Explain EXPLAIN AST (children 1)
 SelectWithUnionQuery (children 1)
  ExpressionList (children 1)
   SelectQuery (children 1)
    ExpressionList (children 1)
     Function isNotDistinctFrom (children 1)
      ExpressionList (children 2)
       Function isNull (children 1)
        ExpressionList (children 1)
         Literal NULL
       Literal NULL
