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From: Andrei Lepikhov <a.lepikhov@postgrespro.ru>
To: Robert Haas <robertmhaas@gmail.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Richard Guo <guofenglinux@gmail.com>
Cc: Alexander Lakhin <exclusion@gmail.com>
Cc: Gregory Stark (as CFM) <stark.cfm@gmail.com>
Cc: Michał Kłeczek <michal@kleczek.org>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: Removing unneeded self joins
Date: Sun, 5 May 2024 09:45:47 +0700
Message-ID: <24e93efd-eeac-43a6-a75a-ad68eacb32ee@postgrespro.ru> (raw)
In-Reply-To: <CA+TgmoYSRwjeTDeLY+OfMkKZOg0M6bBOh-L9UfZExu0xZVY_bA@mail.gmail.com>
References: <64486b0b-0404-e39e-322d-0801154901f3@postgrespro.ru>
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On 3/5/2024 20:55, Robert Haas wrote:
> One of my most embarrassing gaffes in this area personally was
> a448e49bcbe40fb72e1ed85af910dd216d45bad8. I don't know how I managed
> to commit the original patch without realizing it was going to cause
> an increase in the WAL size, but I can tell you that when I realized
> it, my heart sank through the floor.
I discovered this feature and agree that it looks like a severe problem.
Unfortunately, in the case of the SJE patch, the committer and reviewers 
don't provide negative feedback. We see the only (I'm not sure I use the 
proper English phrase) 'negative feelings' from people who haven't 
reviewed or analysed it at all (at least, they didn't mention it).

Considering the situation, I suggest setting the default value of 
enable_self_join_removal to false in PG17 for added safety and then 
changing it to true in early PG18.

Having no objective negative feedback, we have no reason to change 
anything in the design or any part of the code. It looks regrettable and 
unusual.

After designing the feature, fixing its bugs, and reviewing joint 
patches on the commitfest, the question more likely lies in the planner 
design. For example, I wonder if anyone here knows why exactly the 
optimiser makes a copy of the whole query subtree in some places. 
Another example is PlannerInfo. Can we really control all the 
consequences of introducing, let's say, a new JoinDomain entity?

You also mentioned 2024.pgconf.dev. Considering the current migration 
policy in some countries, it would be better to work through the online 
presence as equivalent to offline. Without an online part of the 
conference, the only way to communicate and discuss is through this 
mailing list.

-- 
regards,
Andrei Lepikhov






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