| From: | Teodor Sigaev <teodor(at)sigaev(dot)ru> |
|---|---|
| To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com>, Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: POC: GROUP BY optimization |
| Date: | 2018-06-29 14:51:56 |
| Message-ID: | 3bbdd1ee-544d-a549-b0c2-30485d9f6a16@sigaev.ru |
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| Lists: | pgsql-hackers |
>> 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!
I hope, now we have a better cost_sort(). The obvious way is a try all
combination of pathkeys in get_cheapest_group_keys_order() and choose cheapest
one by cost_sort(). But it requires N! operations and potentially could be very
expensive in case of large number of pathkeys and doesn't solve the issue with
user-knows-what-he-does pathkeys. We could suggest an order of pathkeys as patch
suggests now and if cost_sort() estimates cost is less than 80% (arbitrary
chosen) cost of user-suggested pathkeys then it use our else user pathkeys.
--
Teodor Sigaev E-mail: teodor(at)sigaev(dot)ru
WWW: http://www.sigaev.ru/
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