RE: Allow logical replication to copy tables in binary format - Mailing list pgsql-hackers

Hi,


On Monday, October 3, 2022 8:50 PM Melih Mutlu <m.melihmutlu@gmail.com> wrote:
>     (4) Error message of the binary format copy
>
>     I've gotten below message from data type contradiction (between
> integer and bigint).
>     Probably, this is unclear for the users to understand the direct cause
>     and needs to be adjusted ?
>     This might be a similar comment Euler mentioned in [1].
>
>     2022-09-16 11:54:54.835 UTC [4570] ERROR:  insufficient data left in
> message
>     2022-09-16 11:54:54.835 UTC [4570] CONTEXT:  COPY tab, line 1,
> column id
>
> It's already unclear for users to understand what's the issue if they're copying
> data between different column types via the COPY command.
> This issue comes from COPY, and logical replication just utilizes COPY.
> I don't think it would make sense to adjust an error message from a functionality
> which logical replication only uses and has no direct impact on.
> It might be better to do this in a separate patch. What do you think?
Yes, makes sense. It should be a separate patch.


>     I agree with the direction to support binary copy for v16 and later.
>
>     IIUC, the binary format replication with different data types fails even
> during apply phase on HEAD.
>     I thought that means, the upgrade concern only applies to a scenario
> that the user executes
>     only initial table synchronizations between the publisher and subscriber
>     and doesn't replicate any data at apply phase after that. I would say
>     this isn't a valid scenario and your proposal makes sense.
>
> No, logical replication in binary does not fail on apply phase if data types are
> different.
With HEAD, I observe in some case we fail at apply phase because of different data types like
integer vs. bigint as written scenario in [1]. In short, I think we can slightly
adjust your documentation and make it more general so that the description applies to
both table sync phase and apply phase.

I'll suggest a below change for your sentence of logical-replication.sgml.
FROM:
In binary case, it is not allowed to replicate data between different types due to restrictions inherited from COPY.
TO:
Binary format is type specific and does not allow to replicate data between different types according to its
restrictions.


If my idea above is correct, then I feel we can remove all the fixes for create_subscription.sgml.
I'm not sure if I should pursue this perspective of the document improvement
any further after this email, since this isn't essentially because of this patch.



> The concern with upgrade (if data types are not the same) would be not being
> able to create a new subscription with binary enabled or replicate new tables
> added into publication.
> Replication of tables from existing subscriptions would not be affected by this
> change since they will already be in the apply phase, not tablesync.
> Do you think this would still be an issue?
Okay, thanks for explaining this. I understand that
the upgrade concern applies to the table sync that is executed
between text format (before the patch) and binary format (after the patch).




[1] - binary format test that we fail for different types on apply phase on HEAD

<publisher>
create table tab (id integer);
insert into tab values (100);
create publication mypub for table tab;

<subscriber>
create table tab (id bigint);
create subscription mysub connection '...' publication mypub with (copy_data = false, binary = true, disable_on_error =
true);

-- wait for several seconds

<subscriber>
select srsubid, srrelid, srrelid::regclass, srsubstate, srsublsn from pg_subscription_rel; -- check the status as 'r'
forthe relation 
select * from tab; -- confirm we don't copy the initial data on the pub

<publisher>
insert into tab values (1), (2);

-- wait for several seconds

<subscriber>
select subname, subenabled from pg_subscription; -- shows 'f' for the 2nd column because of an error
select * from tab -- no records

This error doesn't happen when we adopt 'integer' on the subscriber aligned with the publisher
and we can see the two records on the subscriber.



Best Regards,
    Takamichi Osumi