pg.ddx.io  pgsql-hackers@postgresql.org mailing list archive  

help / color / mirror / Atom feed
From: a.rybakina <a.rybakina@postgrespro.ru>
To: 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>
Cc: Alexander Korotkov <aekorotkov@gmail.com>
Subject: Re: Removing unneeded self joins
Date: Fri, 13 Oct 2023 12:42:35 +0300
Message-ID: <69a74357-102c-4591-9619-f672076135db@postgrespro.ru> (raw)
In-Reply-To: <f052df9a-c714-4c60-92ba-9b5ae7e258b0@postgrespro.ru>
References: <64486b0b-0404-e39e-322d-0801154901f3@postgrespro.ru>
	<d9493d5a-b950-465e-f09b-62c92a23fba1@postgrespro.ru>
	<20221206184643.zya44h5auv6f45zp@awork3.anarazel.de>
	<36579124-f9d9-a51e-38b3-af3511921c7e@postgrespro.ru>
	<88079CBD-B5BB-4C61-A68B-A8C67599E1C7@kleczek.org>
	<CAM-w4HMZXW+hKc8C+WLQ+gdpEkoh9378a2UGKyZRz7mZVerG+Q@mail.gmail.com>
	<c97574c4-13e6-9107-26ab-af2bd626fc85@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>
	<9eb86ca9-da3c-417f-8ce1-29c0ef5a4f50@postgrespro.ru>
	<29ddec15-a83f-4f24-b069-6e93c189b169@postgrespro.ru>
	<f052df9a-c714-4c60-92ba-9b5ae7e258b0@postgrespro.ru>

On 13.10.2023 12:03, Andrei Lepikhov wrote:
> On 13/10/2023 15:56, a.rybakina wrote:
>>
>>>> Also I've incorporated improvements from Alena Rybakina except one for
>>>> skipping SJ removal when no SJ quals is found.  It's not yet clear for
>>>> me if this check fix some cases. But at least optimization got skipped
>>>> in some useful cases (as you can see in regression tests).
>>>
>>> Agree. I wouldn't say I like it too. But also, I suggest skipping 
>>> some unnecessary assertions proposed in that patch:
>>> Assert(toKeep->relid != -1); - quite strange. Why -1? Why not all 
>>> the negative numbers, at least?
>>> Assert(is_opclause(orinfo->clause)); - above we skip clauses with 
>>> rinfo->mergeopfamilies == NIL. Each mergejoinable clause is already 
>>> checked as is_opclause.
>>> All these changes (see in the attachment) are optional.
>>>
>> I don't mind about asserts, maybe I misunderstood something in the 
>> patch.
>>
>> About skipping SJ removal when no SJ quals is found, I assume it is 
>> about it:
>>
>> split_selfjoin_quals(root, restrictlist, &selfjoinquals,
>>                                    &otherjoinquals, inner->relid, 
>> outer->relid);
>>
>> +            if (list_length(selfjoinquals) == 0)
>> +             {
>> +                 /*
>> +                  * XXX:
>> +                  * we would detect self-join without quals like 
>> 'x==x' if we had
>> +                  * an foreign key constraint on some of other quals 
>> and this join
>> +                  * haven't any columns from the outer in the target 
>> list.
>> +                  * But it is still complex task.
>> +                  */
>> +                 continue;
>> +             }
>>
>> as far as I remember, this is the place where it is checked that the 
>> SJ list is empty and it is logical, in my opinion, that no 
>> transformations should be performed if no elements are found for them.
> You forget we have "Degenerate" case, as Alexander mentioned above. 
> What if you have something like that:
> SELECT ... FROM A a1, A a2 WHERE a1.id=1 AND a2.id=1;
> In this case, uniqueness can be achieved by the baserestrictinfo 
> "A.id=1", if we have an unique index on this column.
>
Yes, sorry, I missed it. thanks again for the explanation 🙂





view thread (247+ messages)  latest in thread

Message-ID: <69a74357-102c-4591-9619-f672076135db@postgrespro.ru>
Permalink:  ../69a74357-102c-4591-9619-f672076135db@postgrespro.ru/
Also on:    postgresql.org/message-id/69a74357-102c-4591-9619-f672076135db@postgrespro.ru

 · 

reply
Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: pgsql-hackers@postgresql.org
  Cc: a.rybakina@postgrespro.ru, a.lepikhov@postgrespro.ru, stark.cfm@gmail.com, michal@kleczek.org, pgsql-hackers@lists.postgresql.org, aekorotkov@gmail.com
  Subject: Re: Removing unneeded self joins
  In-Reply-To: <69a74357-102c-4591-9619-f672076135db@postgrespro.ru>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by DDX for PostgreSQL; see mirroring instructions
for how to clone and mirror all data and code used for this inbox