Message-ID: <982de4a4-71b6-4d1d-afe2-35b1c5d43529@vondra.me>
Date: Thu, 11 Dec 2025 18:07:54 +0100
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: Add a greedy join search algorithm to handle large join problems
To: Pavel Stehule <pavel.stehule@gmail.com>,
 John Naylor <johncnaylorls@gmail.com>
Cc: Chengpeng Yan <chengpeng_yan@outlook.com>,
 "pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
References: <3FF63E99-AB4F-41A9-BC78-AAB28823FBD0@Outlook.com>
 <6db6d2ec-7529-4add-9a95-178fc318311d@vondra.me>
 <313ACE5A-CBF1-43B3-9181-10D3E8ADF424@Outlook.com>
 <5abd6054-413c-4f48-9172-d8b31062b266@vondra.me>
 <cb313155-24c4-4838-a46b-44968993a6e2@vondra.me>
 <938E2286-9B0D-4F8D-A916-8E0E35D55034@Outlook.com>
 <e82d4302-2450-4915-93a5-7df75f69c385@vondra.me>
 <CANWCAZY529EPHyo1kLnEzjFBq-UaDPc3KErK=ApqDZZ1Oc-XHg@mail.gmail.com>
 <CAFj8pRCO5ocbr-wFWx5QsKdfkW-=XuQ6zkW5FES7ERQZQHtpwQ@mail.gmail.com>
Content-Language: en-US
From: Tomas Vondra <tomas@vondra.me>
In-Reply-To: 
 <CAFj8pRCO5ocbr-wFWx5QsKdfkW-=XuQ6zkW5FES7ERQZQHtpwQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Archived-At: 
 <https://www.postgresql.org/message-id/982de4a4-71b6-4d1d-afe2-35b1c5d43529%40vondra.me>
Precedence: bulk

On 12/11/25 07:12, Pavel Stehule wrote:
> 
> 
> čt 11. 12. 2025 v 3:53 odesílatel John Naylor <johncnaylorls@gmail.com
> <mailto:johncnaylorls@gmail.com>> napsal:
> 
>     On Wed, Dec 10, 2025 at 5:20 PM Tomas Vondra <tomas@vondra.me
>     <mailto:tomas@vondra.me>> wrote:
>     > I did however notice an interesting thing - running EXPLAIN on the 99
>     > queries (for 3 scales and 0/4 workers, so 6x 99) took this much time:
>     >
>     > master:       8s
>     > master/geqo: 20s
>     > master/goo:   5s
> 
>     > It's nice that "goo" seems to be faster than "geqo" - assuming the
>     plans
>     > are comparable or better. But it surprised me switching to geqo
>     makes it
>     > slower than master. That goes against my intuition that geqo is
>     meant to
>     > be cheaper/faster join order planning. But maybe I'm missing
>     something.
> 
>     Yeah, that was surprising. It seems that geqo has a large overhead, so
>     it takes a larger join problem for the asymptotic behavior to win over
>     exhaustive search.
> 
> 
> If I understand correctly to design - geqo should be slower for any
> queries with smaller complexity. The question is how many queries in the
> tested model are really complex.
> 

Depends on what you mean by "really complex". TPC-DS queries are not
trivial, but the complexity may not be in the number of joins.

Of course, setting geqo_threshold to 2 may be too aggressive. Not sure.


regards

-- 
Tomas Vondra