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Re:Postgresql 9.6: Support parallel aggregation by kazagistar on Monday March 21, 2016 @06:05PM (#11332523)
Blog post with some details and benchmarks by one of the authors of the patch:

http://rhaas.blogspot.com/2015/11/parallel-sequential-scan-i...

Re:Postgresql 9.6: Support parallel aggregation by pgaddict on Monday March 21, 2016 @06:25PM (#11332676)
That's not about parallel aggregate but about parallel scan.

Also, Robert is not the author of the patch, he did a review and committed it. The actual author are listed in the commit message: David Rowley and Haribabu Kommi.