Re: Should array_length() Return NULL

Brendan Jurd <direvus@gmail.com> writes:
> On 17 March 2013 05:19, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> Perhaps not.  I think for most uses, a 1-D zero-length array would be
>> just as good.  I guess what I'd want to know is whether we also need
>> to support higher-dimensional zero-size arrays, and if so, what does
>> the I/O syntax for those look like?

> If I'm reading right, in our current implementation of array
> dimensionality, there can be no such thing as a higher-dimensional
> zero-length array anyhow.  Postgres doesn't care about how many
> dimensions you define for an array, it uses the "quacks like a duck"
> test for number of dimensions.  For example:

> postgres=# SELECT ARRAY[1]::int[][], array_dims(ARRAY[1]::int[][]);
>  array | array_dims
> -------+------------
>  {1}   | [1:1]

Um, this seems to be conflating the issue with a different one, which
is that the type system doesn't care how many dimensions arrays have.
So "int[]" and "int[][]" are the same type.  That's slightly annoying
but I'm not sure it's really worth changing.

What I'm concerned about here is whether these expressions shouldn't
be yielding different data values:

regression=# select array[]::int[];array 
-------{}
(1 row)

regression=# select array[array[]]::int[];array 
-------{}
(1 row)

regression=# select array[array[],array[]]::int[];array 
-------{}
(1 row)

Right now, if we did make them produce what they appear to mean, the
array I/O functions would have a problem with representing the results:

regression=# select '{}'::int[];int4 
------{}
(1 row)

regression=# select '{{}}'::int[];
ERROR:  malformed array literal: "{{}}"
LINE 1: select '{{}}'::int[];              ^
regression=# select '{{},{}}'::int[];
ERROR:  malformed array literal: "{{},{}}"
LINE 1: select '{{},{}}'::int[];              ^

So I think we'd need to fix that before we could go very far in this
direction.
        regards, tom lane