Indexing Policy

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Category: Optimization

Summary: Defines which properties are indexed, trading off query performance vs. write RU cost.

Overview

An indexing policy defines which properties of items in a container are indexed and how, controlling the trade-off between query performance (read efficiency) and write RU cost/storage (since more indexed paths mean more write overhead). By default, Azure Cosmos DB automatically indexes every property, but you can customize the policy to include/exclude specific paths, favoring either a read-heavy or write-heavy workload.

{
  "indexingMode": "consistent",
  "automatic": true,
  "includedPaths": [
    {
      "path": "/*"
    }
  ],
  "excludedPaths": [
    {
      "path": "/\"_etag\"/?"
    }
  ]
}

var containerDefinition = new ContainerDefinition
{
    Id = "Products",
    PartitionKeyPath = "/category",
    IndexingPolicy = new IndexingPolicy
    {
        Automatic = true,
        IndexingMode = IndexingMode.Consistent,
        IncludedPaths = 
        {
            new IncludedPath { Path = "/*" } // Optional when automatic is true
        },
        ExcludedPaths =
        {
            new ExcludedPath { Path = "/description/*" } // Exclude description
        }
    }
};

  string sqlQueryText = "SELECT TOP 10 c.id FROM c WHERE c.Item = 'value1234' AND c.Price > 2";

    QueryDefinition query = new QueryDefinition(sqlQueryText);

    FeedIterator resultSetIterator = container.GetItemQueryIterator(
                query, requestOptions: new QueryRequestOptions
        {
            PopulateIndexMetrics = true
        });

    FeedResponse response = null;

    while (resultSetIterator.HasMoreResults)
        {
          response = await resultSetIterator.ReadNextAsync();
          Console.WriteLine(response.IndexMetrics);
        }
  

More Info & Links

• Microsoft Learn: Indexing policies in Azure Cosmos DB

DP-420 Style Practice Questions

Q1. What is indexed by default in a new Azure Cosmos DB for NoSQL container?

• A. Every property of every item, using the default automatic indexing policy
• B. No properties, until explicitly configured
• C. Only the partition key property
• D. Only properties marked as unique keys

Q2. Why might you exclude certain paths from indexing in a write-heavy workload?

• A. To reduce the RU cost and latency of write operations by avoiding unnecessary index maintenance
• B. To increase the maximum logical partition size
• C. To enable multi-region writes
• D. To allow TTL to be configured

Answer Key

Answers are listed at the very end so you can attempt the questions first.

• Q1 (Indexing Policy): A. Every property of every item, using the default automatic indexing policy — By default, Azure Cosmos DB automatically indexes all properties unless a custom indexing policy excludes some paths.
• Q2 (Indexing Policy): A. To reduce the RU cost and latency of write operations by avoiding unnecessary index maintenance — Excluding unused paths from indexing reduces the RU cost of writes, which is valuable for write-heavy workloads.

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