Re: BUG #7730: intarray representation of empty arrays

On Wed, Dec  5, 2012 at 12:44:39AM +0000, elein@varlena.com wrote:
> The following bug has been logged on the website:
>
> Bug reference:      7730
> Logged by:          elein
> Email address:      elein@varlena.com
> PostgreSQL version: 9.2.1
> Operating system:   Linux
> Description:
>
> select NULLIF('{1,2,3}'::integer[] - '{3,2,1}'::integer[], '{}'::integer[]);
> This returns an empty array.  It should return NULL.
>
> Per RhodiumToad: the core code represents '{}' as an array with 0
> dimensions, whereas intarray represents it as an array with 1 dimension but
> 0 elements
>
> intarray should use the same standards as the core code if possible.  I
> peered at the code and don't see anything untoward but did not have time to
> spend on it.

I just got time to look at this, and it is certainly easier to see when
you use array_dims():

    SELECT '{1,2,3}'::integer[] - '{3,2,1}'::integer[];
     ?column?
    ----------
     {}

    SELECT array_dims('{1,2,3}'::integer[] - '{3,2,1}'::integer[]);
     array_dims
    ------------
     [1:0]

    SELECT array_dims('{}'::integer[]);
     array_dims
    ------------
     (null)

This is part of the larger TODO item of how to handle empty
>=1-dimensional empty arrays vs. zero-dimensional empty arrays, which is
discussed here:

    https://wiki.postgresql.org/wiki/Todo#Arrays
    Improve handling of empty arrays

In that thread, no one could find a way to create a 1-dimensional empty
array at the SQL level, but thanks to intarray, you found a way.  It is
natural that intarray, being mostly used for one-dimensional arrays,
would return a 1-dimensional empty array.  However, besides being
inconsistent, as you mentioned, there is also no way to dump/restore
one-dimensional empty arrays, which is a larger concern.

I have developed the attached patch to force empty intarray results to
be zero-dimensional empty arrays, rather than 1-dimensional empty
arrays.  With this patch, a zero-dimensional empty array is returned:

    SELECT array_dims('{1,2,3}'::integer[] - '{3,2,1}'::integer[]);
     array_dims
    ------------
     (null)

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +