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From: Alexander Lakhin <exclusion@gmail.com>
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Andrei Lepikhov <a.lepikhov@postgrespro.ru>
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: Wed, 1 May 2024 14:00:00 +0300
Message-ID: <735b5cd4-568d-fbf7-ad8c-dcfb4c3af86e@gmail.com> (raw)
In-Reply-To: <CAPpHfdsHWbdPVzUdmoKv3dpo8RHJOa=LQR-ZAT+89uvktMs9XA@mail.gmail.com>
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	<CAPpHfduoAHTp6O7Q8PDe_w=30v0n_06pGe5M65iR4DYcfaYnRw@mail.gmail.com>
	<0ed2cf02-e655-6ec7-a4fe-52fd8d572f65@gmail.com>
	<CAPpHfdsHWbdPVzUdmoKv3dpo8RHJOa=LQR-ZAT+89uvktMs9XA@mail.gmail.com>

30.04.2024 13:20, Alexander Korotkov wrote:
> On Tue, Apr 30, 2024 at 9:00 AM Alexander Lakhin <exclusion@gmail.com> wrote:
>> I've discovered another failure, introduced by d3d55ce57.
>> Please try the following:
>> CREATE TABLE t (a int unique, b float);
>> SELECT * FROM t NATURAL JOIN LATERAL
>>    (SELECT * FROM t t2 TABLESAMPLE SYSTEM (t.b)) t2;
> I think we should just forbid SJE in case when relations to be merged
> have cross-references with lateral vars.  The draft patch for this is
> attached.  I'd like to ask Alexander to test it, Richard and Andrei to
> review it.  Thank you!

Beside LATERAL vars, it seems that SJR doesn't play well with TABLESAMPLE
in general. For instance:
CREATE TABLE t (a int unique);
INSERT INTO t SELECT * FROM generate_series (1,100);

SELECT COUNT(*) FROM (SELECT * FROM t TABLESAMPLE BERNOULLI(1)) t1
  NATURAL JOIN (SELECT * FROM t TABLESAMPLE BERNOULLI(100)) t2;
returned 100, 100, 100 for me, though with enable_self_join_removal = off,
I got 4, 0, 1...

Best regards,
Alexander





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