CreateQuery nullable_set_index (children 3)
 Identifier nullable_set_index
 Columns definition (children 2)
  ExpressionList (children 2)
   ColumnDeclaration a (children 1)
    DataType UInt64
   ColumnDeclaration b (children 1)
    DataType Nullable (children 1)
     ExpressionList (children 1)
      DataType String
  ExpressionList (children 1)
   Index (children 2)
    Identifier b
    Function set (children 1)
     ExpressionList (children 1)
      Literal UInt64_0
 Storage definition (children 2)
  Function MergeTree
  Identifier a
