Re: unnest on multi-dimensional arrays

Thanks for the explanation and examples!


Zev

On 11/28/2013 10:03 AM, Tom Lane wrote:
> David Johnston <polobo@yahoo.com> writes:
>> Zev Benjamin wrote
>>> It appears that unnest, when called on a multi-dimensional array,
>>> effectively flattens the array first.  For example: ...
>
>> Multidimensional arrays do have shortcomings in the current implementation
>> of which this is one.  I'm not sure, though, if there is anything
>> substantial and centralized in the docs so pertaining.
>
> It might be worth explaining that this is a consequence of the fact that
> Postgres treats all arrays over the same element type as being of the
> same data type --- that is, 1-D and 2-D arrays are not distinguished
> by the type system.  Thus, when the polymorphic function "unnest(anyarray)
> returns setof anyelement" is applied to an integer array, it must return
> a series of integers; not a series of lower-dimensional arrays.
>
> There have been some discussions over whether this could be changed
> without a backwards-compatibility disaster, but nobody sees how.
>
>             regards, tom lane
>
>