Feature Request: DynamoDB GSI multi-attribute key support

[theomantz-luna]

Is there an existing issue for this?

• I have searched the existing issues

Description

LocalStack does not currently support multi-attribute keys for DynamoDB Global Secondary Indexes. Deploying a table that defines a GSI with a multi-attribute key fails.

Environment

• LocalStack version: 4.4
• DynamoDB

Expected behavior

Creating a DynamoDB table with a GSI that uses a multi-attribute key should succeed, matching the behavior of AWS DynamoDB.

Actual behavior

The deploy fails when attempting to create the table.

Reproduction steps

import boto3

client = boto3.client("dynamodb", endpoint_url="http://localhost:4566")

client.create_table(
    TableName="example-table",
    AttributeDefinitions=[
        {"AttributeName": "PK", "AttributeType": "S"},
        {"AttributeName": "SK", "AttributeType": "S"},
        {"AttributeName": "GSI1PK", "AttributeType": "S"},
        {"AttributeName": "GSI1SK", "AttributeType": "S"},
    ],
    KeySchema=[
        {"KeyType": "HASH", "AttributeName": "PK"},
        {"KeyType": "RANGE", "AttributeName": "SK"},
    ],
    GlobalSecondaryIndexes=[
        {
            "IndexName": "GSI1",
            "KeySchema": [
                {"KeyType": "HASH", "AttributeName": "GSI1PK"},
                {"KeyType": "RANGE", "AttributeName": "GSI1SK"},
            ],
            "Projection": {"ProjectionType": "ALL"},
            "OnDemandThroughput": {"MaxReadRequestUnits": 0, "MaxWriteRequestUnits": 0},
            "MultiAttributeKeys": ["GSI1PK", "GSI1SK"],
        }
    ],
    BillingMode="PAY_PER_REQUEST",
)

Additional context

Multi-attribute keys for GSIs were introduced by AWS to allow multiple table attributes to be combined into a single index key attribute. This is useful for creating flexible access patterns without concatenating attributes at the application layer. See the AWS documentation for details.

[HarshCasper]

Hi,

Thanks for the report. I looked into this, and the good news is that multi-attribute GSI keys do work.

Multi-attribute keys for GSIs were implemented inside DynamoDB Local, and AWS only added them in DynamoDB Local 3.3.0 (released 19 January 2026). LocalStack 4.4 ships an older 2.x build, which is why you got the "Key Schema too big" error. Newer LocalStack versions automatically pull a 3.3.0+ build, so the resolution is to upgrade.

Let me know if it still fails after upgrading, and I will take another look.