MongoDB vs PostgreSQL in 2026: When to Use Which

The Honest Answer

In 2026, PostgreSQL is the default choice for nearly every application. MongoDB has valid use cases, but they are narrower than most developers think. This article explains when each database genuinely excels, without the marketing spin.

<div style="margin:2.5rem auto;max-width:600px;width:100%;text-align:center;"><svg viewBox="0 0 600 180" xmlns="http://www.w3.org/2000/svg" style="width:100%;height:auto;"><rect width="600" height="180" rx="12" fill="#1a1a2e"/><ellipse cx="150" cy="55" rx="60" ry="18" fill="#6366f1" opacity="0.8"/><rect x="90" y="55" width="120" height="50" fill="#6366f1" opacity="0.8"/><ellipse cx="150" cy="105" rx="60" ry="18" fill="#6366f1" opacity="0.9"/><text x="150" y="85" text-anchor="middle" fill="#ffffff" font-size="12" font-family="system-ui" font-weight="bold">Primary</text><text x="150" y="140" text-anchor="middle" fill="#94a3b8" font-size="10" font-family="system-ui">Read + Write</text><ellipse cx="400" cy="30" rx="50" ry="14" fill="#a855f7" opacity="0.7"/><rect x="350" y="30" width="100" height="35" fill="#a855f7" opacity="0.7"/><ellipse cx="400" cy="65" rx="50" ry="14" fill="#a855f7" opacity="0.8"/><text x="400" y="52" text-anchor="middle" fill="#ffffff" font-size="10" font-family="system-ui">Replica 1</text><ellipse cx="400" cy="110" rx="50" ry="14" fill="#a855f7" opacity="0.7"/><rect x="350" y="110" width="100" height="35" fill="#a855f7" opacity="0.7"/><ellipse cx="400" cy="145" rx="50" ry="14" fill="#a855f7" opacity="0.8"/><text x="400" y="132" text-anchor="middle" fill="#ffffff" font-size="10" font-family="system-ui">Replica 2</text><defs><marker id="arrow8" markerWidth="8" markerHeight="6" refX="8" refY="3" orient="auto"><path d="M0,0 L8,3 L0,6" fill="#2dd4bf"/></marker></defs><path d="M212,65 Q280,30 348,48" stroke="#2dd4bf" stroke-width="1.5" fill="none" marker-end="url(#arrow8)"/><path d="M212,90 Q280,130 348,128" stroke="#2dd4bf" stroke-width="1.5" fill="none" marker-end="url(#arrow8)"/><text x="280" y="55" text-anchor="middle" fill="#2dd4bf" font-size="9" font-family="system-ui">WAL stream</text><text x="280" y="130" text-anchor="middle" fill="#2dd4bf" font-size="9" font-family="system-ui">WAL stream</text><text x="500" y="52" text-anchor="start" fill="#94a3b8" font-size="9" font-family="system-ui">Read-only</text><text x="500" y="132" text-anchor="start" fill="#94a3b8" font-size="9" font-family="system-ui">Read-only</text></svg><p style="margin-top:0.75rem;font-size:0.85rem;color:#94a3b8;font-style:italic;line-height:1.4;">Database replication: the primary handles writes while replicas serve read queries via WAL streaming.</p></div>

At TechSaaS, we run both — PostgreSQL 16 as our primary database for most services, and MongoDB 7 for specific workloads like logging and event storage. Here is what we have learned.

PostgreSQL's Superpower: It Does Everything

PostgreSQL is not just a relational database. It is a relational database that also does:

-- JSON document storage in PostgreSQL (yes, this works)
CREATE TABLE products (
    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
    data JSONB NOT NULL,
    created_at TIMESTAMPTZ DEFAULT NOW()
);

INSERT INTO products (data) VALUES ('{
    "name": "Widget Pro",
    "price": 29.99,
    "tags": ["electronics", "gadgets"],
    "specs": {"weight": "150g", "color": "blue"}
}');

-- Query nested JSON with indexes
CREATE INDEX idx_products_name ON products ((data->>'name'));
CREATE INDEX idx_products_tags ON products USING GIN ((data->'tags'));

