Horton

Re: logical decoding and replication of sequences

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
Cc: vignesh C <vignesh21@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Petr Jelinek <petr.jelinek@enterprisedb.com>
Date: 2021-07-30T18:58:16Z
Lists: pgsql-hackers 
On Tue, Jul 20, 2021 at 5:41 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
> I think the crucial aspect of this that needs discussion/feedback the
> most is the transactional vs. non-transactional behavior. All the other
> questions are less important / cosmetic.

Yeah, it seems really tricky to me to get this right. The hard part
is, I think, mostly figuring out what the right behavior really is.

DDL in PostgreSQL is transactional. Non-DDL operations on sequences
are non-transactional. If a given transaction does only one of those
things, it seems clear enough what to do, but when the same
(sub)transaction does both, it gets messy. I'd be tempted to think
about something like:

1. When a transaction performs only non-transactional operations on
sequences, they are emitted immediately.

2. If a transaction performs transactional operations on sequences,
the decoded operations acquire a dependency on the transaction and
cannot be emitted until that transaction is fully decoded. When commit
or abort of that XID is reached, emit the postponed non-transactional
operations at that point.

I think this is similar to what you've designed, but I'm not sure that
it's exactly equivalent. I think in particular that it may be better
to insist that all of these operations are non-transactional and that
the debate is only about when they can be sent, rather than trying to
sort of convert them into an equivalent series of transactional
operations. That approach seems confusing especially in the case where
some (sub)transactions abort.

But this is just my $0.02.

-- 
Robert Haas
EDB: http://www.enterprisedb.com

Commits

  1. Fix cache invalidation bug in recovery_prefetch.

  2. Revert "Logical decoding of sequences"

  3. Minor improvements in sequence decoding code and docs

  4. Handle sequences in preprocess_pubobj_list

  5. Update tab-completion for CREATE PUBLICATION with sequences

  6. Add decoding of sequences to built-in replication

  7. Stabilize test_decoding touching with sequences

  8. Call ReorderBufferProcessXid from sequence_decode

  9. Add decoding of sequences to test_decoding

  10. Replace Test::More plans with done_testing

  11. Logical decoding of sequences

  12. Respect permissions within logical replication.