Horton

Re: row filtering for logical replication

Dilip Kumar <dilipbalaut@gmail.com>

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From: Dilip Kumar <dilipbalaut@gmail.com>
To: Ajin Cherian <itsajin@gmail.com>
Cc: Amit Kapila <amit.kapila16@gmail.com>, Euler Taveira <euler@eulerto.com>, 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: 2021-09-21T04:23:44Z
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 Tue, Sep 21, 2021 at 8:58 AM Ajin Cherian <itsajin@gmail.com> wrote:
> > I understand why this is done, but I have 2 concerns here 1) We are
> > having extra deform and copying the field from new to old in case it
> > is unchanged replica identity.  2) The same unchanged attribute values
> > get qualified in the old tuple as well as in the new tuple.  What
> > exactly needs to be done is that the only updated field should be
> > validated as part of the old as well as the new tuple, the unchanged
> > field does not make sense to have redundant validation.   For that we
> > will have to change the filter for the old tuple to just validate the
> > attributes which are actually modified and remaining unchanged and new
> > values will anyway get validated in the new tuple.
> >
> But what if the filter expression depends on multiple columns, say (a+b) > 100
> where a is unchanged while b is changed. Then we will still need both
> columns for applying

In such a case, we need to.

> the filter even though one is unchanged. Also, I am not aware of any
> mechanism by which
> we can apply a filter expression on individual attributes. The current
> mechanism does it
> on a tuple. Do let me know if you have any ideas there?

What I suggested is to modify the filter for the old tuple, e.g.
filter is (a > 10 and b < 20 and c+d = 20), now only if a and c are
modified then we can process the expression and we can transform this
filter to (a > 10 and c+d=20).

>
> Even if it were done, there would still be the overhead of deforming the tuple.

Suppose filter is just (a > 10 and b < 20) and only if the a is
updated, and if we are able to modify the filter for the oldtuple to
be just (a>10) then also do we need to deform?  Even if we have to we
can save a lot on avoiding duplicate expression evaluation.

> I will run some performance tests like Amit suggested and see what the
> overhead is and
> try to minimise it.

It is good to know,  I think you must try with some worst-case
scenarios, e.g. we have 10 text column and 1 int column in the REPLICA
IDENTITY and only the int column get updated and all the text column
are not updated, and you have a filter on all the columns.

Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com