Re: BUG #7730: intarray representation of empty arrays - Mailing list pgsql-bugs

Applied.

---------------------------------------------------------------------------

On Wed, Sep  4, 2013 at 03:01:50PM -0400, Bruce Momjian wrote:
> 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. +

> diff --git a/contrib/intarray/_int_tool.c b/contrib/intarray/_int_tool.c
> new file mode 100644
> index 8635576..fc462b2
> *** a/contrib/intarray/_int_tool.c
> --- b/contrib/intarray/_int_tool.c
> *************** resize_intArrayType(ArrayType *a, int nu
> *** 246,251 ****
> --- 246,258 ----
>       int            nbytes = ARR_DATA_OFFSET(a) + sizeof(int) * num;
>       int            i;
>
> +     /* if no elements, return a zero-dimensional array */
> +     if (num == 0)
> +     {
> +         ARR_NDIM(a) = 0;
> +         return a;
> +     }
> +
>       if (num == ARRNELEMS(a))
>           return a;
>

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

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