RE: pg_upgrade and logical replication

Dear Amit, Michael,

> > I am not sure to get the reason why get_old_cluster_logical_slot_infos()
> > could not be optimized, TBH.  LogicalReplicationSlotHasPendingWal()
> > uses the fast forward mode where no changes are generated, hence there
> > should be no need for a dependency to a connection to a specific
> > database :)
> >
> > Combined to a hash table based on the database name and/or OID to know
> > to which dbinfo to attach the information of a slot, then it should be
> > possible to use one query, making the slot info gathering closer to
> > O(N) rather than the current O(N^2).
> >
> 
> The point is that unlike subscriptions logical slots are not
> cluster-level objects. So, this needs more careful design decisions
> rather than a fix-up patch for PG-17. One more thing after collecting
> slot-level, we also want to consider the creation of slots which again
> are created at per-database level.

I also considered the combination with the optimization (parallelization) of
pg_upgrade [1]. IIUC, the patch tries to connect to some databases in parallel
and run commands. The current style of create_logical_replication_slots() can be
easily adapted because tasks are divided per database.

However, if we change like get_old_cluster_logical_slot_infos() to do in a single
pass, we may have to shift LogicalSlotInfoArr to cluster-wide data and store the
database name in LogicalSlotInfo. Also, in create_logical_replication_slots(),
we may have to check the located database for every slot and connect to the
appropriate database. These changes make it difficult to parallelize the operation.

[1]: https://www.postgresql.org/message-id/flat/20240516211638.GA1688936@nathanxps13

Best regards,
Hayato Kuroda
FUJITSU LIMITED