RE: [PATCH] Speedup truncates of relation forks

On Tuesday, June 11, 2019 7:23 PM (GMT+9), Adrien Nayrat wrote:

> > Attached is a patch to speed up the performance of truncates of relations.
> 
> Thanks for working on this!

Thank you also for taking a look at my thread. 

> > If you want to test with large number of relations,
> > you may use the stored functions I used here:
> > http://bit.ly/reltruncates
> 
> You should post these functions in this thread for the archives ;)
This is noted. Pasting it below: 

create or replace function create_tables(numtabs int)
returns void as $$
declare query_string text;
begin
  for i in 1..numtabs loop
    query_string := 'create table tab_' || i::text || ' (a int);';
    execute query_string;
  end loop;
end;
$$ language plpgsql;

create or replace function delfrom_tables(numtabs int)
returns void as $$
declare query_string text;
begin
  for i in 1..numtabs loop
    query_string := 'delete from tab_' || i::text;
    execute query_string;
  end loop;
end;
$$ language plpgsql;

create or replace function insert_tables(numtabs int)
returns void as $$
declare query_string text;
begin
  for i in 1..numtabs loop
    query_string := 'insert into tab_' || i::text || ' VALUES (5);' ;
    execute query_string;
  end loop;
end;
$$ language plpgsql;


> From a user POW, the main issue with relation truncation is that it can block
> queries on standby server during truncation replay.
> 
> It could be interesting if you can test this case and give results of your
> path.
> Maybe by performing read queries on standby server and counting wait_event
> with pg_wait_sampling?

Thanks for the suggestion. I tried using the extension pg_wait_sampling,
But I wasn't sure that I could replicate the problem of blocked queries on standby server.
Could you advise?
Here's what I did for now, similar to my previous test with hot standby setup,
but with additional read queries of wait events on standby server.

128MB shared_buffers
SELECT create_tables(10000);
SELECT insert_tables(10000);
SELECT delfrom_tables(10000);

[Before VACUUM]
Standby: SELECT the following view from pg_stat_waitaccum

wait_event_type |   wait_event    | calls | microsec
-----------------+-----------------+-------+----------
 Client          | ClientRead      |     2 | 20887759
 IO              | DataFileRead    |   175 |     2788
 IO              | RelationMapRead |     4 |       26
 IO              | SLRURead        |     2 |       38

Primary: Execute VACUUM (induces relation truncates)

[After VACUUM]
Standby:
 wait_event_type |   wait_event    | calls | microsec
-----------------+-----------------+-------+----------
 Client          | ClientRead      |     7 | 77662067
 IO              | DataFileRead    |   284 |     4523
 IO              | RelationMapRead |    10 |       51
 IO              | SLRURead        |     3 |       57

Regards,
Kirk Jamison