Re: Conflict detection for update_deleted in logical replication - Mailing list pgsql-hackers

On Thu, Nov 14, 2024 at 8:24 AM Zhijie Hou (Fujitsu)
<houzj.fnst@fujitsu.com> wrote:
>
> Attach the V9 patch set which addressed above comments.
>

Reviewed v9 patch-set and here are my comments for below changes:

@@ -1175,10 +1189,29 @@ ApplyLauncherMain(Datum main_arg)
  long elapsed;

  if (!sub->enabled)
+ {
+ can_advance_xmin = false;
+ xmin = InvalidFullTransactionId;
  continue;
+ }

  LWLockAcquire(LogicalRepWorkerLock, LW_SHARED);
  w = logicalrep_worker_find(sub->oid, InvalidOid, false);
+
+ if (can_advance_xmin && w != NULL)
+ {
+ FullTransactionId nonremovable_xid;
+
+ SpinLockAcquire(&w->relmutex);
+ nonremovable_xid = w->oldest_nonremovable_xid;
+ SpinLockRelease(&w->relmutex);
+
+ if (!FullTransactionIdIsValid(xmin) ||
+ !FullTransactionIdIsValid(nonremovable_xid) ||
+ FullTransactionIdPrecedes(nonremovable_xid, xmin))
+ xmin = nonremovable_xid;
+ }
+

1) In Patch-0002, could you please add a comment above "+ if
(can_advance_xmin && w != NULL)" to briefly explain the purpose of
finding the minimum XID at this point?

2) In Patch-0004, with the addition of the 'detect_update_deleted'
option, I see the following two issues in the above code:
2a) Currently, all enabled subscriptions are considered when comparing
and finding the minimum XID, even if detect_update_deleted is disabled
for some subscriptions.
I suggest excluding the oldest_nonremovable_xid of subscriptions where
detect_update_deleted=false by updating the check as follows:

    if (sub->detectupdatedeleted && can_advance_xmin && w != NULL)

2b) I understand why advancing xmin is not allowed when a subscription
is disabled. However, the current check allows a disabled subscription
with detect_update_deleted=false to block xmin advancement, which
seems incorrect. Should the check also account for
detect_update_deleted?, like :
  if (sub->detectupdatedeleted &&  !sub->enabled)
+ {
+ can_advance_xmin = false;
+ xmin = InvalidFullTransactionId;
  continue;
+ }

However, I'm not sure if this is the right fix, as it could lead to
inconsistencies if the detect_update_deleted is set to false after
disabling the sub.
Thoughts?

--
Thanks,
Nisha