Horton

Re: row filtering for logical replication

Greg Nancarrow <gregn4422@gmail.com>

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From: Greg Nancarrow <gregn4422@gmail.com>
To: Amit Kapila <amit.kapila16@gmail.com>
Cc: Tomas Vondra <tomas.vondra@enterprisedb.com>, Euler Taveira <euler@eulerto.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Peter Smith <smithpb2250@gmail.com>, Rahila Syed <rahilasyed90@gmail.com>, Peter Eisentraut <peter.eisentraut@enterprisedb.com>, Önder Kalacı <onderkalaci@gmail.com>, japin <japinli@hotmail.com>, Michael Paquier <michael@paquier.xyz>, David Steele <david@pgmasters.net>, Craig Ringer <craig@2ndquadrant.com>, Tomas Vondra <tomas.vondra@2ndquadrant.com>, Amit Langote <amitlangote09@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Date: 2021-07-15T11:50:46Z
Lists: pgsql-hackers

Commits

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  1. Release cache tuple when no longer needed

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

  3. Fix one of the tests introduced in commit 52e4f0cd47.

  4. Allow specifying row filters for logical replication of tables.

  5. Move scanint8() to numutils.c

  6. Replace Test::More plans with done_testing

  7. Reduce relcache access in WAL sender streaming logical changes

  8. Small cleanups related to PUBLICATION framework code

  9. Add a view to show the stats of subscription workers.

  10. Allow publishing the tables of schema.

  11. Doc: improve documentation of CREATE/ALTER SUBSCRIPTION.

  12. Add PublicationTable and PublicationRelInfo structs

  13. Remove unused argument "txn" in maybe_send_schema().

  14. Add prepare API support for streaming transactions in logical replication.

  15. Unify PostgresNode's new() and get_new_node() methods

  16. Use l*_node() family of functions where appropriate

  17. Add support for prepared transactions to built-in logical replication.

  18. Restore the portal-level snapshot after procedure COMMIT/ROLLBACK.

  19. Rename a parse node to be more general

  20. Remove unused column atttypmod from initial tablesync query

  21. SEARCH and CYCLE clauses

On Wed, Jul 14, 2021 at 10:50 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
>
> I think apart from the above, it might be good if we can find what
> some other databases does in this regard?
>

I did a bit of investigation in the case of Oracle Database and SQL Server.
(purely from my interpretation of available documentation; I did not
actually use the replication software)

For Oracle (GoldenGate), it appears that it provides the ability for
filters to reference both OLD and NEW rows in replication of UPDATEs:
"For update operations, it can be advantageous to retrieve the before
values of source columns: the values before the update occurred. These
values are stored in the trail and can be used in filters and column
mappings"
It provides @BEFORE and @AFTER functions for this.

For SQL Server, the available replication models seem quite different
to that in PostgreSQL, and not all seem to support row filtering.
For "snapshot replication", it seems that it effectively supports
filtering rows on the NEW values.
It seems that the snapshot is taken at a transactional boundary and
rows included according to any filtering, and is then replicated.
So to include the result of a particular UPDATE in the replication,
the replication row filtering would effectively be done on the result
(NEW) rows.
Another type of replication that supports row filtering is "merge
replication", which again seems to be effectively based on NEW rows:
"For merge replication to process a row, the data in the row must
satisfy the row filter, and it must have changed since the last
synchronization"
It's not clear to me if there is ANY way to filter on the OLD row
values by using some option.

If anybody has experience with the replication software for these
other databases and I've interpreted the documentation for these
incorrectly, please let me know.

Regards,
Greg Nancarrow
Fujitsu Australia