What is Secondary Index in DynamoDB: Explanation and Example

A secondary index in DynamoDB is a way to create an alternate view of your data to query it efficiently using different keys than the primary key. It lets you search the table by attributes other than the main key without scanning the entire table.

How It Works

Think of a DynamoDB table like a big phone book sorted by last name (the primary key). If you want to find someone by their phone number instead, you would have to look through every page, which is slow. A secondary index is like having a separate mini phone book sorted by phone numbers, so you can quickly find the person using that attribute.

Technically, a secondary index creates a copy of selected attributes from the main table, organized by a different key you choose. This lets DynamoDB quickly find items based on that key without scanning the whole table. There are two types: Global Secondary Index (GSI) which can use any attribute as a key, and Local Secondary Index (LSI) which shares the same partition key but uses a different sort key.

Example

This example shows how to create a Global Secondary Index on a DynamoDB table to query by a different attribute.

python

import boto3

dynamodb = boto3.resource('dynamodb')

# Create table with primary key 'UserId'
# and a Global Secondary Index on 'Email'

table = dynamodb.create_table(
    TableName='Users',
    KeySchema=[
        {'AttributeName': 'UserId', 'KeyType': 'HASH'}  # Partition key
    ],
    AttributeDefinitions=[
        {'AttributeName': 'UserId', 'AttributeType': 'S'},
        {'AttributeName': 'Email', 'AttributeType': 'S'}
    ],
    GlobalSecondaryIndexes=[
        {
            'IndexName': 'EmailIndex',
            'KeySchema': [
                {'AttributeName': 'Email', 'KeyType': 'HASH'}
            ],
            'Projection': {
                'ProjectionType': 'ALL'
            },
            'ProvisionedThroughput': {
                'ReadCapacityUnits': 5,
                'WriteCapacityUnits': 5
            }
        }
    ],
    ProvisionedThroughput={
        'ReadCapacityUnits': 5,
        'WriteCapacityUnits': 5
    }
)

table.wait_until_exists()

print('Table and GSI created successfully')

Output

Table and GSI created successfully

When to Use

Use a secondary index when you need to query your DynamoDB table by attributes other than the primary key. For example, if your table's primary key is UserId but you want to find users by their Email or PhoneNumber, a secondary index lets you do this efficiently.

Secondary indexes are useful in real-world cases like searching customers by email, orders by status, or products by category without scanning the entire table, which saves time and cost.

Key Points

• A secondary index provides an alternate way to query data in DynamoDB.
• Global Secondary Index (GSI) can use any attribute as a key.
• Local Secondary Index (LSI) shares the partition key but uses a different sort key.
• Indexes improve query speed but add some storage and write cost.
• Use indexes to avoid slow full table scans when querying by non-primary keys.

Key Takeaways

A secondary index lets you query DynamoDB by attributes other than the primary key.

Global Secondary Indexes (GSI) allow flexible keys, while Local Secondary Indexes (LSI) share the partition key.

Indexes speed up queries but may increase storage and write costs.

Use secondary indexes to efficiently find data without scanning the whole table.

Plan indexes based on your query patterns to optimize performance.