Re: what is good solution for support NULL inside string_to_array function?

Hello

I understand why we don't support expression 'null'::sometype. But it
does problems with array deserialisation.

postgres=# select array_to_string(ARRAY[10,20,30,NULL,30], '|');
array_to_string
-----------------
10|20|30|30
(1 row)

quietly removing NULL is maybe good for compatibility but is wrong for
functionality. Can we enhance function array_to_string and
string_to_array like:

CREATE OR REPLACE FUNCTION array_to_string(dta anyarray, sep text,
nullsym text)
RETURNS text AS $$
SELECT array_to_string(ARRAY(SELECT coalesce(v::text,$3)
FROM unnest($1) g(v)),$2)
$$ LANGUAGE sql;
CREATE FUNCTION
Time: 231.445 ms
postgres=# select array_to_string(ARRAY[10,20,30,NULL,30], '|', '');
array_to_string
-----------------
10|20|30||30
(1 row)

Time: 230.879 ms
postgres=# select array_to_string(ARRAY[10,20,30,NULL,30], '|', 'NULL');
array_to_string
------------------
10|20|30|NULL|30
(1 row)

Time: 2.031 ms

CREATE OR REPLACE FUNCTION string_to_array(str text, sep text, nullsym text)
RETURNS text[] AS $$
SELECT ARRAY(SELECT CASE WHEN v <> $3 THEN v ELSE NULL END
FROM unnest(string_to_array($1,$2)) g(v))
$$ LANGUAGE sql;
CREATE FUNCTION
Time: 29.044 ms

postgres=# SELECT string_to_array('10,20,30,,40',',','');
string_to_array
--------------------
{10,20,30,NULL,40}
(1 row)

postgres=# SELECT string_to_array('10,20,30,,40',',','')::int[];
string_to_array
--------------------
{10,20,30,NULL,40}
(1 row)

it is correct?

other ideas?

Regards
Pavel Stehule

> quietly removing NULL is maybe good for compatibility but is wrong for
> functionality.

I agree. I wasn't aware of this little misfeature.

Default display for NULL should be a zero-length string.

--
-- Josh Berkus
PostgreSQL Experts Inc.
http://www.pgexperts.com

Josh Berkus <josh(at)agliodbs(dot)com> writes:
>> quietly removing NULL is maybe good for compatibility but is wrong for
>> functionality.

> I agree. I wasn't aware of this little misfeature.

> Default display for NULL should be a zero-length string.

That's just as broken as Pavel's suggestion. Unless you have something
that is guaranteed distingishable from the output of any non-null value,
you really can't make a significant improvement here.

regards, tom lane

Tom Lane wrote:
>> Default display for NULL should be a zero-length string.
>>
>
> That's just as broken as Pavel's suggestion. Unless you have something
> that is guaranteed distingishable from the output of any non-null value,
> you really can't make a significant improvement here.
>
>
>

Right. This is the problem we solved in CSV processing by distinguishing
between quoted and unquoted values that could be null - the CSV rule is
that a null value isn't quoted.

cheers

andrew

2010/5/4 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
>>> quietly removing NULL is maybe good for compatibility but is wrong for
>>> functionality.
>
>> I agree.  I wasn't aware of this little misfeature.
>
>> Default display for NULL should be a zero-length string.
>
> That's just as broken as Pavel's suggestion.  Unless you have something
> that is guaranteed distingishable from the output of any non-null value,
> you really can't make a significant improvement here.
>

I wouldn't modify current two params string_to_array and
array_to_string function. So there are not any default string (maybe
empty string) for NULL. My proposal is new three params functions with
>>>explicit<<< "null string" definition. This cannot break
compatibility and enhance functionality - It is just short cut for
code from my proposal - in C this functionality can by implemented
much faster.

