Re: parallel distinct union and aggregate support patch

I also had a quick look at the patch and the comments made so far. Summary:

1. The performance results are promising.

2. The code needs comments.

Regarding the design:

Thomas Munro mentioned the idea of a "Parallel Repartition" node that
would redistribute tuples like this. As I understand it, the difference
is that this BatchSort implementation collects all tuples in a tuplesort
or a tuplestore, while a Parallel Repartition node would just
redistribute the tuples to the workers, without buffering. The receiving
worker could put the tuples to a tuplestore or sort if needed.

I think a non-buffering Reparttion node would be simpler, and thus
better. In these patches, you have a BatchSort node, and batchstore, but
a simple Parallel Repartition node could do both. For example, to
implement distinct:

Gather
- > Unique
-> Sort
-> Parallel Redistribute
-> Parallel Seq Scan

And a Hash Agg would look like this:

Gather
- > Hash Agg
-> Parallel Redistribute
-> Parallel Seq Scan

I'm marking this as Waiting on Author in the commitfest.

- Heikki