Re: ARRAY[1] || NULL <> array_append(ARRAY[1], NULL)

On Mon, Oct 26, 2015 at 11:04:40AM +0100, Michael Herold wrote:
> I just got confused by another behavior of the || operator. I
> thought it might be appropriate to report it here.
>
> The docs say "the result retains the lower bound subscript of the
> left-hand operand’s outer dimension" [1]. That's again not true for
> corner cases.

You didn't quote the entire sentence:

    When two arrays with an equal number of dimensions are concatenated, the
    -------------------------------------------------------------------
    result retains the lower bound subscript of the left-hand operand's
    outer dimension.

> Expected:
> # SELECT '{0}'::int[] || '[15:16]={1,2}';
> ----------
>  {0,1,2}
>
> Unexpected (lower bound is untouched):
> # SELECT '{}'::int[] || '[15:16]={1,2}';
> ---------------
>  [15:16]={1,2}

I would argue that '{}'::int[] is zero dimmensions, so there is no
documented behavior for this.

The C code is:

    /*
     * short circuit - if one input array is empty, and the other is not, we
     * return the non-empty one as the result
     *
     * if both are empty, return the first one
     */
    if (ndims1 == 0 && ndims2 > 0)
        PG_RETURN_ARRAYTYPE_P(v2);

I doubt we want to change this.

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