Horton

Re: [HACKERS] Partition-wise aggregation/grouping

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Jeevan Chalke <jeevan.chalke@enterprisedb.com>
Cc: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>, Dilip Kumar <dilipbalaut@gmail.com>, David Rowley <david.rowley@2ndquadrant.com>, Rajkumar Raghuwanshi <rajkumar.raghuwanshi@enterprisedb.com>, PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Date: 2018-02-02T13:59:29Z
Lists: pgsql-hackers 
On Fri, Feb 2, 2018 at 8:25 AM, Jeevan Chalke
<jeevan.chalke@enterprisedb.com> wrote:
>> The problem is that create_partition_agg_paths() is doing *exactly*
>> same thing that add_paths_to_grouping_rel() is already doing inside
>> the blocks that say if (grouped_rel->partial_pathlist).  We don't need
>> two copies of that code.  Both of those places except to take a
>> partial path that has been partially aggregated and produce a
>> non-partial path that is fully aggregated.  We do not need or want two
>> copies of that code.
>
> OK. Got it.
>
> Will try to find a common place for them and will also check how it goes
> with your suggested design change.
>
>> Here's another way to look at it.  We have four kinds of things.
>>
>> 1. Partially aggregated partial paths
>> 2. Partially aggregated non-partial paths
>> 3. Fully aggregated partial paths
>> 4. Fully aggregated non-partial paths

So in the new scheme I'm proposing, you've got a partially_grouped_rel
and a grouped_rel.  So all paths of type #1 go into
partially_grouped_rel->partial_pathlist, paths of type #2 go into
partially_grouped_rel->pathlist, type #3 (if we have any) goes into
grouped_rel->partial_pathlist, and type #4 goes into
grouped_rel->pathlist.

> add_paths_to_append_rel() -> generate_mergeappend_paths() does not consider
> partial_pathlist. Thus we will never see MergeAppend over parallel scan
> given by partial_pathlist. And thus plan like:
> -> Gather Merge
>   -> MergeAppend
> is not possible with current HEAD.
>
> Are you suggesting we should implement that here? I think that itself is a
> separate task.

Oh, I didn't realize that wasn't working already.  I agree that it's a
separate task from this patch, but it's really too bad that it doesn't
already work.

-- 
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.