Defect Tracker #25035: The current structuring of Symbol objects is confusing and makes planning more difficult than necessary. Changes that should be made:
AggregateSymbol should become AggregateFunction (which should not inherit from Function) and not be a Symbol.
SingleElementSymbol, ExpressionSymbol, and AliasSymbol should be merged to a single class DerivedSymbol
DerivedSymbol and MultipleElementSymbol should implement SelectSymbol (as an interface rather than base class) and SelectSymbol should provide a method for getting the projected symbols.
ElementSymbol should not directly be a DerivedSymbol.