anchor.diff:      {Field, FieldType, _OldVal} = lists:keyfind(Field, 1, TupleList),
       lists:keyreplace(Field, 1, TupleList, {Field, FieldType, NewVal}).
   
  +-record(filtercheck, {name,'='}).
  +
   check_where_clause_is_possible(DDL, WhereProps) ->
       Filter1 = proplists:get_value(filter, WhereProps),
       {Filter2, _} = riak_ql_ddl:mapfold_where_tree(
  
positive.code:     #riak_field_v1{optional = Optional} = lists:keyfind(FieldName, #riak_field_v1.name, Fields),
      (Optional == true).
  
  
  %% tl;dr put filters that test equality of columns in the local key but not in
  %% in the partition key, in the star and end key.
  
negative.code:         false ->
              throw({error, {impossible_where_clause, << >>}})
      end;
  check_where_clause_is_possible_fold(DDL, _, {'!=',{field,FieldName,_},{const,?SQL_NULL}} = F, Acc) ->
      %% `NOT NULL` filters
      %% if column is marked NOT NULL, and the WHERE clause has IS NOT NULL for
      %% that column then we can eliminate it, and reduce the filter, maybe to
  