Re: So what's an "empty" array anyway?

2008/10/21 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Currently, the constructs
> '{}'::arraytype
> ARRAY[]::arraytype
> return zero-dimensional arrays, as does the underlying function
> construct_empty_array(). I can't immediately find any way at SQL
> level to produce an empty array with one or more dimensions.
> However, construct_array and construct_md_array will happily build
> zero-length arrays of dimension 1 or higher, leading to weirdnesses
> such as illustrated here:
> http://archives.postgresql.org/pgsql-general/2008-10/msg00915.php
>

> Seems like we ought to clean this up. I'm not sure which way to jump
> though: should we decree that arrays of no elements must always have
> zero dimensions, or should we get rid of that and standardize on, say,
> 1-D array with lower bound 1 and upper bound 0?
>

I believe so zero dimensions for empty array should be more clean and
more simple. This solve question about array_dims too. But this empty
dimensionless array should be simple cast to dimensional empty array.

array_ndims(array[]) --> 0
array[1,2] || array[] = array[1,2]
array[[1,2],[1,3]] || array[] = array[[1,2],[1,3]]

or
array[1,2] = array[] -> false
array[[1,2],[1,3]] = array[] -> false

Regards
Pavel Stehule

> A somewhat related issue that I noticed while poking at this is that
> array_dims() returns NULL for a zero-dimension array. That seems a bit
> bogus too; wouldn't an empty string be saner? Of course the issue
> goes away if we get rid of zero-dimension arrays.
>
> Thoughts?
>
> regards, tom lane
>
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