Re: Removing unneeded self joins - Mailing list pgsql-hackers

On 12/10/2023 18:32, Alexander Korotkov wrote:
> On Thu, Oct 5, 2023 at 12:17 PM Andrei Lepikhov
> <a.lepikhov@postgrespro.ru> wrote:
>> On 4/10/2023 14:34, Alexander Korotkov wrote:
>>>   > Relid replacement machinery is the most contradictory code here. We used
>>>   > a utilitarian approach and implemented a simplistic variant.
>>>
>>>   > > 2) It would be nice to skip the insertion of IS NOT NULL checks when
>>>   > > they are not necessary.  [1] points that infrastructure from [2] might
>>>   > > be useful.  The patchset from [2] seems committed mow.  However, I
>>>   > > can't see it is directly helpful in this matter.  Could we just skip
>>>   > > adding IS NOT NULL clause for the columns, that have
>>>   > > pg_attribute.attnotnull set?
>>>   > Thanks for the links, I will look into that case.
>> To be more precise, in the attachment, you can find a diff to the main
>> patch, which shows the volume of changes to achieve the desired behaviour.
>> Some explains in regression tests shifted. So, I've made additional tests:
>>
>> DROP TABLE test CASCADE;
>> CREATE TABLE test (a int, b int not null);
>> CREATE UNIQUE INDEX abc ON test(b);
>> explain SELECT * FROM test t1 JOIN test t2 ON (t1.a=t2.a)
>> WHERE t1.b=t2.b;
>> CREATE UNIQUE INDEX abc1 ON test(a,b);
>> explain SELECT * FROM test t1 JOIN test t2 ON (t1.a=t2.a)
>> WHERE t1.b=t2.b;
>> explain SELECT * FROM test t1 JOIN test t2 ON (t1.a=t2.a)
>> WHERE t1.b=t2.b AND (t1.a=t2.a OR t2.a=t1.a);
>> DROP INDEX abc1;
>> explain SELECT * FROM test t1 JOIN test t2 ON (t1.a=t2.a)
>> WHERE t1.b=t2.b AND (t1.b=t2.b OR t2.b=t1.b);
>>
>> We have almost the results we wanted to have. But in the last explain
>> you can see that nothing happened with the OR clause. We should use the
>> expression mutator instead of walker to handle such clauses. But It
>> doesn't process the RestrictInfo node ... I'm inclined to put a solution
>> of this issue off for a while.
> 
> OK.  I think it doesn't worth to eliminate IS NULL quals with this
> complexity (at least at this stage of work).
> 
> I made improvements over the code.  Mostly new comments, grammar
> corrections of existing comments and small refactoring.
> 
> Also, I found that the  suggestion from David Rowley [1] to qsort
> array of relations to faster find duplicates is still unaddressed.
> I've implemented it.  That helps to evade quadratic complexity with
> large number of relations.
> 
> 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).

I would like to propose one more minor improvement (see in attachment). 
The idea here is that after removing a self-join and changing clauses we 
should re-probe the set of relids with the same Oid, because we can find 
more removable self-joins (see the demo test in join.sql).

-- 
regards,
Andrey Lepikhov
Postgres Professional