This is lying to the matcher, and will result in incorrect results of matches.

It's better not to define these methods if you can't return a bound - in that case the default implementations in backends/database.cc will be used:

```
Xapian::termcount 
Database::Internal::get_wdf_upper_bound(const string & term) const 
{ 
    // Not a very tight bound in general, but this is only a fall-back for 
    // backends which don't store these stats. 
    return get_collection_freq(term); 
}
```