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