Horton

Re: row filtering for logical replication

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Ajin Cherian <itsajin@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: 2021-09-21T05:11:08Z
Lists: pgsql-hackers

Commits

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

  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 9:54 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
>
> 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).
>

If you have only a and c in the old tuple, how will it evaluate
expression c + d? I think the point is if for some expression some
values are in old tuple and others are in new then the idea proposed
in the patch seems sane. Moreover, I think in your idea for each tuple
we might need to build a new expression and sometimes twice that will
beat the purpose of cache we have kept in the patch and I am not sure
if it is less costly.

See another example where splitting filter might not give desired results:

Say filter expression: (a = 10 and b = 20 and c = 30)

Now, old_tuple has values for columns a and c and say values are 10
and 30. So, the old_tuple will match the filter if we split it as per
your suggestion. Now say new_tuple has values (a = 5, b = 15, c = 25).
In such a situation dividing the filter will give us the result that
the old_tuple is matching but new tuple is not matching which seems
incorrect. I think dividing filter conditions among old and new tuples
might not retain its sanctity.

> >
> > 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?
>

Without deforming, how will you determine which columns are part of
the old tuple?

-- 
With Regards,
Amit Kapila.