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Date: Sun, 23 Feb 2025 23:12:36 +0100
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Subject: Re: Removing unneeded self joins
To: Alexander Korotkov <aekorotkov@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>
Cc: Alena Rybakina <a.rybakina@postgrespro.ru>,
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 "Gregory Stark (as CFM)" <stark.cfm@gmail.com>,
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From: Andrei Lepikhov <lepihov@gmail.com>
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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 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.

-- 
regards, Andrei Lepikhov