I need to create a filter which gets all the possible values for a concrete field, but I'm only getting the values which appears in the current page.
Result
What can I do to get all the values?
It looks like you want to filter table basing on value in the dropdown. In order to do this you need to bind your dropdown's value to datasource's filter:
@datasources.MyData.query.filters.Country._equals
then you need to reload your datasource every time when user changes dropdown's value
// onValueEdit
app.datasources.MyData.load();
You can play with this sample app to learn more: https://developers.google.com/appmaker/samples/project-list/