Horton

Re: [HACKERS] Partition-wise aggregation/grouping

Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>

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From: Ashutosh Bapat <ashutosh.bapat@enterprisedb.com>
To: Robert Haas <robertmhaas@gmail.com>
Cc: Jeevan Chalke <jeevan.chalke@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: 2017-12-04T02:14:43Z
Lists: pgsql-hackers 
On Sat, Dec 2, 2017 at 4:08 AM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Fri, Dec 1, 2017 at 7:41 AM, Ashutosh Bapat
> <ashutosh.bapat@enterprisedb.com> wrote:
>> This code creates plans where there are multiple Gather nodes under an Append
>> node.
>
> We should avoid that.  Starting and stopping workers is inefficient,
> and precludes things like turning the Append into a Parallel Append.

Ah, I didn't think about it. Thanks for bringing it up.

>
>> AFAIU, the workers assigned to one gather node can be reused until that
>> Gather node finishes. Having multiple Gather nodes under an Append mean that
>> every worker will be idle from the time that worker finishes the work till the
>> last worker finishes the work.
>
> No, workers will exit as soon as they finish.  They don't hang around idle.

Sorry, I think I used wrong word "idle". I meant that if a worker
finishes and exists, the query can't use it that worker slot until the
next Gather node starts. But as you pointed out, starting and stopping
a worker is costlier than the cost of not using the slot. So we should
avoid such plans.

>
>> index b422050..1941468 100644
>> --- a/src/tools/pgindent/typedefs.list
>> +++ b/src/tools/pgindent/typedefs.list
>> @@ -2345,6 +2345,7 @@ UnlistenStmt
>>  UnresolvedTup
>>  UnresolvedTupData
>>  UpdateStmt
>> +UpperPathExtraData
>>  UpperRelationKind
>>  UpperUniquePath
>>  UserAuth
>>
>> Do we commit this file as part of the feature?
>
> Andres and I regularly commit such changes; Tom rejects them.
>

We will leave it to the committer to decide what to do with this hunk.

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database 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.