RE: speed up a logical replica setup

Dear Euler,

> Hayato already mentioned one of the solution in a previous email [2].
> AFAICS we can use any solution that creates a WAL record. I tested the
> following options:

Yes, my point was that we should add an arbitrary record after the recovery_target_lsn.

> (a) temporary replication slot: requires an additional replication slot.
> small payload. it is extremely slow in comparison with the other
> options.
> (b) logical message: can be consumed by logical replication when/if it
> is supported some day. big payload. fast.
> (c) snapshot of running txn: small payload. fast.
> (d) named restore point: biggest payload. fast.

Another PoV is whether they trigger the flush of the generated WAL record. You've
tested pg_logical_emit_message() with flush=false, but this meant that the WAL does
not flush so that the wait_for_end_recovery() waits a time. IIUC, it may wait 15
seconds, which is the time duration bgwriter works. If flush is set to true, the
execution time will be longer.
pg_create_restore_point() also does not flush the generated record so that it may
be problematic. Moreover, the name of the restore point might be a conflict that
users will use.

Overall, I could agree to use pg_log_standby_snapshot(). This flushes the WAL
asynchronously but ensures the timing is not so delayed.

Best regards,
Hayato Kuroda
FUJITSU LIMITED