Re: Initial COPY of Logical Replication is too slow

Run first time and will create 10 thousand tables, publish them and measure how much time to get all tables Ready on pg_subscription_rel.

Run again to add more 10 thousand tables and see that time will increase, more tables and more time.

This is just to show that if you create a subscription with a high number of tables it spends more time doing select on pg_get_publication_tables than the time spent actually copying. My use case I have 50 thousand tables, and it takes 5 seconds every time it needs to get next table to copy.

--Create a empty publication

create publication my_pub;

--Run these 3 following anonymous blocks to create schemas, tables and add them to publication.

--Need to have 3 blocks because I cannot create a table in a schema that is not committed. And the same for a publication.
do $$ declare Schemas_Add integer = 100; Actual_Schema text; begin
  for Actual_Schema in select 'test_'||(select to_char(coalesce(max(substring(nspname,'test_(\d+)')::integer),0)+g,'FM00000')
                                       from pg_namespace where nspname ~ 'test_\d+') from generate_series(1,Schemas_Add) g loop
    execute format('create schema %s',Actual_Schema);
  end loop;
end;$$;

do $$ declare Tables_Add integer = 100; Actual_Schema text; begin
  for Actual_Schema in select nspname from pg_namespace where nspname  ~ 'test_\d+' and
                      not exists(select from pg_class where relnamespace = pg_namespace.oid) loop
    for j in 1..Tables_Add loop
      execute format('create table %s.test_%s as select generate_series(1,random(0,10))::integer id;',
                     Actual_Schema,to_char(j,'FM00000'));
    end loop;
  end loop;
end;$$;

do $$ declare Schemas_To_Add text = (select string_agg(nspname,',') from pg_namespace n where nspname ~ 'test_\d+' and
                              not exists(select from pg_publication_namespace where pnnspid = n.oid)); begin
  execute format('alter publication my_pub add tables in schema %s;',Schemas_To_Add);
end;$$;

--Then you can see what was generated and go to the subscriber side to refresh the subscription and measure time spent to synchronize.
select * from pg_Namespace where nspname ~ 'test_\d+';
select pnnspid::regnamespace, * from pg_publication_namespace;
select oid::regclass, * from pg_Class where relnamespace::regnamespace::text ~ 'test_\d+' and relkind = 'r';

--Later just clean what you do.
drop publication my_pub;

do $$ declare Schema_Drop text; begin
  for Schema_Drop in select nspname from pg_Namespace where nspname ~ 'test_\d+' loop
    execute format ('drop schema %s cascade;',Schema_Drop);
  end loop;
end;$$;

regards

Marcos