What are Cosmos DB materialized views and how do they differ from containers?

Cosmos DB materialized views (in preview as of 2024) are automatically maintained read-only containers that are derived from a source container via a defined query or projection. When data changes in the source container, Cosmos DB updates the materialized view automatically and asynchronously — similar to how the analytical store works but for operational read patterns rather than analytical ones.

The problem they solve: Cosmos DB forces you to choose a single partition key per container. If your source container is partitioned by /userId (efficient for user-centric queries), a query like "find all orders for product X" becomes a cross-partition scan. Without materialized views, you have to either build a Change Feed processor that maintains a second container keyed by /productId, or accept the expensive cross-partition query.

With materialized views, you declare the derived container's partition key and projection, and Cosmos DB handles the Change Feed-based maintenance automatically:

// Source container: partitioned by /userId
// Materialized view: same data partitioned by /productId

{
  "materializedViewDefinition": {
    "sourceCollectionName": "orders",
    "definition": "SELECT c.id, c.productId, c.userId, c.amount FROM orders c",
    "partitionKey": { "paths": ["/productId"] }
  }
}

Key differences from a regular container:

• Materialized views are read-only — writes go only to the source container.
• They are automatically maintained — no Change Feed processor code required.
• They may have a short lag between source write and view update (eventual consistency).
• Storage and throughput for the view are billed separately.

Compared to manually maintaining a secondary container via the Change Feed, materialized views eliminate significant operational code and failure-handling complexity.

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