If I issue a SOQL query that includes aggregate functions, I get back a list of AggregateResult objects. From each of those, I can use ar.get('fieldname') to get the various values. But that assumes I know the names of the field. If I do a Dynamic SOQL query from a VF page controller, where the query is based on input from the user, I may not know the field names ahead of time. How can I determine the fieldnames to use when I want to access the contents of an AggregateResult object?

Having read that an AggregateResult is a special type of SObject, I tried using Dynamic Apex to discover the fields:

List<AggregateResult> lst = database.query('select count(id), grp__c from myobj__c group by grp__c');

Schema.SObjectType sot = lst.getSObjectType();
Schema.DescribeSObjectResult res = sot.getDescribe();
Set<String> setFields = res.fields.getMap().keyset();
System.debug('Fields are: ' + setFields);

But the only field I get back is "id."  

Thanks for your help!