When does an aggregate with the Fetch property set to "only on demand" run?

An aggregate with the Fetch property set to "only on demand" runs programmatically, which is typically invoked using a Refresh Data node in a Screen Action. This means that such an aggregate does not execute during the screen initialization or automatically based on other actions, but rather only when specifically triggered by an explicit request from the developer.

This approach allows developers to control when data is fetched, which can be beneficial for performance reasons, as it avoids loading data that may not be needed immediately. By ensuring that the aggregate runs only when called by a Refresh Data node, the application can optimize resource usage and enhance responsiveness by fetching data at the right moment, based on user interactions or specific conditions.

This selective fetching is particularly useful in scenarios where data may change frequently or when dealing with large datasets, where loading everything at once could lead to slower performance or increased load times. Thus, the correct understanding of the fetch property being set to "only on demand" highlights the importance of explicit triggers for data retrieval in OutSystems applications.