Horton

Re: Partition-wise aggregation/grouping

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Antonin Houska <ah@cybertec.at>
Cc: "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Date: 2017-05-01T14:33:36Z
Lists: pgsql-hackers 
On Fri, Apr 28, 2017 at 3:03 AM, Antonin Houska <ah@cybertec.at> wrote:
> I think this is not generic enough because the result of the Append plan can
> be joined to another relation. As such a join can duplicate the
> already-aggregated values, the aggregates should not be finalized below the
> top-level plan.

If the grouping key matches the partition key, then it's correct to
push the entire aggregate down, and there's probably a large
performance advantage from avoiding aggregating twice.  If the two
don't match, then pushing the aggregate down necessarily involves a
"partial" and a "finalize" stage, which may or may not be cheaper than
doing the aggregation all at once.  If you have lots of 2-row groups
with 1 row in the first branch of the append and 1 row in the second
branch of the append, breaking the aggregate into two steps is
probably going to be a loser.  If the overall number of groups is
small, it's probably going to win.  But when the grouping key matches
the partition key, so that two-stage aggregation isn't required, I
suspect the pushdown should almost always win.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Commits

  1. postgres_fdw: Push down partition-wise aggregation.

  2. Remove 'target' from GroupPathExtraData.

  3. Implement partition-wise grouping/aggregation.

  4. Don't pass the grouping target around unnecessarily.

  5. Determine grouping strategies in create_grouping_paths.

  6. Defer creation of partially-grouped relation until it's needed.

  7. Split create_grouping_paths into degenerate and non-degenerate cases.

  8. Pass additional arguments to a couple of grouping-related functions.

  9. Fix logic error in add_paths_to_partial_grouping_rel.

  10. Minor cleanup of code related to partially_grouped_rel.

  11. Add a new upper planner relation for partially-aggregated results.

  12. Charge cpu_tuple_cost * 0.5 for Append and MergeAppend nodes.

  13. Rename enable_partition_wise_join to enable_partitionwise_join

  14. Factor some code out of create_grouping_paths.

  15. Pad XLogReaderState's main_data buffer more aggressively.

  16. Prevent int128 from requiring more than MAXALIGN alignment.

  17. Fix DROP SUBSCRIPTION hang

  18. Inject $(ICU_LIBS) regardless of platform.

  19. Some preliminary refactoring towards partitionwise join.