Horton

Re: row filtering for logical replication

Tomas Vondra <tomas.vondra@enterprisedb.com>

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From: Tomas Vondra <tomas.vondra@enterprisedb.com>
To: Amit Kapila <amit.kapila16@gmail.com>, Petr Jelinek <petr.jelinek@enterprisedb.com>
Cc: Alvaro Herrera <alvherre@alvh.no-ip.org>, Euler Taveira <euler@eulerto.com>, Greg Nancarrow <gregn4422@gmail.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: 2021-07-13T19:15:48Z
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 7/13/21 12:57 PM, Amit Kapila wrote:
> On Tue, Jul 13, 2021 at 10:24 AM Amit Kapila <amit.kapila16@gmail.com> wrote:
>>
>> On Mon, Jul 12, 2021 at 3:01 PM Tomas Vondra
>> <tomas.vondra@enterprisedb.com> wrote:
>>
>>> In terms of implementation, I think there are two basic options - either
>>> we can define a new "expression" type in gram.y, which would be a subset
>>> of a_expr etc. Or we can do it as some sort of expression walker, kinda
>>> like what the transform* functions do now.
>>>
>>
>> I think it is better to use some form of walker here rather than
>> extending the grammar for this. However, the question is do we need
>> some special kind of expression walker here or can we handle all
>> required cases via transformWhereClause() call as the patch is trying
>> to do. AFAIU, the main things we want to prohibit in the filter are:
>> (a) it doesn't refer to any relation other than catalog in where
>> clause, (b) it doesn't use UDFs in any way (in expressions, in
>> user-defined operators, user-defined types, etc.), (c) the columns
>> referred to in the filter should be part of PK or Replica Identity.
>> Now, if all such things can be detected by the approach patch has
>> taken then why do we need a special kind of expression walker? OTOH,
>> if we can't detect some of this then probably we can use a special
>> walker.
>>
>> I think in the long run one idea to allow UDFs is probably by
>> explicitly allowing users to specify whether the function is
>> publication predicate safe and if so, then we can allow such functions
>> in the filter clause.
>>
> 
> Another idea here could be to read the publication-related catalog
> with the latest snapshot instead of a historic snapshot. If we do that
> then if the user faces problems as described by Petr [1] due to
> missing dependencies via UDFs then she can Alter the Publication to
> remove/change the filter clause and after that, we would be able to
> recognize the updated filter clause and the system will be able to
> move forward.
> 
> I might be missing something but reading publication catalogs with
> non-historic snapshots shouldn't create problems as we use the
> historic snapshots are required to decode WAL.
> 

IMHO the best option for v1 is to just restrict the filters to 
known-safe expressions. That is, just built-in operators, no UDFs etc. 
Yes, it's not great, but both alternative proposals (allowing UDFs or 
using current snapshot) are problematic for various reasons.

Even with those restrictions the row filtering seems quite useful, and 
we can relax those restrictions later if we find acceptable compromise 
and/or decide it's worth the risk. Seems better than having to introduce 
new restrictions later.

> I think the problem described by Petr[1] is also possible today if the
> user drops the publication and there is a corresponding subscription,
> basically, the system will stuck with error: "ERROR:  publication
> "mypub" does not exist. I think allowing to use non-historic snapshots
> just for publications will resolve that problem as well.
> 
> [1] - https://www.postgresql.org/message-id/92e5587d-28b8-5849-2374-5ca3863256f1%402ndquadrant.com
> 

That seems like a completely different problem, TBH. For example the 
slot is dropped too, which means the WAL is likely gone etc.


regards

-- 
Tomas Vondra
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