Re: onlyvalue aggregate (was: First Aggregate Funtion?)

On Wed, Oct 28, 2015 at 1:38 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:

> Marko Tiikkaja <marko(at)joh(dot)to> writes:
> > Here's a patch for the aggregate function outlined by Corey Huinker in
> > CADkLM=foA_oC_Ri23F9PbfLnfwXFbC3Lt8bBzRu3=CB77G9_qw(at)mail(dot)gmail(dot)com . I
> > called it "onlyvalue", which is a horrible name, but I have nothing
> > better to offer. (Corey called it "only", but that doesn't really work
> > since ONLY is a fully reserved keyword.)
>
> On the name front, maybe think "single" rather than "only"? This might
> lead to "single()" or "single_value()", or "singleton()" if you want to
> sound highbrow.
>
> On the semantics front, I'm not sure that I like excluding nulls from the
> input domain. I'd rather that it acted like IS NOT DISTINCT, ie, nulls
> are fine as long as all the input values are nulls. The implementation
> would need some work for that.
>

​homogeneous() ?

It is basically an assertion function since in most cases where you would
use this you needn't use a function at all but instead simply place the
column in the GROUP BY clause.

I have at various times desired having various assertion functions that
return the input value if the assertion is met but causes a query error if
it is not. That said functions can be both scalar and aggregate oriented
makes sense. Adding just this one function in seems like it adds a partial
feature to -core. I'd rather it bake more in PGXN before considering
putting it into core. There doesn't seem to be any hard need or benefit to
doing so at this time.

I would probably stick to the concept of assertion and call it something
like
"assert_nongrouping()"
​ to denote that the input does not cause multiple groups to be created due
to their being multiple values.

David J.
​