To summarize the current state of parallel grouping sets, we now have two available implementations for it.
1) Each worker performs an aggregation step, producing a partial result for each group of which that process is aware. Then the partial results are gathered to the leader, which then performs a grouping sets aggregation, as in patch [1]. This implementation is not very efficient sometimes, because the group key for Partial Aggregate has to be all the columns involved in the grouping sets. 2) Each worker performs a grouping sets aggregation on its partial data, and tags 'GroupingSetId' for each tuple produced by partial aggregate. Then the partial results are gathered to the leader, and the leader performs a modified grouping aggregate, which dispatches the partial results into different pipe according to 'GroupingSetId', as in patch [2], or instead as another method, the leader performs a normal aggregation, with 'GroupingSetId' included in the group keys, as discussed in [3]. The second implementation would be generally better than the first one in performance, and we have decided to concentrate on it. [1] https://www.postgresql.org/message-id/CAN_9JTx3NM12ZDzEYcOVLFiCBvwMHyM0gENvtTpKBoOOgcs=k...@mail.gmail.com [2] https://www.postgresql.org/message-id/can_9jtwtttnxhbr5ahuqvcriz3hxvppx1jwe--dcsdjyuhr...@mail.gmail.com [3] https://www.postgresql.org/message-id/CAN_9JTwtzttEmdXvMbJqXt=51kxibtckepkq6kk2pz6xz6m...@mail.gmail.com Thanks Richard >
