need to be able to specify connections for scheduled refresh

[jayang]

Currently when you set up auto refresh, all connections in the data model will be refreshed, which sometimes can be a huge waste of time and resources. I can have 100 data connections but only one of them needs to be refreshed daily. We need to be able to specify which connections to auto refresh and the interval for each. Otherwise, as long as one connection doesn't work, the whole auto refresh is useless.