Regards
Pavel

>                        regards, tom lane
>

2010/5/4 Josh Berkus <josh(at)agliodbs(dot)com>:
>
>> quietly removing NULL is maybe good for compatibility but is wrong for
>> functionality.
>
> I agree.  I wasn't aware of this little misfeature.
>
> Default display for NULL should be a zero-length string.
>

I disagree - NULL is NULL, not empty string (Oracle is different)

if array_to_string is equivalent to

x[1] || sep || x[2] || sep || x[3] || sep ....

then correct result is NULL

and then string_to_array and array_to_string are correct, because
string_to_array cannot contain any NULL symbol.

Regards
Pavel Stehule

> --
>                                  -- Josh Berkus
>                                     PostgreSQL Experts Inc.
>                                     http://www.pgexperts.com
>

2010/5/4 Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>:
> 2010/5/4 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
>> Josh Berkus <josh(at)agliodbs(dot)com> writes:
>>>> quietly removing NULL is maybe good for compatibility but is wrong for
>>>> functionality.
>>
>>> I agree.  I wasn't aware of this little misfeature.
>>
>>> Default display for NULL should be a zero-length string.
>>
>> That's just as broken as Pavel's suggestion.  Unless you have something
>> that is guaranteed distingishable from the output of any non-null value,
>> you really can't make a significant improvement here.
>>
>
> I wouldn't modify current two params string_to_array and
> array_to_string function. So there are not any default string (maybe
> empty string) for NULL. My proposal is new three params functions with
>>>>explicit<<< "null string" definition. This cannot break
> compatibility and enhance functionality - It is just short cut for
> code from my proposal - in C this functionality can by implemented
> much faster.

I did some coding - the patch can be very simple

postgres=# select array_to_string(array[1,2,3,4,5,null],',','*');
array_to_string
-----------------
1,2,3,4,5,*
(1 row)

Time: 0,501 ms
postgres=# select
string_to_array(array_to_string(array[1,2,3,4,5,null],',','*'),',','*');
string_to_array
------------------
{1,2,3,4,5,NULL}
(1 row)

Time: 0,617 ms

postgres=# select string_to_array('1,2,3,4,5,*',',','*')::int[];
string_to_array
------------------
{1,2,3,4,5,NULL}
(1 row)

Time: 0,652 ms

and then string_to_array and array_to_string are orthogonal with NULL.

Pavel

>
> Regards
> Pavel
>
>>                        regards, tom lane
>>
>

On Tue, May 4, 2010 at 10:05 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
> and then string_to_array and array_to_string are orthogonal with NULL.

I like the behavior, but should it share the name with the 2 argument
version given the incompatibility? Maybe make a new function
to_string(anyarray, sep, nullsym='') and deprecate the old one?

merlin

2010/5/4 Merlin Moncure <mmoncure(at)gmail(dot)com>:
> On Tue, May 4, 2010 at 10:05 AM, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>> and then string_to_array and array_to_string are orthogonal with NULL.
>
> I like the behavior, but should it share the name with the 2 argument
> version given the incompatibility? Maybe make a new function
> to_string(anyarray, sep, nullsym='') and deprecate the old one?
>

maybe to_string X to_array ... Why not? It shorter, maybe it is cleaner

Regards
Pavel

> merlin
>

Tom Lane wrote:
> Josh Berkus <josh(at)agliodbs(dot)com> writes:
>
>>> quietly removing NULL is maybe good for compatibility but is wrong for
>>> functionality.
>>>
>
>
>> I agree. I wasn't aware of this little misfeature.
>>
>
>
>> Default display for NULL should be a zero-length string.
>>
>
> That's just as broken as Pavel's suggestion. Unless you have something
> that is guaranteed distingishable from the output of any non-null value,
> you really can't make a significant improvement here.
>
> regards, tom lane
>
>
Is this, perhaps, a generalized case of this long-running discussion
from last year?:
http://archives.postgresql.org/pgsql-hackers/2009-03/msg01350.php

Cheers,
Steve