I'm unhappy with MongoDB for transactional app data. Is PostgreSQL a better default?

Yes — for most transactional application data, PostgreSQL is usually the better default.

Why PostgreSQL is often the better choice

• Strong ACID transactions: excellent for money, orders, inventory, user state, and anything that must stay consistent.
• Relational model fits app data well: most business data is naturally related.
• Powerful constraints: foreign keys, uniqueness, check constraints, etc. help prevent bad data.
• Great query flexibility: joins, aggregates, subqueries, window functions.
• Mature ecosystem: tooling, extensions, migrations, replication, backups, observability.

Where MongoDB can be a good fit

MongoDB is often fine when:

• your data is highly document-shaped
• schemas vary a lot between records
• you mostly fetch/update whole documents
• you want to optimize for developer speed in a document model
• the app doesn’t need many relational constraints

The common trap

Many teams choose MongoDB because they expect “schema-less” to make development easier, but later run into:

• awkward relationships
• duplicate data
• inconsistent documents
• hard-to-enforce invariants
• complex application-side transaction logic

PostgreSQL usually reduces those problems.

Practical default recommendation

If you’re building a normal web/app backend and your data has:

• users
• accounts
• permissions
• orders
• payments
• events
• inventory
• workflows

then PostgreSQL should be your default.

When I’d still pick MongoDB

Use MongoDB if:

• your core entities are naturally document-oriented
• nested documents are the main access pattern
• you rarely need joins or strong cross-entity constraints
• you’re storing semi-structured content, logs, or rapidly evolving payloads

Short version

For transactional app data, PostgreSQL is generally the safer, more reliable default.
MongoDB is better as a specialized choice, not the default for most business applications.

If you want, I can also give you a decision checklist or a Postgres-vs-MongoDB comparison for your specific app.