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: Peter Smith <smithpb2250@gmail.com>
Cc: Euler Taveira <euler@eulerto.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: 2021-08-26T11:12:56Z
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 Thu, Aug 26, 2021 at 3:41 PM Peter Smith <smithpb2250@gmail.com> wrote:
>
> On Thu, Aug 26, 2021 at 3:00 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> >
> > On Thu, Aug 26, 2021 at 9:51 AM Peter Smith <smithpb2250@gmail.com> wrote:
> > >
> > > On Thu, Aug 26, 2021 at 1:20 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > >
> > > > On Thu, Aug 26, 2021 at 7:37 AM Peter Smith <smithpb2250@gmail.com> wrote:
> > > > >
> > > > > On Wed, Aug 25, 2021 at 3:28 PM Amit Kapila <amit.kapila16@gmail.com> wrote:
> > > > > >
> > > > > ...
> > > > > >
> > > > > > Hmm, I think the gain via caching is not visible because we are using
> > > > > > simple expressions. It will be visible when we use somewhat complex
> > > > > > expressions where expression evaluation cost is significant.
> > > > > > Similarly, the impact of this change will magnify and it will also be
> > > > > > visible when a publication has many tables. Apart from performance,
> > > > > > this change is logically correct as well because it would be any way
> > > > > > better if we don't invalidate the cached expressions unless required.
> > > > >
> > > > > Please tell me what is your idea of a "complex" row filter expression.
> > > > > Do you just mean a filter that has multiple AND conditions in it? I
> > > > > don't really know if few complex expressions would amount to any
> > > > > significant evaluation costs, so I would like to run some timing tests
> > > > > with some real examples to see the results.
> > > > >
> > > >
> > > > I think this means you didn't even understand or are convinced why the
> > > > patch has cache in the first place. As per your theory, even if we
> > > > didn't have cache, it won't matter but that is not true otherwise, the
> > > > patch wouldn't have it.
> > >
> > > I have never said there should be no caching. On the contrary, my
> > > performance test results [1] already confirmed that caching ExprState
> > > is of benefit for the millions of times it may be used in the
> > > pgoutput_row_filter function. My only doubts are in regard to how much
> > > observable impact there would be re-evaluating the filter expression
> > > just a few extra times by the get_rel_sync_entry function.
> > >
> >
> > I think it depends but why in the first place do you want to allow
> > re-evaluation when there is a way for not doing that?
>
> Because the current code logic of having the "delayed" ExprState
> evaluation does come at some cost.
>

So, now you mixed it with the second point. Here, I was talking about
the need for correct invalidation but you started discussing when to
first time evaluate the expression, both are different things.

>  And the cost is -
> a. Needing an extra condition and more code in the function pgoutput_row_filter
> b. Needing to maintain the additional Node list
>

I am not sure you need (b) above and I think (a) should make the
overall code look clean.

> If we chose not to implement a delayed ExprState cache evaluation then
> there would still be a (one-time) ExprState cache evaluation but it
> would happen whenever get_rel_sync_entry is called (regardless of if
> pgoputput_row_filter is subsequently called). E.g. there can be some
> rebuilds of the ExprState cache if the user calls TRUNCATE.
>

Apart from Truncate, it will also be a waste if any error happens
before actually evaluating the filter, tomorrow there could be other
operations like replication of sequences (I have checked that proposed
patch for sequences uses get_rel_sync_entry) where we don't need to
build ExprState (as filters might or might not be there). So, it would
be better to avoid cache lookups in those cases if possible. I still
think doing expensive things like preparing expressions should ideally
be done only when it is required.
--
With Regards,
Amit Kapila.