Re: tuple radix sort

>I did some more self-review after a couple weeks and made some more
>minor cosmetic adjustments for v7. I will commit 0001 and 0002 in a
>few days unless there are objections.

Hi John,

I'm wondering if we should replace `state->memtuples` with `data` in the `sort_byvalue_datum()` function here.
```
    if (nulls_first)
    {
        null_start = state->memtuples;
        null_count = d1;
        not_null_start = state->memtuples + d1;
        not_null_count = d2;
    }
    else
    {
        not_null_start = state->memtuples;
        not_null_count = d1;
        null_start = state->memtuples + d1;
        null_count = d2;
    }
```
If my understanding is wrong, please ignore this.

--
regards,
Man Zeng