Explain EXPLAIN QUERY TREE (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 2)
    ExpressionList (children 4)
     Function length (children 1)
      ExpressionList (children 1)
       Identifier arr
     Function empty (children 1)
      ExpressionList (children 1)
       Identifier arr
     Function notEmpty (children 1)
      ExpressionList (children 1)
       Identifier arr
     Function empty (children 1)
      ExpressionList (children 1)
       Identifier n
    TablesInSelectQuery (children 1)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier t_func_to_subcolumns
