Re: tuple radix sort

[cca5507]

Hi John,

One additional comment:

```
        /* presorted check */
        for (SortTuple *st = data + 1; st < data + n; st++)
        {
                CHECK_FOR_INTERRUPTS();

                if (COMPARETUP(state, st - 1, st) > 0)
                {
                        presorted = false;
                        break;
                }
        }
        if (presorted)
                return;
```

I think we need to add a comment to explain why we do the
check. The cost of this check is not small.

--
Regards,
ChangAo Chen