Re: [HACKERS] Group By, NULL values and inconsistent behaviour.

Where are we on this.  It appears this NULL group by is seriously
broken.

Can we have some tests on commercial databases, and get a patch
generated?

>
>
> While looking thru the nodeGroup code, I noticed the following
> that I'm not sure is correct.
>
> -- Using 01-09 snapshot
> create table t1 (a int4, b char(2), c char(2));
> CREATE
> insert into t1 (a,c) values (1,'x');
> INSERT 149419 1
> insert into t1 (a,c) values (2,'x');
> INSERT 149420 1
> insert into t1 (a,c) values (3,'z');
> INSERT 149421 1
> insert into t1 (a,c) values (2,'x');
> INSERT 149422 1
> select * from t1;
> a|b|c
> -+-+--
> 1| |x
> 2| |x
> 3| |z
> 2| |x
> (4 rows)
>
> select b,c,sum(a) from t1 group by b,c;
> b|c |sum
> -+--+---
>  |x |  3
>  |z |  3
>  |x |  2
> (3 rows)
>
> select b,c,sum(a) from t1 group by b,c order by c;
> b|c |sum
> -+--+---
>  |x |  3
>  |x |  2
>  |z |  3
> (3 rows)
>
> In the second query, the first two rows have been grouped, but shouldn't
> they not be since b is NULL?  I thought that NULL != NULL?
>
> If so, is the third query wrong?  The first two rows are different, but
> only because of the aggregated column that is the source of the group by.
> According to the logic from the second query, these should have been
> grouped, no?
>
> What does the standard say about comparing two NULL values?
>
> The fixes for these inconsistencies appear to be simple.  To cause a new
> group to be started if NULL != NULL, simply change the "continue;" in the
> sameGroup function in nodeGroup.c to "return FALSE;"  Ignoring aggregated
> columns would also then be added to sameGroup().
>
> darrenk
>
>


--
Bruce Momjian
maillist@candle.pha.pa.us