What is a primary key in a document database?

A primary key in a document database is a unique identifier for a document within a collection. Unlike relational databases, where primary keys often reference columns in structured tables, document databases use a field—typically named _id—to ensure each document can be uniquely located. This key is mandatory; if you don’t provide one when creating a document, the database will automatically generate it (e.g., MongoDB uses an ObjectId). The primary key’s core role is to prevent duplicate documents and enable efficient retrieval, serving as the foundation for operations like updates, deletions, or lookups.

The primary key’s uniqueness is enforced by the database, and it often doubles as the default index for optimizing query performance. For example, in MongoDB, querying a document by its _id is fast because the database maintains an index on this field. This avoids full collection scans, which would slow down operations. While the primary key is usually a single field, some document databases allow composite keys (combining multiple fields), though this is less common. Developers can also assign custom values to the primary key (e.g., a username or email) if they align with uniqueness requirements, but care must be taken to avoid conflicts.

A practical example: Suppose you’re building a user profile system. Each user document might include fields like name, email, and age. By setting email as the _id, you ensure no two users share the same email. Alternatively, relying on an auto-generated ObjectId lets the database handle uniqueness. A key consideration is that primary keys in distributed systems may influence data partitioning. For instance, in sharded MongoDB clusters, the _id determines how documents are distributed across servers. Choosing a meaningful primary key (like a timestamp or geohash) can optimize data locality, while poorly chosen keys might lead to uneven storage or performance bottlenecks.