SELECT * FROM products WHERE data->>'name' = 'Widget Pro';
SELECT * FROM products WHERE data->'tags' ? 'electronics';
SELECT * FROM products WHERE (data->>'price')::numeric < 50;

MongoDB's Genuine Strengths

MongoDB is better than PostgreSQL when you have:

1. Truly Schema-Less Data

When document structures vary wildly and unpredictably:

// IoT device data — each device sends different fields
db.telemetry.insertOne({
  deviceId: "sensor-001",
  type: "temperature",
  value: 23.5,
  unit: "celsius",
  battery: 0.87
});

db.telemetry.insertOne({
  deviceId: "camera-042",
  type: "motion",
  detected: true,
  confidence: 0.94,
  frameUrl: "s3://bucket/frame.jpg",
  objects: ["person", "car"]
});

2. Massive Write Throughput

MongoDB's append-oriented storage engine handles write-heavy workloads better at extreme scale:

// Millions of log entries per hour
db.logs.insertMany([
  { ts: new Date(), level: "info", service: "api", msg: "Request processed" },
  { ts: new Date(), level: "error", service: "worker", msg: "Connection refused" },
  // ... millions more
]);

3. Horizontal Scaling (Sharding)

Native sharding for datasets that genuinely exceed single-server capacity:

sh.shardCollection("mydb.events", { "tenantId": "hashed" });
// Now events are distributed across shards by tenant

Performance Comparison

Real benchmarks on identical hardware (Ryzen 5, 16GB RAM, NVMe SSD):

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Simple Key-Value Lookups

PostgreSQL (indexed): ~0.2ms
MongoDB (indexed):    ~0.3ms
Winner: PostgreSQL (slightly)

Complex Aggregations

-- PostgreSQL: 45ms for 10M rows
SELECT category, COUNT(*), AVG(price), PERCENTILE_CONT(0.95)
WITHIN GROUP (ORDER BY price)
FROM products
GROUP BY category;

// MongoDB: 120ms for 10M documents
db.products.aggregate([
  { $group: {
    _id: "$category",
    count: { $sum: 1 },
    avgPrice: { $avg: "$price" },
    p95Price: { ... }  // More complex in Mongo
  }}
]);
// Winner: PostgreSQL (significantly)

Write Throughput (Bulk Insert)

PostgreSQL (COPY): ~150K rows/sec
MongoDB (insertMany): ~200K docs/sec
Winner: MongoDB (for unstructured bulk writes)

JSON Document Queries

PostgreSQL JSONB (GIN indexed): ~1ms
MongoDB (indexed): ~0.8ms
Winner: Roughly equal

Schema Design Comparison

E-Commerce Order

PostgreSQL (Relational):

CREATE TABLE orders (
    id UUID PRIMARY KEY,
    customer_id UUID REFERENCES customers(id),
    status VARCHAR(20) NOT NULL,
    total NUMERIC(10,2) NOT NULL,
    created_at TIMESTAMPTZ DEFAULT NOW()
);

CREATE TABLE order_items (
    id UUID PRIMARY KEY,
    order_id UUID REFERENCES orders(id),
    product_id UUID REFERENCES products(id),
    quantity INTEGER NOT NULL,
    price NUMERIC(10,2) NOT NULL
);

-- Join to get full order
SELECT o.*, json_agg(oi.*) as items
FROM orders o
JOIN order_items oi ON oi.order_id = o.id
WHERE o.id = $1
GROUP BY o.id;

MongoDB (Document):

db.orders.insertOne({
  _id: ObjectId(),
  customerId: ObjectId("..."),
  status: "confirmed",
  items: [
    { productId: ObjectId("..."), name: "Widget", quantity: 2, price: 29.99 },
    { productId: ObjectId("..."), name: "Gadget", quantity: 1, price: 49.99 }
  ],
  total: 109.97,
  createdAt: new Date()
});

// Single read, no join needed
db.orders.findOne({ _id: orderId });

MongoDB's embedded documents are faster for single-document reads. PostgreSQL's normalized design is better for complex queries across relationships.

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The Decision Framework

Choose PostgreSQL when:

Choose MongoDB when:

Use both when:

This is exactly what we do at TechSaaS, and it works well. The key is picking the right tool for each workload rather than forcing one database to do everything.