midCosmos DB
What is indexing in Cosmos DB and why is it important?
Updated Feb 20, 2026
Short answer
Indexing in Azure Cosmos DB automatically creates indexes on data to make queries faster without manual configuration.
Deep explanation
Unlike traditional databases where indexing is manually configured, Cosmos DB automatically indexes all fields by default. This allows fast query performance without extra setup.
However, indexing increases write cost because every insert or update must also update the index. Developers can customize indexing policies to:
- Exclude certain paths
- Use range or spatial indexes
- Reduce RU costs
Efficient indexing balances query speed and write performance.
Real-world example
In a blog platform:
- Searching posts by title is fast because the title field is indexed.
- If large text fields are excluded from indexing, storage and RU costs decrease.
Common mistakes
- - Disabling indexing completely (hurts query performance).
- - Not tuning indexing policies for workload patterns.
- - Assuming indexing has no cost.
Follow-up questions
- What is a composite index?
- How does indexing affect RU consumption?
- When should you exclude fields from indexing?