When to Use DynamoDB Secondary Indexes vs Primary Keys?

DynamoDB primary keys uniquely identify items and provide the most efficient, cost-effective access pattern for single-item lookups or query operations within a specific partition. However, applications need secondary indexes—Global Secondary Indexes (GSIs) or Local Secondary Indexes (LSIs)—when query requirements demand searching or filtering data by attributes other than the table's defined primary key.

Primary Keys: The Default and Most Efficient Access Path

Every DynamoDB table requires a primary key upon creation, configured as either a simple Partition Key (PK) or a composite key consisting of a Partition Key and a Sort Key (SK).

Primary keys are designed for high-throughput, predictable access patterns:

  • Direct Lookups (GetItem): Retrieving an item via its exact Partition Key (and Sort Key, if applicable) provides the fastest response times and consumes the fewest Read Capacity Units (RCUs).
  • Sorted Querying (Query): For composite keys, applications can efficiently query items sharing the same Partition Key while filtering or ordering by the Sort Key using comparison operators like BEGINS_WITH, BETWEEN, or range conditions.
  • Cost Efficiency: Operations using primary keys avoid the additional storage and write costs associated with maintaining auxiliary indexes.

When all application queries can be satisfied using the primary key schema, secondary indexes are unnecessary.

When Secondary Indexes Become Necessary

Secondary indexes serve as alternate projections of table data, enabling queries on non-key attributes without resorting to full table scans. An application should introduce secondary indexes in several core scenarios:

1. Alternate Query Patterns

If a table stores user accounts with UserId as the Partition Key, primary key lookups work perfectly for loading profile pages. However, if the application needs to support logging in via EmailAddress or searching by PhoneNumber, querying these attributes using the primary key is impossible. Creating a Global Secondary Index (GSI) with EmailAddress or PhoneNumber as the index partition key enables efficient, targeted lookups for these secondary access patterns.

2. Multi-Attribute Range Queries

Composite primary keys limit range queries to a single attribute (the table's Sort Key). If an application needs to query data using different sorting or range conditions—for example, querying orders by OrderDate in one view and by TotalAmount in another—a secondary index allows the application to project a new Partition Key and Sort Key combination to support the additional ordering requirement.

3. Sparse Indexing for Filtering Subsets

In large datasets, applications often need to query a small subset of items that meet specific criteria, such as "all pending orders" or "users requiring verification." By projecting only items that contain a specific attribute into a secondary index (a sparse index), DynamoDB automatically omits items lacking that attribute. Queries against a sparse index evaluate only relevant records, significantly reducing read costs and improving response times compared to scanning the primary table.

4. Overloading Partition Keys in Single-Table Design

In advanced DynamoDB single-table designs, multiple entity types (such as Users, Orders, and Products) reside in a single table. Secondary indexes allow applications to invert or rearrange relationships (e.g., swapping PK and SK in an index) to support reverse lookups, such as finding all orders belonging to a customer versus loading a specific order directly.

Choosing Between Global and Local Secondary Indexes

Once the need for an alternate access path is identified, choose the appropriate index type based on consistency and key constraints:

  • Global Secondary Indexes (GSIs): Can be defined with partition and sort keys different from the base table. They operate asynchronously, support eventual consistency, and can be created or deleted at any time.
  • Local Secondary Indexes (LSIs): Share the same partition key as the base table but use a different sort key. They offer optional strong consistency but must be defined at table creation time and enforce a 10 GB limit per partition key value.

By leveraging primary keys for core access patterns and reserving secondary indexes for alternate lookups, applications maintain optimal performance and cost efficiency in DynamoDB.