Horton

RE: Conflict detection for update_deleted in logical replication

Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>

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From: "Hayato Kuroda (Fujitsu)" <kuroda.hayato@fujitsu.com>
To: "Zhijie Hou (Fujitsu)" <houzj.fnst@fujitsu.com>
Cc: Nisha Moond <nisha.moond412@gmail.com>, Masahiko Sawada <sawada.mshk@gmail.com>, shveta malik <shveta.malik@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, Amit Kapila <amit.kapila16@gmail.com>
Date: 2024-11-29T10:35:20Z
Lists: pgsql-hackers 
Dear Hou,

Thanks for updating the patch! Here are my comments mainly for 0001.

01. protocol.sgml

I think the ordering of attributes in "Primary status update" seems not correct.
The second entry is LSN, not the oldest running xid.

02. maybe_advance_nonremovable_xid

```
+        case RCI_REQUEST_PUBLISHER_STATUS:
+            request_publisher_status(data);
+            break;
```

I think the part is not reachable because the transit
RCI_REQUEST_PUBLISHER_STATUS->RCI_WAIT_FOR_PUBLISHER_STATUS is done in
get_candidate_xid()->request_publisher_status().
Can we remove this?

03. RetainConflictInfoData

```
+    Timestamp   xid_advance_attempt_time;   /* when the candidate_xid is
+                                             * decided */
+    Timestamp   reply_time;     /* when the publisher responds with status */
+
+} RetainConflictInfoData;
```

The datatype should be TimestampTz.

04. get_candidate_xid

```
+    if (!TimestampDifferenceExceeds(data->xid_advance_attempt_time, now,
+                                    wal_receiver_status_interval * 1000))
+        return;
```

I think data->xid_advance_attempt_time can be accessed without the initialization
at the first try. I've found the patch could not pass test for 32-bit build
due to the reason.


05. request_publisher_status

```
+    if (!reply_message)
+    {
+        MemoryContext oldctx = MemoryContextSwitchTo(ApplyContext);
+
+        reply_message = makeStringInfo();
+        MemoryContextSwitchTo(oldctx);
+    }
+    else
+        resetStringInfo(reply_message);
```

Same lines exist in two functions: can we provide an inline function?

06. wait_for_publisher_status

```
+    if (!FullTransactionIdIsValid(data->last_phase_at))
+        data->last_phase_at = FullTransactionIdFromEpochAndXid(data->remote_epoch,
+                                                               data->remote_nextxid);
+
```

Not sure, is there a possibility that data->last_phase_at is valid here? It is initialized
just before transiting to RCI_WAIT_FOR_PUBLISHER_STATUS.

07. wait_for_publisher_status

I think all calculations and checking in the function can be done even on the
walsender. Based on this, I come up with an idea to reduce the message size:
walsender can just send a status (boolean) whether there are any running transactions
instead of oldest xid, next xid and their epoch. Or, it is more important to reduce the
amount of calc. on publisher side?

Best regards,
Hayato Kuroda
FUJITSU LIMITED

Commits

  1. Fix intermittent BF failures in 035_conflicts.

  2. Resume conflict-relevant data retention automatically.

  3. Fix intermittent test failure introduced in 6456c6e2c4.

  4. Fix Coverity issue reported in commit a850be2fe.

  5. Add test to prevent premature removal of conflict-relevant data.

  6. Post-commit review fixes for 228c370868.

  7. Add max_retention_duration option to subscriptions.

  8. Detect and report update_deleted conflicts.

  9. Preserve conflict-relevant data during logical replication.

  10. Integrate FullTransactionIds deeper into two-phase code

  11. Improve checks for GUC recovery_target_timeline

  12. Prevent excessive delays before launching new logrep workers.

  13. Keep WAL segments by slot's last saved restart LSN

  14. Rework some code handling pg_subscription data in psql and pg_dump

  15. Use generateClonedIndexStmt to propagate CREATE INDEX to partitions.