final Set<SingleAggregateFunction> aggFuncSet = Sets.newHashSetWithExpectedSize(context.getExpressionManager().getExpressionCount());
Iterator<Expression> expressions = context.getExpressionManager().getExpressions();
while (expressions.hasNext()) {
Expression expression = expressions.next();
expression.accept(new SingleAggregateFunctionVisitor() {
@Override
public Iterator<Expression> visitEnter(SingleAggregateFunction function) {
aggFuncSet.add(function);
return Iterators.emptyIterator();
}