Message-ID: <735b5cd4-568d-fbf7-ad8c-dcfb4c3af86e@gmail.com>
Date: Wed, 1 May 2024 14:00:00 +0300
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.4.2
Subject: Re: Removing unneeded self joins
Content-Language: en-US
To: Alexander Korotkov <aekorotkov@gmail.com>
Cc: Andrei Lepikhov <a.lepikhov@postgrespro.ru>,
 "Gregory Stark (as CFM)" <stark.cfm@gmail.com>,
 =?UTF-8?Q?Micha=c5=82_K=c5=82eczek?= <michal@kleczek.org>,
 PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
References: <64486b0b-0404-e39e-322d-0801154901f3@postgrespro.ru>
 <bcf2f3c5-fbdc-f85b-af3e-ebec616160f6@postgrespro.ru>
 <05469e80-454b-41d7-d04f-421f42bbd4a9@postgrespro.ru>
 <CAPpHfdt9G3EU+vXuqexhweEQoMF-acPUUFuB6hV2Kg0gCy7AjA@mail.gmail.com>
 <ebe66358-428c-439f-85dd-da72c9b2b0c2@postgrespro.ru>
 <CAPpHfdt-0kVV7O==aJEbjY2iGYBu+XBzTHEbPv_6sVNeC7fffQ@mail.gmail.com>
 <48c11b23-9c23-440e-9a3c-020e05196449@postgrespro.ru>
 <CAPpHfdsQkY0Gu7bQ9Ekdyzo_5Xxz74TYKbwmk1on1WAASpxXcQ@mail.gmail.com>
 <8800ccf2-ae82-461c-92a1-d4c0a22f8cf2@postgrespro.ru>
 <CAPpHfdvJ3jBQaJQKrf69K++kXi+Tvzn6TXO-DKXj0KVJjvMebg@mail.gmail.com>
 <1bd80083-a13e-4e9f-a7ce-73c3faab521c@postgrespro.ru>
 <CAPpHfdsz8E085YF2VoJe2+h4YAShsh7p--t3kQfQTntiCy63Xg@mail.gmail.com>
 <922623af-872e-4cdc-9238-129b18bee42d@postgrespro.ru>
 <CAPpHfduoAHTp6O7Q8PDe_w=30v0n_06pGe5M65iR4DYcfaYnRw@mail.gmail.com>
 <0ed2cf02-e655-6ec7-a4fe-52fd8d572f65@gmail.com>
 <CAPpHfdsHWbdPVzUdmoKv3dpo8RHJOa=LQR-ZAT+89uvktMs9XA@mail.gmail.com>
From: Alexander Lakhin <exclusion@gmail.com>
In-Reply-To: 
 <CAPpHfdsHWbdPVzUdmoKv3dpo8RHJOa=LQR-ZAT+89uvktMs9XA@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Archived-At: 
 <https://www.postgresql.org/message-id/735b5cd4-568d-fbf7-ad8c-dcfb4c3af86e%40gmail.com>
Precedence: bulk

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