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Re: Column Filtering in Logical Replication

Peter Smith <smithpb2250@gmail.com>

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From: Peter Smith <smithpb2250@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: vignesh C <vignesh21@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>
Date: 2022-09-02T03:15:33Z
Lists: pgsql-hackers

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On Thu, Sep 1, 2022 at 7:53 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> On Fri, Aug 26, 2022 at 7:33 AM Peter Smith <smithpb2250@gmail.com> wrote:
> >
>
> Few comments on both the patches:
> v4-0001*
> =========
> 1.
> Furthermore, if the table uses
> +   <literal>REPLICA IDENTITY FULL</literal>, specifying a column list is not
> +   allowed (it will cause publication errors for <command>UPDATE</command> or
> +   <command>DELETE</command> operations).
>
> This line sounds a bit unclear to me. From this like it appears that
> the following operation is not allowed:
>
> postgres=# create table t1(c1 int, c2 int, c3 int);
> CREATE TABLE
> postgres=# Alter Table t1 replica identity full;
> ALTER TABLE
> postgres=# create publication pub1 for table t1(c1);
> CREATE PUBLICATION
>
> However, what is not allowed is the following:
> postgres=# delete from t1;
> ERROR:  cannot delete from table "t1"
> DETAIL:  Column list used by the publication does not cover the
> replica identity.
>
> I am not sure if we really need this line but if so then please try to
> make it more clear. I think the similar text is present in 0002 patch
> which should be modified accordingly.
>

The "Furthermore…" sentence came from the commit message [1]. But I
agree it seems redundant/ambiguous, so I have removed it (and removed
the same in patch 0002).


> V4-0002*
> =========
> 2.
> However, if a
> +   <firstterm>column list</firstterm> is specified then only the columns named
> +   in the list will be replicated. This means the subscriber-side table only
> +   needs to have those columns named by the column list. A user might choose to
> +   use column lists for behavioral, security or performance reasons.
> +  </para>
> +
> +  <sect2 id="logical-replication-col-list-rules">
> +   <title>Column List Rules</title>
> +
> +   <para>
> +    A column list is specified per table following the table name, and enclosed
> +    by parentheses. See <xref linkend="sql-createpublication"/> for details.
> +   </para>
> +
> +   <para>
> +    When a column list is specified, only the named columns are replicated.
> +    The list order is not important.
>
> It seems like "When a column list is specified, only the named columns
> are replicated." is almost a duplicate of the line in the first para.
> So, I think we can remove it. And if we do so then the second line
> could be changed to something like: "While specifying column list, the
> order of columns is not important."
>

Modified as suggested.

> 3. It seems information about initial table synchronization is
> missing. We copy only columns specified in the column list. Also, it
> would be good to add a Note similar to Row Filter to indicate that
> this list won't be used by pre-15 publishers.
>

Done as suggested. Added a new "Initial Data Synchronization" section
with content similar to that of the Row Filters section.

~~~

Thanks for your review comments.

PSA v5* patches where all the above have been addressed.

------
[1] https://github.com/postgres/postgres/commit/923def9a533a7d986acfb524139d8b9e5466d0a5

Kind Regards,
Peter Smith.
Fujitsu Australia

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