Re: [HACKERS] Partition-wise aggregation/grouping

Hi,

Attached is the patch to implement partition-wise aggregation/grouping.

As explained earlier, we produce a full aggregation for each partition when
partition keys are leading group by clauses and then append is performed.
Else we do a partial aggregation on each partition, append them and then add
finalization step over it.

I have observed that cost estimated for partition-wise aggregation and cost
for the plans without partition-wise aggregation is almost same. However,
execution time shows significant improvement (as explained my in the very
first email) with partition-wise aggregates. Planner chooses a plan according
to the costs, and thus most of the time plan without partition-wise
aggregation is chosen. Hence, to force partition-wise plans and for the
regression runs, I have added a GUC named partition_wise_agg_cost_factor to
adjust the costings.

This feature is only used when enable_partition_wise_agg GUC is set to on.

Here are the details of the patches in the patch-set:

0001 - Refactors sort and hash final grouping paths into separate functions.
Since partition-wise aggregation too builds paths same as that of
create_grouping_paths(), separated path creation for sort and hash agg into
separate functions. These functions later used by main partition-wise
aggregation/grouping patch.

0002 - Passes targetlist to get_number_of_groups().
We need to estimate groups for individual child relations and thus need to
pass targetlist corresponding to the child rel.

0003 - Adds enable_partition_wise_agg and partition_wise_agg_cost_factor GUCs.

0004 - Implements partition-wise aggregation.

0005 - Adds test-cases.

0006 - postgres_fdw changes which enable pushing aggregation for other upper
relations.

Since this patch is highly dependent on partition-wise join [1], one needs to
apply all those patches on HEAD (my repository head was at:
66ed3829df959adb47f71d7c903ac59f0670f3e1) before applying these patches in
order.

Suggestions / feedback / inputs ?

[1] https://www.postgresql.org/message-id/CAFjFpRd9Vqh_=-Ldv-XqWY006d07TJ+VXuhXCbdj=P1jukYBrw@mail.gmail.com