATAM engineers, with a vetted shortlist in 72 hours. See available engineers.
