@Override
  protected Void visitSubqueryExpression(SubqueryExpression node, C context) {
    // Don't traverse into Subqueries within an Expression
    return null;
  }
When walking Expressions, don't traverse into SubqueryExpressions
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/sql/ast/DefaultExpressionTraversalVisitor.java