Re: tuple radix sort

Hi,

> I'm wondering if we should replace `state->memtuples` with `data` in the `sort_byvalue_datum()` function here.

+1. Now data == state->memtuples, how about remove the parameter "data" and "n" and just
get them from "Tuplesortstate"?

Some comments for v7:

1)

```
/*
 * Retrieve byte from datum, indexed by 'level': 0 for LSB, 7 for MSB
 */
static inline uint8
current_byte(Datum key, int level)
{
	int			shift = (SIZEOF_DATUM - 1 - level) * BITS_PER_BYTE;

	return (key >> shift) & 0xFF;
}
```

Maybe "0 for MSB, 7 for LSB"? If level == 0, this function will return the Most Significant Byte.

2) radix_sort_tuple()

```
		size_t		end_offset = partitions[*rp].next_offset;
		SortTuple  *partition_end = begin + end_offset;
		ptrdiff_t	num_elements = end_offset - start_offset;
```

Why the type of "num_elements" is "ptrdiff_t"? Maybe just "size_t"?

3) tuplesort_sort_memtuples()

```
		/*
		 * Do we have the leading column's value or abbreviation in datum1?
		 */
		if (state->base.haveDatum1 && state->base.sortKeys)
		{
			SortSupportData ssup = state->base.sortKeys[0];
```

I think we should avoid the copy of SortSupportData. We can just use a pointer.

4)

Many places just consider "Datum" as integer, do we need to add a DatumGetUInt**() for them?

--
Regards,
ChangAo Chen