Postgres vs MongoDB

The Postgres vs MongoDB question comes up often enough that it has become almost tribal. In practice, both are capable operational databases, and the right choice depends on data shape, team background, and what you plan to build next.

This post covers the genuine trade-offs rather than the talking points, including where each database has real weaknesses and what happens when analytics needs grow beyond what either can handle.

The fundamental difference: data model

Postgres is a relational database. Data lives in tables with defined columns and types. Relationships between entities are expressed as foreign keys. The schema enforces what data can exist.

MongoDB is a document database. Data lives in collections of BSON documents. Each document can have different fields. The schema is optional and enforced at the application layer.

This difference is real and matters:

Relational wins when: your data has consistent structure, you need joins across entities, you need strict data integrity, or you're running SQL queries for analytics.

Document wins when: your data shape varies across records, you're frequently reading or writing entire nested objects, or your schema changes rapidly during development.

Where Postgres wins

Joins and relational queries

Postgres handles multi-table joins natively with the query planner optimizing execution order, join type, and index usage. A query joining orders, customers, products, and discounts runs efficiently with proper indexes:

SELECT
    c.email,
    p.name                                  AS product_name,
    o.created_at,
    o.total_amount,
    d.pct_discount
FROM orders AS o
INNER JOIN customers AS c ON o.customer_id = c.id
INNER JOIN order_items AS oi ON oi.order_id = o.id
INNER JOIN products AS p ON oi.product_id = p.id
LEFT JOIN discount_codes AS d ON o.discount_code = d.code
WHERE o.created_at >= now() - INTERVAL '30 days'
  AND c.tier = 'enterprise'
ORDER BY o.total_amount DESC;

MongoDB can model the same data, but joins (called $lookup) are slower and less expressive. Teams using MongoDB for relational data patterns often end up denormalizing everything into documents, which creates update anomalies and storage bloat.

Schema enforcement and data integrity

Postgres enforces column types, NOT NULL constraints, foreign key constraints, and check constraints at the database level. Bad data can't enter. You don't need application-level validation for data integrity rules.

CREATE TABLE orders (
    id          UUID DEFAULT gen_random_uuid() PRIMARY KEY,
    customer_id UUID NOT NULL REFERENCES customers(id),
    amount      NUMERIC(12,2) CHECK (amount > 0),
    status      TEXT NOT NULL CHECK (status IN ('pending', 'confirmed', 'shipped', 'cancelled')),
    created_at  TIMESTAMPTZ NOT NULL DEFAULT now()
);

MongoDB's schema validation ($jsonSchema) exists but is opt-in and less ergonomic. Teams often skip it, leading to inconsistent documents that require defensive coding in every query.

Transactions across multiple collections

Postgres transactions are ACID across any combination of tables. Multi-statement operations either fully succeed or fully roll back. This is the fundamental building block of reliable financial and inventory systems.

MongoDB added multi-document transactions in version 4.0, but they come with significant performance overhead and are generally avoided in write-heavy workloads. The recommended pattern for MongoDB is to design documents so that operations are atomic within a single document, which constrains your data model significantly.

JSONB for flexible data within a structured schema

Postgres's JSONB gives you MongoDB-style document flexibility within a relational schema. A properties JSONB column stores arbitrary structured data per row with full GIN indexing, while the surrounding table keeps its relational integrity:

-- Relational structure + flexible JSON properties
CREATE TABLE events (
    id          UUID PRIMARY KEY,
    user_id     UUID NOT NULL REFERENCES users(id),
    event_type  TEXT NOT NULL,
    occurred_at TIMESTAMPTZ NOT NULL,
    properties  JSONB
);

CREATE INDEX ON events USING GIN (properties);
CREATE INDEX ON events (user_id, occurred_at DESC);

For many teams considering MongoDB for schema flexibility, Postgres with JSONB columns achieves the same result without giving up joins, constraints, or SQL.

Where MongoDB wins

Genuinely variable document structure

If different records genuinely need different fields, not just nullable columns but structurally different shapes, MongoDB's document model fits more naturally. Content management systems where article types have completely different metadata structures, or product catalogs where electronics and clothing have nothing in common, benefit from the document model.

Forcing variable-structure data into a relational table usually results in nullable columns or a separate properties table with EAV (Entity-Attribute-Value) patterns that are slow to query and hard to maintain.

Deeply nested writes

For data with deep nesting that is always read and written as a unit, MongoDB's document model avoids the join overhead on reads and the multi-table transaction overhead on writes. A user profile with nested addresses, preferences, and notification settings stored as a single MongoDB document is simpler to work with than 5 normalized Postgres tables.

Horizontal sharding at extreme write scale

MongoDB's built-in sharding (mongos router, config servers, sharded collections) is more operationally accessible than Postgres horizontal sharding options (Citus, manual partitioning). For write throughput that genuinely exceeds what a single Postgres primary can handle, MongoDB's sharding story is more mature.

