That's a good idea.  

Technically, the current version exploits a vagary of the ```IdFormatter```, which is that the entire ID is always written out, even when it exceeds the maximum length; the formatter only ensures that strings that are less than that length get padded out with underscores:

```scala
def format(entry: SpatioTemporalIndexEntry): Text = new Text(entry.sid.padTo(maxLength, "_").mkString)
```