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: 2018-03-21T14:16:08Z
Lists: pgsql-hackers 
On Wed, Mar 21, 2018 at 8:01 AM, Jeevan Chalke
<jeevan.chalke@enterprisedb.com> wrote:
>> In the patch as proposed, create_partial_grouping_paths() can get
>> called even if GROUPING_CAN_PARTIAL_AGG is not set.  I think that's
>> wrong.
>
> I don't think so. For parallel case, we do check that. And for partitionwise
> aggregation check, it was checked inside can_partitionwise_grouping()
> function and flags were set accordingly. Am I missing something?

Well, one of us is missing something somewhere.  If
GROUPING_CAN_PARTIAL_AGG means that we're allowed to do partial
grouping, and if create_partial_grouping_paths() is where partial
grouping happens, then we should only be calling the latter if the
former is set.  I mean, how can it make sense to create
partially-grouped paths if we're not allowed to do partial grouping?

> I have tweaked these conditions and posted in a separate patch (0006).
> However, I have merged all your three patches in one (0005).

OK, thanks.  I wasn't sure I had understood what was going on, so
thanks for checking it.

Thanks also for keeping 0004-0006 separate here, but I think you can
flatten them into one patch in the next version.

>> I think that if the last test in can_partitionwise_grouping were moved
>> before the previous test, it could be simplified to test only
>> (extra->flags & GROUPING_CAN_PARTIAL_AGG) == 0 and not
>> *perform_partial_partitionwise_aggregation.
>
> I think we can't do this way. If *perform_partial_partitionwise_aggregation
> found to be true then only we need to check whether partial aggregation
> itself is possible or not. If we are going to perform a full partitionwise
> aggregation then test for can_partial_agg is not needed. Have I misread your
> comments?

It seems you're correct, because when I change it the tests fail.  I
don't yet understand why.

Basically, the main patch seems to use three Boolean signaling mechanisms:

1. GROUPING_CAN_PARTITIONWISE_AGG
2. is_partial_aggregation
3. perform_partial_partitionwise_aggregation

Stuff I don't understand:

- Why is one of them a Boolean shoved into "flags", even though it's
not static across the whole hierarchy like the other flags, and the
other two are separate Booleans?
- What do they all do, anyway?

-- 
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.