What is the role of collections in document databases?

In document databases, collections serve as containers that group related documents together, similar to how tables organize rows in relational databases. Each collection holds documents that typically share a common purpose or structure, but unlike relational tables, they don’t enforce a rigid schema. For example, in MongoDB, a collection might store product data where each document represents a product with fields like name, price, and category. However, documents in the same collection can vary in structure—some might include optional fields like discount, while others omit them. This flexibility allows developers to adapt data models over time without requiring database-wide schema changes.

Collections simplify data organization and querying. By grouping related documents, queries can target specific collections, reducing the scope of data scanned and improving performance. For instance, a users collection might store all user profiles, enabling queries like “find users with a last_login date older than 30 days.” Collections also support indexing, which speeds up searches. In Couchbase, you might create an index on the email field within a customers collection to optimize login lookups. Additionally, collections often align with access control rules, letting administrators restrict permissions (e.g., allowing read access to a reports collection but not write access).

While collections provide structure, their use differs from relational tables. Developers must decide how granular to make collections—too many can complicate management, while too few might reduce query efficiency. A common approach is to group documents that are queried together or share lifecycle patterns. For example, an e-commerce app might use separate orders and inventory collections to isolate transactional data from stock updates. Some document databases, like MongoDB, also support sub-collections or nested documents for hierarchical data. However, over-nesting can lead to performance issues, so it’s important to balance flexibility with practical design.