Explain EXPLAIN QUERY TREE (children 2)
 Set
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 1)
    ExpressionList (children 1)
     Function extract (children 1)
      ExpressionList (children 2)
       Function identity (children 1)
        ExpressionList (children 1)
         Literal \'abc123\'
       Literal \'(abc)\'
