Re: crosstab/repivot...any interest?

On Fri, Jan 25, 2019 at 04:31:00PM -0600, Merlin Moncure wrote:
> 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?

I believe the constructs can nest and/or refer to each other, so yes.

Best,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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