What are secondary indexes in document databases?

Secondary indexes in document databases are additional data structures that improve query performance on fields other than the primary key. Unlike the primary index, which is automatically created for the document’s unique identifier (like _id in MongoDB), secondary indexes are explicitly defined by developers on specific fields. These indexes act as shortcuts, allowing the database to locate data without scanning every document in a collection. For example, if you frequently query a users collection by email, creating a secondary index on the email field lets the database retrieve matching documents efficiently. Secondary indexes are essential for optimizing read operations in schemaless databases, where documents can have varying structures.

Secondary indexes work by maintaining a sorted or hashed representation of the indexed field’s values, along with pointers to the original documents. This structure enables faster lookups, sorting, and filtering. For instance, in a product catalog stored in a document database, a secondary index on price allows queries like “find all products under $50” to execute quickly. Compound secondary indexes (indexes on multiple fields, like category and price) further optimize queries that filter or sort using those fields together. However, indexes require storage and computational overhead: each write operation (insert, update, delete) must update the index, which can slow down writes. This trade-off means developers must strategically choose which fields to index based on query patterns.

When using secondary indexes, consider their impact on database performance and maintenance. Over-indexing can degrade write throughput and increase storage costs, while under-indexing may lead to slow queries. For example, an e-commerce app might index product_category and creation_date to speed up category-based browsing and time-based filtering but avoid indexing rarely queried fields like supplier_notes. Some document databases also support features like partial indexes (indexing only a subset of documents) or TTL (time-to-live) indexes for auto-expiring data, which help reduce overhead. By analyzing query requirements and monitoring performance, developers can balance the benefits of secondary indexes with their costs, ensuring efficient data access without unnecessary resource consumption.