Feature Request: DynamoDB GSI multi-attribute key support

[theomantz-luna]

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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.