From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andrei Lepikhov <a.lepikhov@postgrespro.ru>
cc: Robert Haas <robertmhaas@gmail.com>,
        "pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
Subject: Re: post-freeze damage control
In-reply-to: <38494264-62c1-4b37-be16-84197018e8f7@postgrespro.ru>
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Comments: In-reply-to Andrei Lepikhov <a.lepikhov@postgrespro.ru>
	message dated "Tue, 09 Apr 2024 12:37:31 +0700"
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Date: Tue, 09 Apr 2024 01:55:39 -0400
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Andrei Lepikhov <a.lepikhov@postgrespro.ru> writes:
> On 9/4/2024 09:12, Tom Lane wrote:
>> I have another one that I'm not terribly happy about:
>> Author: Alexander Korotkov <akorotkov@postgresql.org>
>> Branch: master [72bd38cc9] 2024-04-08 01:27:52 +0300
>>     Transform OR clauses to ANY expression

>> * What the medical community would call off-label usage of
>> query jumbling.  I'm not sure this is even correct as-used,
>> and for sure it's using that code for something never intended.
>> Nor is the added code adequately (as in, at all) documented.

> I agree with documentation and disagree with critics on the expression 
> jumbling. It was introduced in the core. Why don't we allow it to be 
> used to speed up machinery with some hashing?

I would back up from that a good deal: why do we need to hash here in
the first place?  There's no evidence I'm aware of that it's needful
from a performance standpoint.

>> * I really, really dislike jamming this logic into prepqual.c,
>> where it has no business being.  I note that it was shoved
>> into process_duplicate_ors without even the courtesy of
>> expanding the header comment:

> Yeah, I preferred to do it in parse_expr.c with the assumption of some 
> 'minimal' or 'canonical' tree form.

That seems quite the wrong direction to me.  AFAICS, the argument
for making this transformation depends on being able to convert
to an indexscan condition, so I would try to apply it even later,
when we have a set of restriction conditions to apply to a particular
baserel.  (This would weaken the argument that we need hashing
rather than naive equal() tests even further, I think.)  Applying
the transform to join quals seems unlikely to be a win.

			regards, tom lane