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

On Tue, 2008-10-21 at 13:50 -0400, Tom Lane wrote:
> 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?
> 
> 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.

Please remove zero-dimension arrays.

The number of dimensions of an empty array really ought to be NULL, or
if we fix it to be non-NULL then 1+. Zero just makes a weird case for no
reason. An empty string only makes sense in the context of that
particular function, it doesn't really help with other maths.

-- Simon Riggs           www.2ndQuadrant.comPostgreSQL Training, Services and Support