presto-main/src/main/java/io/prestosql/sql/planner/SubqueryPlanner.java/SubqueryPlanner/extractColumnReferences
class SubqueryPlanner:    private static Set<NodeRef<Expression>> extractColumnReferences(Expression expression, Set<NodeRef<Expression>> columnReferences)
    {
        ImmutableSet.Builder<NodeRef<Expression>> expressionColumnReferences = ImmutableSet.builder();
        new ColumnReferencesExtractor(columnReferences).process(expression, expressionColumnReferences);
        return expressionColumnReferences.build();
    }