Re: Removing unneeded self joins

From: Andrei Lepikhov <a(dot)lepikhov(at)postgrespro(dot)ru>
To: Alexander Korotkov <aekorotkov(at)gmail(dot)com>
Cc: "Gregory Stark (as CFM)" <stark(dot)cfm(at)gmail(dot)com>, Michał Kłeczek <michal(at)kleczek(dot)org>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: Removing unneeded self joins
Date: 2023-10-23 03:43:13
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On 22/10/2023 05:01, Alexander Korotkov wrote:
> On Thu, Oct 19, 2023 at 6:16 AM Andrei Lepikhov
> <a(dot)lepikhov(at)postgrespro(dot)ru> wrote:
>> On 19/10/2023 01:50, Alexander Korotkov wrote:
>>> This query took 3778.432 ms with self-join removal disabled, and
>>> 3756.009 ms with self-join removal enabled. So, no measurable
>>> overhead. Similar to the higher number of joins. Can you imagine
>>> some extreme case when self-join removal could introduce significant
>>> overhead in comparison with other optimizer parts? If not, should we
>>> remove self_join_search_limit GUC?
>> Thanks,
>> It was Zhihong Yu who worried about that case [1]. And my purpose was to
>> show a method to avoid such a problem if it would be needed.
>> I guess the main idea here is that we have a lot of self-joins, but only
>> few of them (or no one) can be removed.
>> I can't imagine a practical situation when we can be stuck in the
>> problems here. So, I vote to remove this GUC.
> I've removed the self_join_search_limit. Anyway there is
> enable_self_join_removal if the self join removal algorithm causes any
> problems. I also did some grammar corrections for the comments. I
> think the patch is getting to the committable shape. I noticed some
> failures on I'd like to check how this
> version will pass it.

I have observed the final patch. A couple of minor changes can be made
(see attachment).
Also, I see room for improvement, but it can be done later. For example,
we limit the optimization to only ordinary tables in this patch. It can
be extended at least with partitioned and foreign tables soon.

Andrey Lepikhov
Postgres Professional

Attachment Content-Type Size
minor_changes.diff text/plain 685 bytes

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