Re: [PATCH] Introduce array_shuffle() and array_sample()

> On 3 Apr 2023, at 23:46, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Daniel Gustafsson <daniel(at)yesql(dot)se> writes:
>> On 29 Sep 2022, at 21:33, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> I find this behavior a bit surprising:
>>>
>>> +SELECT array_dims(array_sample('[-1:2][2:3]={{1,2},{3,NULL},{5,6},{7,8}}'::int[], 3));
>>> + array_dims
>>> +-------------
>>> + [-1:1][2:3]
>>> +(1 row)
>>>
>>> I can buy preserving the lower bound in array_shuffle(), but
>>> array_sample() is not preserving the first-dimension indexes of
>>> the array, so ISTM it ought to reset the first lower bound to 1.
>
>> I might be daft but I'm not sure I follow why not preserving here, can you
>> explain?
>
> Because array_sample selects only some of the (first level) array
> elements, those elements are typically not going to have the same
> indexes in the output as they did in the input. So I don't see why
> it would be useful to preserve the same lower-bound index. It does
> make sense to preserve the lower-order index bounds ([2:3] in this
> example) because we are including or not including those array
> slices as a whole.

Ah, ok, now I see what you mean, thanks! I'll try to fix up the patch like
this tomorrow.

--
Daniel Gustafsson