I'm Trong, a developer on the AWS Development Team here at Mendix. Thank you for bringing up this issue concerning the BatchGetItem action and DynamoDB integration. Your feedback is invaluable to us as we aim to improve and expand our features.
I understand that you are looking to retrieve a list of items using only the partition key from a DynamoDB table within a Mendix microflow. While it's unfortunate that the Query function isn't readily available in our Amazon DynamoDB Connector, I want you to know that I have added an item to our backlog to investigate and develop the connector action Query. We're eager to learn more about your use case or that of your client. Your insights will help our team prioritize the development of this requested feature. Would you be willing to elaborate on the use case?
In the meantime, I would recommend you explore constructing this action using the REST API interface of Amazon DynamoDB service. A colleague of mine, Ayça Öner, has written an extensive blog post on how to achieve this, which you can find here: Building Your Own REST API Based AWS Connector. You might also find AWS' DynamoDB API reference useful for understanding the Query API call: AWS DynamoDB API Query Reference.
If you have further questions or need more clarification, please don't hesitate to respond to this thread. Alternatively, you can send me a private message by going to my developer profile and clicking on "Contact."
AWS Development Team at Mendix