At moderate scale (hundreds of millions of rows, thousands of writes per second), Postgres handles it. Beyond that, both databases need careful architecture. MongoDB's default path to horizontal scale is more straightforward.

Development speed for schema-less prototyping

During early development when your data model changes frequently, MongoDB's schema-less nature means you don't update migrations for every field addition. This is a genuine advantage in the prototype phase, though the benefits often disappear in production when inconsistent documents become a maintenance burden.

Analytics: where both hit limits

Neither Postgres nor MongoDB is built for large-scale analytical queries. Both are OLTP databases optimized for transactional reads and writes, not for aggregating millions of rows to answer business questions.

Postgres analytics limits:

• Row storage means reading all columns for aggregation queries
• Long analytical queries block OLTP traffic on shared instances
• No columnar compression, so storage costs scale linearly with data

MongoDB analytics limits:

• Aggregation pipeline is expressive but slow on large collections
• No columnar storage, no vectorized execution
• $lookup (join) on large collections is notoriously slow
• Collection scans for analytics queries compete with OLTP reads

For both databases, the production pattern at analytics scale is to stream data into a columnar store. Real-time change data capture from Postgres or MongoDB into ClickHouse® keeps the analytical layer fresh without taxing the OLTP database with analytical queries.

Migration and operational considerations

Identifying scale problems in Postgres before they become production incidents is straightforward: watch pg_stat_activity for long-running queries, monitor index bloat, and track autovacuum lag. The signals are clear.

MongoDB operations are less transparent. Diagnosing a slow aggregation pipeline, understanding WiredTiger cache pressure, or debugging a sharding imbalance requires MongoDB-specific expertise that is less commonly available than Postgres operational knowledge.

For teams without dedicated DBA resources, Postgres tends to behave more predictably under load and has more accessible tooling for diagnosis.

When to choose each

The analytics layer that works with both

For teams using real-time databases for OLTP and needing analytics at scale, ClickHouse fits alongside both Postgres and MongoDB without replacing them. CDC streams capture changes from either database, transform them into append-optimized ClickHouse tables, and expose analytics queries as HTTP endpoints with sub-100ms latency.

The specific performance difference is architectural. MongoDB stores documents, so even an aggregation that touches only two fields (event_type, user_id) reads the entire document for every row in the collection. ClickHouse stores columns separately, so the same aggregation reads only the two columns it needs, compressed, in vectorized batches. Tinybird's own engineering team documented that simple aggregations on traditional databases take 20-30 seconds, while ClickHouse with 5-minute pre-aggregated rollups returns the same computation in 250ms on a 500M-row dataset. Resend, running 100TB/month on Tinybird, measured 62ms p90 query latency in production without caching.

The best real-time analytics tools consistently distinguish between OLTP databases (Postgres, MongoDB) and OLAP databases (ClickHouse, BigQuery, Redshift). Using an OLAP database for your analytics layer means your OLTP database doesn't need to do both jobs, which is when both it and your analytics layer start to perform well. Postgres handles your order writes and customer profile reads. MongoDB handles your content documents. ClickHouse handles the cross-cutting analytics that neither OLTP database was designed for.

Tinybird for Postgres and MongoDB teams

Tinybird is managed ClickHouse that connects to both Postgres and MongoDB. Data engineering for developers is the pattern: CDC connectors sync changes from your OLTP database into Tinybird in real time. SQL Pipes define the analytics queries. HTTP endpoints expose them to your product.

For Postgres teams, Postgres CDC uses logical replication slots to capture every committed row change. Inserts, updates, and deletes all flow into Tinybird datasources within seconds. Your ClickHouse tables stay current without batch exports, without polling, and without any changes to your existing Postgres configuration.

For MongoDB teams, change stream connectors capture the oplog and transform document mutations into append-optimized ClickHouse rows. Nested document fields are flattened or stored as JSON columns depending on query patterns. The end result is the same: your analytical queries run against ClickHouse, not MongoDB, with sub-second latency instead of the full collection scans MongoDB runs for analytics workloads.

The contrast with running analytics directly on either database: production Postgres instances running analytical queries show measurable query latency increases for concurrent OLTP traffic. MongoDB aggregation pipelines on large collections frequently hit the 16MB cursor document limit and require disk-based sorting that blocks the collection. Moving analytics to ClickHouse via Tinybird eliminates both problems: your OLTP database handles what it was designed for, and your analytics queries run where they run fast.

Resend built their complete Marketing Analytics feature in two weeks using Tinybird, with 62ms p90 query latency in production on 100TB/month. The pipeline from database change to queryable HTTP endpoint takes hours to configure rather than weeks to build. SQL Pipes handle authentication, parameter validation, and rate limiting at the endpoint layer. Real-time change data capture keeps the data fresh. Your team writes SQL and ships dashboards instead of building and maintaining a CDC pipeline.