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Re: Column Filtering in Logical Replication

Rahila Syed <rahilasyed90@gmail.com>

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From: Rahila Syed <rahilasyed90@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Peter Smith <smithpb2250@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2021-08-12T03:10:00Z
Lists: pgsql-hackers 
Hi Amit,

Thanks for your review.


> Can you please explain why you have the restriction for including
> replica identity columns and do we want to put a similar restriction
> for the primary key? As far as I understand, if we allow default
> values on subscribers for replica identity, then probably updates,
> deletes won't work as they need to use replica identity (or PK) to
> search the required tuple. If so, shouldn't we add this restriction
> only when a publication has been defined for one of these (Update,
> Delete) actions?
>

Yes, like you mentioned they are needed for Updates and Deletes to work.
The restriction for including replica identity columns in column filters
exists because
In case the replica identity column values did not change, the old row
replica identity columns
are not sent to the subscriber, thus we would need new replica identity
columns
to be sent to identify the row that is to be Updated or Deleted.
I haven't tested if it would break Insert as well  though. I will update
the patch accordingly.


> Another point is what if someone drops the column used in one of the
> publications? Do we want to drop the entire relation from publication
> or just remove the column filter or something else?
>
>
Thanks for pointing this out. Currently, this is not handled in the patch.
I think dropping the column from the filter would make sense on the lines
of the table being dropped from publication, in case of drop table.


> Do we want to consider that the columns specified in the filter must
> not have NOT NULL constraint? Because, otherwise, the subscriber will
> error out inserting such rows?
>
> I think you mean columns *not* specified in the filter must not have NOT
NULL constraint
on the subscriber, as this will break during insert, as it will try to
insert NULL for columns
not sent by the publisher.
I will look into fixing this. Probably this won't be a problem in
case the column is auto generated or contains a default value.


> Minor comments:
> ================
>   pq_sendbyte(out, flags);
> -
>   /* attribute name */
>   pq_sendstring(out, NameStr(att->attname));
>
> @@ -953,6 +1000,7 @@ logicalrep_write_attrs(StringInfo out, Relation rel)
>
>   /* attribute mode */
>   pq_sendint32(out, att->atttypmod);
> +
>   }
>
>   bms_free(idattrs);
> diff --git a/src/backend/replication/logical/relation.c
> b/src/backend/replication/logical/relation.c
> index c37e2a7e29..d7a7b00841 100644
> --- a/src/backend/replication/logical/relation.c
> +++ b/src/backend/replication/logical/relation.c
> @@ -354,7 +354,6 @@ logicalrep_rel_open(LogicalRepRelId remoteid,
> LOCKMODE lockmode)
>
>   attnum = logicalrep_rel_att_by_name(remoterel,
>   NameStr(attr->attname));
> -
>   entry->attrmap->attnums[i] = attnum;
>
> There are quite a few places in the patch that contains spurious line
> additions or removals.
>
>
Thank you for your comments, I will fix these.

Thank you,
Rahila Syed

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Doc: Explain about Column List feature.

  2. Doc: fix column list vs. replica identity rules.

  3. Prohibit combining publications with different column lists.

  4. Fix the check to limit sync workers.

  5. Wait for subscription to sync in t/031_column_list.sql

  6. Move prattrs to the pg_publication_rel section in docs

  7. Allow specifying column lists for logical replication

  8. Fix row filters with multiple publications

  9. Fix publish_as_relid with multiple publications

  10. Add some additional tests for row filters in logical replication.

  11. Add index on pg_publication_rel.prpubid

  12. Avoid using DefElemAction in AlterPublicationStmt

  13. Small cleanups related to PUBLICATION framework code

  14. Add PublicationTable and PublicationRelInfo structs

  15. Fix various concurrency issues in logical replication worker launching