Re: [HACKERS] Parallel Aggregation support for aggregate functionsthat use transitions not implemented for array_agg

On 2017-06-06 23:32:53 -0400, Peter Eisentraut wrote:
> On 6/6/17 13:52, Regina Obe wrote:
> > It seems CREATE  AGGREGATE was expanded in 9.6 to support parallelization of
> > aggregate functions using transitions, with the addition of serialfunc and
> > deserialfunc to the aggregate definitions.
> > 
> > https://www.postgresql.org/docs/10/static/sql-createaggregate.html
> > 
> > I was looking at the PostgreSQL 10 source code for some example usages of
> > this and was hoping that array_agg and string_agg would support the feature.
> 
> I'm not sure how you would parallelize these, since in most uses you
> want to have a deterministic output order.

Unless you specify ORDER BY you don't really have that anyway, consider
hash-aggregation.  If you want deterministic order, you really need an
ORDER BY inside the aggregate.

- Andres