Overview
Pick Postgres when data has relationships, you need joins and transactions, or documents are one part of a relational model; pick MongoDB when the data is naturally one self-contained document per entity and you want native sharding. As of 2026-10-01 PostgreSQL 18 is current (19 in beta) and MongoDB 9.0 is the latest stable release (8.0 and 7.0 remain listed as stable).
Comparison
| Dimension | Postgres 18 | MongoDB 9 |
|---|---|---|
| Model | Relational tables plus jsonb | BSON documents in collections |
| Schema | Declared; enforced by migrations | Flexible per document; optional schema validation |
| Joins | Native, planned by the optimizer | $lookup in aggregation pipelines |
| Transactions | Default, any statements | Multi-document transactions supported |
| JSON indexing | GIN on jsonb, expression and partial indexes | Native indexes on fields, wildcard, text, geo |
| Horizontal scale | Read replicas; Citus or application sharding | Built-in sharded clusters |
| Search and vectors | tsvector full-text, pgvector | Atlas Search and Atlas Vector Search (Atlas) |
| License | PostgreSQL License | SSPL, not OSI-approved; offering it as a service requires releasing service source |
Pick Postgres when
Pick Postgres for most products.
- Relational data with some flexible attributes: put them in a
jsonbcolumn and index paths you query. See postgres-jsonb. - Reporting, joins, constraints, and foreign keys on the same store as application data.
- One database for relational rows, JSON documents, vectors, and geospatial data.
- Permissive licensing and many managed hosts.
- Multi-TB time series or event streams: partitioning or TimescaleDB before sharded MongoDB.
Pick MongoDB when
Pick MongoDB when the document model is the design, not a workaround.
- Entities are read and written as whole documents, with few cross-document joins.
- Many producers write documents whose shape changes faster than you can migrate (event firehoses, raw analytics layers).
- You need built-in sharding for a very large dataset and want it as a first-class feature.
- You rely on Atlas features such as Atlas Search or Atlas Vector Search, and accept the vendor tie.
- The SSPL is acceptable. Review it with legal if you plan to offer MongoDB itself as a service.
- The team already operates MongoDB well.
Migrate
MongoDB to Postgres: model the relations first, land documents in jsonb, then flatten fields as access patterns settle; dual-write behind a change stream during cutover and compare query results. Postgres to MongoDB is rare and requires denormalizing joins into documents or $lookup pipelines. A cheaper alternative to leaving Postgres is moving hot, simple data (sessions, caches) to a key-value store.
FAQ
Can Postgres replace MongoDB?
For most new workloads, yes: a jsonb column with GIN indexes covers common document patterns beside relational data. Pick MongoDB when data is naturally one self-contained document per entity and you need native sharding.
Is MongoDB open source?
No; MongoDB uses the SSPL, which is not OSI-approved and requires releasing service source if you offer MongoDB as a service. Postgres uses the permissive PostgreSQL License.
Does MongoDB support transactions?
Yes; MongoDB supports multi-document transactions. Postgres runs every statement in a transaction by default.
How do I migrate from MongoDB to Postgres?
Model the relations first, land documents in jsonb, then flatten fields as access patterns settle. Dual-write behind a change stream during cutover and compare query results.