Re: POC: GROUP BY optimization

Tomas Vondra <tomas.vondra@2ndquadrant.com>

From: Tomas Vondra <tomas.vondra@2ndquadrant.com>

To: Teodor Sigaev <teodor@sigaev.ru>, Pgsql Hackers <pgsql-hackers@postgresql.org>

Date: 2018-06-29T13:11:37Z

Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

Restore preprocess_groupclause()
- 505c008ca37c 17.0 landed
Rename PathKeyInfo to GroupByOrdering
- 0c1af2c35c7b 17.0 landed
Add invariants check to get_useful_group_keys_orderings()
- 91143c03d4ca 17.0 landed
Fix asymmetry in setting EquivalenceClass.ec_sortref
- 199012a3d844 17.0 landed
Multiple revisions to the GROUP BY reordering tests
- 874d817baa16 17.0 landed
Get rid of pg_class usage in SJE regression tests
- e1b7fde418f2 17.0 landed
Rename index "abc" in aggregates.sql
- b91f91870828 17.0 landed
Explore alternative orderings of group-by pathkeys during optimization.
- 0452b461bc40 17.0 landed
Generalize the common code of adding sort before processing of grouping
- 7ab80ac1caf9 17.0 landed
Fix out-dated comment in preprocess_groupclause()
- f6c70b81802a 15.0 landed
- 78a9af1a2764 16.0 landed
Force parallelism in partition_aggregate
- 2fe6b2a806f2 16.0 landed
- 01474f56981a 15.0 landed
Optimize order of GROUP BY keys
- db0d67db2401 15.0 landed


On 06/28/2018 06:52 PM, Teodor Sigaev wrote:
>> I'm afraid that's a misunderstanding - my objections did not really go
>> away. I'm just saying my claim that we're not messing with order of
>> grouping keys is not entirely accurate, because we do that in one
>> particular case.
>>
>> I still think we need to be careful when introducing new optimizations
>> in this area - reordering the grouping keys by ndistinct, ORDER BY or
>> whatever. In particular I don't think we should commit these patches
>> that may quite easily cause regressions, and then hope some hypothetical
>> future patch fixes the costing. Those objections still stand.
> 
> Pls, have a look at https://commitfest.postgresql.org/18/1706/
> I tried to attack the cost_sort() issues and hope on that basis we can 
> solve problems with 0002 patch and improve incremental sort patch.
> 

OK, will do. Thanks for working on this!

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
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