Explain EXPLAIN QUERY TREE (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 2)
    ExpressionList (children 2)
     Function length (children 1)
      ExpressionList (children 1)
       Identifier arr
     Function isNull (children 1)
      ExpressionList (children 1)
       Identifier n
    TablesInSelectQuery (children 1)
     TablesInSelectQueryElement (children 1)
      TableExpression (children 1)
       TableIdentifier t_column_names
