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List:       postgresql-admin
Subject:    Re: How to debug logical replication error "columns are missing" if they are not
From:       Thomas Kellerer <shammat () gmx ! net>
Date:       2020-08-25 11:34:05
Message-ID: e5d69628-de66-79a6-b11b-53c105747e30 () gmx ! net
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Thomas Kellerer schrieb am 25.08.2020 um 07:32:
> we have a a logical replication from Postgres 11 to Postgres 12.
> 
> We did some structural changes to the published tables. The changes
> to the subscriber where applied about 60 minutes later then those on
> the publisher. Obviously before the subscriber's tables where synced
> the replication failed.
> But even though the tables are now completely identical, the subscriber still \
> claims: logical replication target relation "public.employee" is missing some \
> replicated columns

So - as explained by Samed - the problem boils down to a setup like this:

The initial table looked like this (on both sides)

    create table test_table (id integer primary key, valid_from date, valid_to date);

Replication is running fine, then we run the following on the publisher

  begin transaction;
    ALTER TABLE test_table ADD COLUMN start_end_date daterange;

    UPDATE test_table SET start_end_date = daterange(valid_from, valid_to, '[]');

    ALTER TABLE test_table ALTER COLUMN start_end_date SET NOT NULL;

    ALTER TABLE test_table
      DROP COLUMN valid_from,
      DROP COLUMN valid_to;
  commit;

If the UPDATE part is removed from the change, everything works smoothly.

So, the UPDATE sends the complete modified row including the to be dropped columns to \
the subscriber. As I ran the same steps on the subscriber, the columns were dropped \
on the subscriber before the UPDATE could be replayed and thus it kept failing.

I do understand now why this happens, and that it is a limitation of the current \
implementation.


However, what I don't understand is, why removing the table from the replication \
doesn't fix this. It seems, that if the table is re-added later, the old WAL segments \
are still considered valid and Postgres tries to replay them.

Which seems a rather strange thing to do to me.
Could anyone enlighten me on that?
Is there a way to mark the no longer needed WAL segments as obsolete?


Regards
Thomas


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