Re: crosstab/repivot...any interest?

On Fri, Jan 25, 2019 at 3:16 PM David Fetter <david(at)fetter(dot)org> wrote:
>
> On Fri, Jan 25, 2019 at 02:21:55PM -0600, Merlin Moncure wrote:
> > Hackers,
> >
> > We have a strong need to make a variant to the crosstab interface so
> > that data that is pivoted one way would be sent through a crosstab
> > like function so that it would be pivoted another way. For example,
> > if you had
> >
> > row 0: a1, a2, a3, k1, c1, c2, ...
> > row 1: a1, a2, a3, k2, c1, c2, ...
> > row 2: a1, a2, a3, k3, c1, c2, ...
> > ...
> >
> > where 'a' columns are uninteresting attribute columns, 'k' is the
> > dimension we want to pivot on, and c1->cN would be stacked vertically,
> > so that we'd end up with,
> > row 0: a1, a2, a3, c1, k1, k2, ...
> > row 1: a1, a2, a3, c2, k1, k2, ...
> > row 2: a1, a2, a3, c3, k1, k2, ...
> >
> > There are various SQL level approaches to this but they tend to be
> > imperformant with large datasets so that I think a crosstab-like C
> > implementation ought to be able to do better (or at least I hope so)
> > since you have to cross product rows and columns in such a way that
> > you can get a clean join. Cribbing from tablefunc.c I don't think
> > this is a terrible challenge to do in hash table style.
> >
> > Questions on the table:
> > *) Has anyone done anything like this or know of any current implementations?
> > *) Would there be any interest in expanding tablefunc along these lines?
>
> There's something in SQL:2016 that I read as crosstabs, or at least as
> enabling crosstabs.
> https://www.iso.org/standard/69776.html
>
> If we're going to put work into crosstabs, it seems to me that the
> "we" needs to be the project as a whole, and the work should be, to
> the extent reasonable, toward standard compliance.

Interesting. Do you see that the spec (it makes my brain hurt) can
handle that kind of repivoting?

merlin