Re: Logical Replication of sequences

shveta malik <shveta.malik@gmail.com>

From: shveta malik <shveta.malik@gmail.com>

To: vignesh C <vignesh21@gmail.com>

Cc: Shlok Kyal <shlok.kyal.oss@gmail.com>, Chao Li <li.evan.chao@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>, Nisha Moond <nisha.moond412@gmail.com>, Dilip Kumar <dilipbalaut@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Peter Smith <smithpb2250@gmail.com>, Peter Eisentraut <peter@eisentraut.org>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Euler Taveira <euler@eulerto.com>, Michael Paquier <michael@paquier.xyz>, "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>, "Jonathan S. Katz" <jkatz@postgresql.org>, shveta malik <shveta.malik@gmail.com>

Date: 2025-09-22T06:33:19Z

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 →

Doc: Add documentation for sequence synchronization.
- 55cefadde874 19 (unreleased) landed
Remove unused assignment in CREATE PUBLICATION grammar.
- bfb7419b0bbe 19 (unreleased) landed
Add seq_sync_error_count to subscription statistics.
- f6a4c498dcf6 19 (unreleased) landed
Fix few issues in commit 5509055d69.
- 5a4eba558aa7 19 (unreleased) landed
Add sequence synchronization for logical replication.
- 5509055d6956 19 (unreleased) landed
Add worker type argument to logical replication worker functions.
- 3e8e05596a02 19 (unreleased) landed
Introduce "REFRESH SEQUENCES" for subscriptions.
- f0b3573c3aac 19 (unreleased) landed
Refactor logical worker synchronization code into a separate file.
- 41c674d2e31e 19 (unreleased) landed
Standardize use of REFRESH PUBLICATION in code and messages.
- 2436b8c047ff 19 (unreleased) landed
Add "ALL SEQUENCES" support to publications.
- 96b378497346 19 (unreleased) landed
Expose sequence page LSN via pg_get_sequence_data.
- b93172ca59f4 19 (unreleased) landed
Resume conflict-relevant data retention automatically.
- 0d48d393d465 19 (unreleased) cited
Post-commit review fixes for 228c370868.
- 1f7e9ba3ac4e 19 (unreleased) cited
Generate GUC tables from .dat file
- 63599896545c 19 (unreleased) cited

On Thu, Sep 18, 2025 at 4:07 PM vignesh C <vignesh21@gmail.com> wrote:
>
> Thanks for the comments, these are handled in the attached patch.
>

Please find a few comments:

patch005:
1)
GetSubscriptionRelations:
+ /* Skip sequences if they were not requested */
+ if (!get_sequences && (relkind == RELKIND_SEQUENCE))
+ continue;
+
+ /* Skip tables if they were not requested */
+ if (!get_tables && (relkind != RELKIND_SEQUENCE))
+ continue;

The use of negated conditions makes the logic harder to follow,
especially in the second if block.

Can we write it as:
   bool is_sequence = (relkind == RELKIND_SEQUENCE);

   /* Skip if the relation type is not requested */
   if ((get_tables && is_sequence) ||
       (get_sequences && !is_sequence))
       continue;

Or at-least:
/* Skip sequences if they were not requested */
if (get_tables && (relkind == RELKIND_SEQUENCE))
  continue;

/* Skip tables if they were not requested */
if (get_sequences && (relkind != RELKIND_SEQUENCE))
  continue;

2)

AlterSubscription_refresh_seq:

+ UpdateSubscriptionRelState(sub->oid, relid, SUBREL_STATE_DATASYNC,
+    InvalidXLogRecPtr, false);

Now it seems we are setting SUBREL_STATE_DATASYNC state as well for
sequences. Earlier it was INIT only.

So we need correction at 2 places:
a)
Comment atop GetSubscriptionRelations() which mentions :
 * If getting sequences and not_ready is false, retrieve all sequences;
 * otherwise, retrieve only sequences that are still in the INIT state
 * (i.e., have not reached the READY state).

We shall change it to that have not reached the READY state.

b)
patch 006's commit message says:
This patch introduces sequence synchronization:
Sequences have 2 states:
   - INIT (needs synchronizing)
   - READY (is already synchronized)

We shall mention the third state as well.

3)
There is some optimization in fetch_relation_list() in patch006. I
think it should be in patch005 itself where we have4 added new logic
to fetch sequeneces and relkind in patch005.
Or do we need those for patch006 specifically?

patch006:
4)
sequencesync.c:

+ * Sequences to be synchronized by the sequencesync worker will
+ * be added to pg_subscription_rel in INIT state when one of the following
+ * commands is executed:
+ * CREATE SUBSCRIPTION
+ * ALTER SUBSCRIPTION ... REFRESH PUBLICATION
+ * ALTER SUBSCRIPTION ... REFRESH PUBLICATION SEQUENCES

I think this comment needs change. 'REFRESH PUBLICATION SEQUENCES' is
not doing that anymore.

5)
 * So the state progression is always just: INIT -> READY.
I see even SUBREL_STATE_DATASYNC being set now by
AlterSubscription_refresh_seq()

thanks
Shveta