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Date: Mon, 24 Feb 2025 13:22:35 +0100
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Subject: Re: Removing unneeded self joins
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Alena Rybakina <a.rybakina@postgrespro.ru>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>,
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 Alexander Lakhin <exclusion@gmail.com>,
 "Gregory Stark (as CFM)" <stark.cfm@gmail.com>,
 =?UTF-8?Q?Micha=C5=82_K=C5=82eczek?= <michal@kleczek.org>,
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From: Andrei Lepikhov <lepihov@gmail.com>
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On 24/2/2025 11:57, Alexander Korotkov wrote:
> Hi, Andrei!
> 
> Thank you for your feedback.
> 
> On Mon, Feb 24, 2025 at 12:12 AM Andrei Lepikhov <lepihov@gmail.com> wrote:
>> On 23/2/2025 22:15, Alexander Korotkov wrote:
>>> There is my attempt to implement this approach.  Getting rid of local
>>> variable (and computation of the same value other way) required to
>>> change arguments of remove_rel_from_eclass() as well.  I'm going to
>>> further polish this tomorrow.
>> I passed through the patch. It works correctly.
>>
>> Multiple ifs in a single routine is not ideal.
> 
> I would say there is a brachcing anyway.  Even if it is inside
> adjust_relid_set().  Patch makes it more explicit.  Ideal or not, I
> don't think it gets worse.
> 
>> I have thought about it
>> already, and it seems the remove_rel_from_query needs refactoring: when
>> I first reused it for self-join removal, we didn't have the 'ojrelid'
>> machinery, and it was implemented smoothly.
>> Right now, this code contains multiple places where we need to remove
>> the outer join relid and separating the removal code for the baserel and
>> outer join may simplify the logic and make it safer.
>> But the way to do it is not apparent now. May be if we implement a new
>> technique of query tree reduction, the approach will become more evident.
> 
> Could you, please, elaborate more on what you mean by "new technique
> of query tree reduction"?
I mean any transformations and optimisations that reduce search space 
for optimisation. Right now, I see the features reduce_unique_semijoins, 
remove_useless_joins, and remove_useless_self_joins.
In practice, I see at least a join on a foreign key, where some cases 
potentially allow the removal of the JOIN operator.

-- 
regards, Andrei Lepikhov