Re: Planning aggregates which require sorted or distinct

On Fri, 19 Jan 2007, Tom Lane wrote:

> Gavin Sherry <swm(at)alcove(dot)com(dot)au> writes:
> > What we want to do is have a kind of 'sub plan' for each aggregate. In
> > effect, the plan might start looking like a directed graph. Here is part
> > of the plan as a directed graph.
>
> > GroupAggregate
> > /-----------------^---------------...
> > | |
> > | |
> > ^ |
> > | Unique
> > | ^
> > | |
> > Sort Sort
> > (saledate) (saledate,prodid)
> > ^ ^
> > | |
> > -------------- Fan Out ------------...
> > ^
> > |
> > Scan
>
> > This idea was presented by Brian Hagenbuch at Greenplum. He calls it a
> > 'Fan Out' plan. It is trivial to rejoin the data because all data input to
> > the aggregates is sorted by the same primary key.
>
> Er, what primary key would that be exactly? And even if you had a key,
> I wouldn't call joining on it trivial; I'd call it expensive ...

I should have used slightly different language. What I meant to say was,
both sets are primarily sorted by saledate so they can be merged back
together. This is why I said it was trivial.

Thanks,

Gavin