From: | Martin Kalcher <martin(dot)kalcher(at)aboutsource(dot)net> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Robert Haas <robertmhaas(at)gmail(dot)com> |
Cc: | John Naylor <john(dot)naylor(at)enterprisedb(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: [PATCH] Introduce array_shuffle() and array_sample() |
Date: | 2022-07-18 21:48:55 |
Message-ID: | 0b5dea2b-37fa-6c81-ea9d-0341ab6f87d6@aboutsource.net |
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Lists: | pgsql-general pgsql-hackers |
Am 18.07.22 um 23:03 schrieb Tom Lane:
> I wrote:
>> Martin had originally proposed (2), which I rejected on the grounds
>> that we don't treat multi-dimensional arrays as arrays-of-arrays for
>> any other purpose.
>
> Actually, after poking at it for awhile, that's an overstatement.
> It's true that the type system doesn't think N-D arrays are
> arrays-of-arrays, but there are individual functions/operators that do.
> Thanks Robert for pointing out the inconsistent behavior of
array_sample(). That needs to be fixed.
As Tom's investigation showed, there is no consensus in the code if
multi-dimensional arrays are treated as arrays-of-arrays or not. We need
to decide what should be the correct treatment.
If we go with (1) array_shuffle() and array_sample() should shuffle each
element individually and always return a one-dimensional array.
select array_shuffle('{{1,2},{3,4},{5,6}}');
-----------
{1,4,3,5,6,2}
select array_sample('{{1,2},{3,4},{5,6}}', 3);
----------
{1,4,3}
If we go with (2) both functions should only operate on the first
dimension and shuffle whole subarrays and keep the dimensions intact.
select array_shuffle('{{1,2},{3,4},{5,6}}');
---------------------
{{3,4},{1,2},{5,6}}
select array_sample('{{1,2},{3,4},{5,6}}', 2);
---------------
{{3,4},{1,2}}
I do not feel qualified to make that decision. (2) complicates the code
a bit, but that should not be the main argument here.
Martin
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