presto-main/src/main/java/io/prestosql/sql/planner/ExpressionInterpreter.java/Visitor/visitSubqueryExpression
class Visitor:        @Override
        protected Object visitSubqueryExpression(SubqueryExpression node, Object context)
        {
            if (!optimize) {
                throw new UnsupportedOperationException("Subquery not yet implemented");
            }
            return node;
        }