Re: Combining Aggregates

On 25 February 2015 at 08:15, Peter Eisentraut <peter_e(at)gmx(dot)net> wrote:

> On 2/20/15 3:32 PM, Tomas Vondra wrote:
>
> > Also, there are aggregate functions like array_agg() or string_agg()
> > that make this impossible, just like for many custom aggregates (like
> > hyperloglog for example). Again, I might not understand the idea
> > correctly ...
>
> How would a combining function work for something like array_agg()? I
> don't think it would, at least if you want to preserve the ordering
> option for the user.
>
>
They just wouldn't work in that case. We'd simply not have a combine
function for that aggregate.

The yet to be written code, (say parallel hash aggregate), the planner
would have to ensure that each aggregate function being used had a combine
function, if any aggregate in the current query level didn't have one then
it would not parallelise the query.

Regards

David Rowley