Re: BUG #6669: unique index w/ multiple columns and NULLs

On Fri, Jun 01, 2012 at 08:58:32AM +0200, jo wrote:
> Hi Tom,
>
> Thanks for the explanation about standard sql.
> The goodness of it must be accepted by faith. :-)
> I still have a doubt about the result of the GROUP BY clause.
> It seems to me that there's an inconsistence between the GROUP BY
> clause and the unique index.
> The GROUP BY clause, consider NULLs as known and equal values
> while the index unique constraint consider NULLs as unknown values
> and not equals between them.
> Don't you think, there's an inconsistence here?

Yes, I can see your point. I think GROUP BY is doing the best it can
with the NULL; having it consider them as different would lead to long
output. Also consider that COUNT(*) counts nulls, while COUNT(col) does
not:

WITH null_test (col) AS
(
SELECT 8
UNION ALL
SELECT NULL
)
SELECT COUNT(*) FROM null_test
UNION ALL
SELECT COUNT(col) FROM null_test;

count
-------
2
1
(2 rows)

COUNT(*) can't skip nulls because there is no specified column, but why
does COUNT(col) skip nulls --- again, inconsistent.

I think NULL is helpful for unknown values, and required as the output
of missing INSERT columns and unjoined outer join columns. I think the
aggregates then did the best they could.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

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