Global Secondary Indexes (GSIs) are a powerful feature in Amazon DynamoDB that allow users to query data efficiently using alternate keys. While DynamoDB’s primary key structure is ideal for fast access using partition and sort keys, many applications require more flexible querying capabilities. This is where GSIs come into play.

What is a Global Secondary Index?

A Global Secondary Index is an index with a partition key and an optional sort key that can be different from those on the base table. Unlike Local Secondary Indexes (LSIs), which share the same partition key as the main table, GSIs offer the ability to query across all data in the table, regardless of the original partition key.

This flexibility makes GSIs essential for building scalable and performant applications that need to retrieve data in multiple ways without duplicating records or creating additional tables.

Benefits of Using Global Secondary Indexes

  • Flexible Querying: Enables queries on non-primary key attributes. 
  • Improved Performance: Allows targeted queries, reducing the need for full table scans. 
  • Scalability: Helps manage large datasets by optimizing access patterns. 
  • Efficient Data Access: Supports use cases like filtering, sorting, and searching across different attributes. 

How to Create and Use a GSI

  1. Define the GSI: Specify the alternate partition and sort keys based on query needs. 
  2. Provision Throughput (Optional): GSIs can be configured with separate read/write capacity, depending on the table’s mode. 
  3. Index Projection: Choose which attributes to project into the GSI—KEYS_ONLY, INCLUDE, or ALL—based on application needs. 
  4. Querying with GSI: Use the Query API with the GSI name to fetch items based on the new key structure. 

Best Practices for GSIs

  • Keep indexes lean by projecting only necessary attributes. 
  • Monitor GSI performance using Amazon CloudWatch metrics. 
  • Avoid hot partitions by using well-distributed partition keys. 
  • Use GSIs for access patterns not supported by the base table’s key schema. 

Common Use Cases

  • Searching orders by customer ID. 
  • Filtering items by status or type. 
  • Sorting blog posts by publish date or category. 
  • Multi-tenant SaaS applications needing flexible access to user data. 

By leveraging GSIs effectively, teams can build more dynamic, responsive, and efficient serverless applications using Amazon DynamoDB.