Re: patch: to_string, to_array functions - Mailing list pgsql-hackers

2010/7/16 Brendan Jurd <direvus@gmail.com>:
> On 17 July 2010 02:15, Pavel Stehule <pavel.stehule@gmail.com> wrote:
>> 2010/7/16 Brendan Jurd <direvus@gmail.com>:
>>> Regarding the behaviour of the third argument (null_string), I was a
>>> little surprised by the results when I passed in a NULL.
>>>
>>
>> I didn't thinking about NULL as separator before. Current behave isn't
>> practical. When default separator is empty string, then NULL can be
>> used as ignore NULLs - so it can emulate current string_to_array and
>> array_to_string behave. It can be, because NULL can't be a separator
>> ever.
>>
>> select to_string(array[1,2,3,null,5], ',') -> 1,2,3,,5
>> select to_string(array[1,2,3,null,5], ',', null) -> 1,2,3,5
>>
>> maybe - next idea and maybe better - we can check NOT NULL for
>> separator and to add other parameter with default = false -
>> ignore_null
>>
>> select to_string(array[1,2,3,null,5], ',', ignore_null := true) -> 1,2,3,5
>>
>> what do you think?
>
> I don't have any problem with null_string = NULL in to_string taking
> the meaning "skip over NULL elements".  It's a slightly strange
> outcome but it's more useful than returning NULL, and I do like that
> it gives us a path to the current array_to_string() treatment even if
> those functions are ultimately deprecated.  I think adding a fourth
> keyword argument might be sacrificing a little too much convenience in
> the calling convention.
>
> As for to_array, null_string = NULL should mean that there is no
> string which should result in a NULL element.  So I would be happy to
> see the following set of behaviours:
>
> to_string(array[1, 2, 3, 4, 5], ',', null) = '1,2,3,4,5'
> to_string(array[1, 2, 3, null, 5], ',', null) = '1,2,3,5'
> to_array('1,2,3,,5', ',', null) = '{1,2,3,"",5}'
>
> Also, if we're going to make the function non-strict, we need to
> consider how to respond when the user specifies NULL for the other
> arguments.  If the field separator is NULL, bearing in mind that NULL
> can't match any string, I would expect that to_array would return the
> undivided string as a single array element, and that to_string would
> throw an error:
>

ok, it has a sense.

other question is empty string as separator - but I think, it can has
same behave like string_to_array and array_to_string functions.

> to_array('1,2,3,4,5', null) = '{"1,2,3,4,5"}'
> to_string(array[1,2,3,4,5], null) = ERROR: the field separator for
> to_string may not be NULL
>
> If the first argument is NULL for either function, I think it would be
> reasonable to return NULL.  But I could be convinced that we should
> throw an error in that case too.
>

I agree - I prefer a NULL

Thank You very much

Pavel

> Cheers,
> BJ
>