Re: BUG #16959: Unnesting null from string_to_array silently removes whole rows from result

Thank you for the answers.  I applied your first suggestion and of course
it worked well.

There's an implicit question in your email, regarding why I would think of
this outcome as a bug.  Not knowing as much of the internals, after
discovering that rows had gone missing my list of things to check was
roughly: no joins, no where clause, no having clause, no grouping, no
distinct, no distinct on, and no union/intersect/except.  After that, I was
down to pure trial and error to find the issue.

I get the message that the outcome was obvious to you.  For me it was
startling to have a function suppress the entire row, absent those other
query elements.  Even having read the note on 9.19, I struggle to see that
as a warning that all rows may disappear.  I also wonder how that outcome
is consistent with this:

\pset null 'nuLL'
select 1, split_part('adfsgasf', '234', 3);
 ?column? | split_part
----------+------------
        1 |
(1 row)

Even if it's perfectly sensible to you, I was caught off guard and I think
a note in the documentation alerting readers to this behavior would go a
long way in saving others from the prolonged confusion that I experienced.

Thanks again,
Pete O'Such


On Sun, Apr 11, 2021 at 8:46 PM Tom Lane <tgl@sss.pgh.pa.us> wrote:

> PG Bug reporting form <noreply@postgresql.org> writes:
> > Sample data:
> > create table test_rows as
> > SELECT * FROM (VALUES (1, null), (2, 'second')) AS t (num,letter);
>
> > Query with the unexpected result (I expected 2 rows):
> > select num, unnest(string_to_array(letter, ',')) from test_rows;
> > num | unnest
> > ----+--------
> >   2 | second
> > (1 row)
>
> Well, you could perhaps argue that string_to_array with NULL input
> should produce an empty array rather than a NULL.  But UNNEST()
> would produce zero rows in either case, and I fail to see why you
> find that surprising, much less buggy.  It would be a bug if it
> manufactured a value out of nothing.
>
> Having said that, you could inject the value you prefer using
> COALESCE, say
>
> # select num, unnest(coalesce(string_to_array(letter, ','), '{""}')) from
> test_rows;
>  num | unnest
> -----+--------
>    1 |
>    2 | second
> (2 rows)
>
> Alternatively, perhaps you'd consider a lateral left join to be
> less-surprising behavior:
>
> # select num, u from test_rows left join lateral
> unnest(string_to_array(letter, ',')) u on true;
>  num |   u
> -----+--------
>    1 |
>    2 | second
> (2 rows)
>
> The behavior you're getting from SRF-in-the-targetlist is basically
> equivalent to a lateral plain join, rather than left join.  See
>
>
> https://www.postgresql.org/docs/current/xfunc-sql.html#XFUNC-SQL-FUNCTIONS-RETURNING-SET
>
>                         regards, tom lane
>