From: | Bruce Momjian <bruce(at)momjian(dot)us> |
---|---|
To: | jo <jose(dot)soares(at)sferacarta(dot)com> |
Cc: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-bugs(at)postgresql(dot)org |
Subject: | Re: BUG #6669: unique index w/ multiple columns and NULLs |
Date: | 2012-06-04 15:05:52 |
Message-ID: | 20120604150552.GB2352@momjian.us |
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Thread: | |
Lists: | pgsql-bugs |
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. +
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