Back in March 2016, e06a38965 added support for parallel aggregation. IIRC, because it was fairly late in the release cycle, I dropped parallel DISTINCT to reduce the scope a little. It's been on my list of things to fix since then. I just didn't get around to it until today.
The patch is just some plumbing work to connect all the correct paths up to make it work. It's all fairly trivial. I thought about refactoring things a bit more to get rid of the additional calls to grouping_is_sortable() and grouping_is_hashable(), but I just don't think it's worth making the code ugly for. We'll only call them again if we're considering a parallel plan, in which case it's most likely not a trivial query. Those functions are pretty cheap anyway. I understand that there's another patch in the September commitfest that does some stuff with Parallel DISTINCT, but that goes about things a completely different way by creating multiple queues to distribute values by hash. I don't think there's any overlap here. We'd likely want to still have the planner consider both methods if we get that patch sometime. David
parallel_distinct.patch
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