What are cross-partition queries in Cosmos DB?

In Cosmos DB, cross-partition queries refer to queries that retrieve data from multiple logical partitions within a container. By default, Cosmos DB treats each logical partition as an isolated unit of data, and queries operate within the context of a single partition. However, there are scenarios where you may need to retrieve data that spans multiple partitions, which is where cross-partition queries come into play.

Here are key points to understand about cross-partition queries:

  1. Default Behavior: By default, queries in Cosmos DB operate within the scope of a single logical partition. When a query is executed, it targets a specific partition based on the partition key values specified in the query predicates.
  2. Cross-Partition Query: A cross-partition query, on the other hand, retrieves data from multiple logical partitions within a container. It allows you to query data that spans across partition boundaries and retrieve a result set that combines data from multiple partitions.
  3. Performance Considerations: Cross-partition queries may have performance implications. Retrieving data from multiple partitions involves additional network round-trips and coordination across the distributed system, potentially impacting query performance and consuming more request units (RUs).
  4. Partitioning Strategy: Choosing an appropriate partitioning strategy and partition key is crucial to minimize the need for cross-partition queries. By designing your data model and queries to operate within a single partition, you can achieve better performance and scalability.
  5. Cross-Partition Query Enablement: To execute cross-partition queries, you need to enable the “AllowScanInQuery” property in the Cosmos DB SDK or query options. This property signifies that you explicitly allow the query to scan multiple partitions. Enabling this property is necessary because cross-partition queries have potential performance and cost implications.
  6. Querying Across All Partitions: If you need to query all partitions within a container, you can use the “CrossPartition” option or set the partition key value to the special value of undefined or null in the query. This approach retrieves data from all partitions but comes with performance considerations.

It’s important to note that cross-partition queries should be used judiciously and optimized for performance. Minimizing the need for cross-partition queries through careful data modeling and query design is generally recommended to achieve the best performance and scalability in Cosmos DB.

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