Re: PL/Python adding support for multi-dimensional arrays

On 10/10/2016 08:42 PM, Pavel Stehule wrote:
> 2016-10-10 12:31 GMT+02:00 Heikki Linnakangas <hlinnaka(at)iki(dot)fi>:
>
>> On 10/01/2016 02:45 AM, Jim Nasby wrote:
>>
>>> On 9/29/16 1:51 PM, Heikki Linnakangas wrote:
>>>
>>>> Now, back to multi-dimensional arrays. I can see that the Sequence
>>>> representation is problematic, with arrays, because if you have a python
>>>> list of lists, like [[1, 2]], it's not immediately clear if that's a
>>>> one-dimensional array of tuples, or two-dimensional array of integers.
>>>> Then again, we do have the type definitions available. So is it really
>>>> ambiguous?
>>>>
>>>
>>> [[1,2]] is a list of lists...
>>> In [4]: b=[[1,2]]
>>>
>>> In [5]: type(b)
>>> Out[5]: list
>>>
>>> In [6]: type(b[0])
>>> Out[6]: list
>>>
>>> If you want a list of tuples...
>>> In [7]: c=[(1,2)]
>>>
>>> In [8]: type(c)
>>> Out[8]: list
>>>
>>> In [9]: type(c[0])
>>> Out[9]: tuple
>>>
>>
>> Hmm, so we would start to treat lists and tuples differently? A Python
>> list would be converted into an array, and a Python tuple would be
>> converted into a composite type. That does make a lot of sense. The only
>> problem is that it's not backwards-compatible. A PL/python function that
>> returns an SQL array of rows, and does that by returning Python list of
>> lists, it would start failing.
>
> is not possible do decision in last moment - on PL/Postgres interface?
> There the expected type should be known.

Unfortunately there are cases that are fundamentally ambiguous.

create type comptype as (intarray int[]);
create function array_return() returns comptype[] as $$
return [[[[1]]]];
$$ language plpython;

What does the function return? It could be two-dimension array of
comptype, with a single-dimension intarray, or a single-dimension
comptype, with a two-dimension intarray.

We could resolve it for simpler cases, but not the general case. The
simple cases would probably cover most things people do in practice. But
if the distinction between a tuple and a list feels natural to Python
programmers, I think it would be more clear in the long run to have
people adjust their applications.

- Heikki