RE: speed up a logical replica setup

Dear Shubham,

Thanks for testing. It seems you ran with v20 patch, but I confirmed
It could reproduce with v21.

>
> I found a couple of issues, while verifying the cascaded replication
> with the following scenarios:
> Scenario 1) Create cascade replication like node1->node2->node3
> without using replication slots (attached
> cascade_3node_setup_without_slots has the script for this):
> Then I ran pg_createsubscriber by specifying primary as node1 and
> standby as node3, this scenario runs successfully. I was not sure if
> this should be supported or not?

Hmm. After the script, the cascading would be broken. The replication would be:

```
Node1 -> node2
|
Node3
```

And the operation is bit strange. The consistent LSN is gotten from the node1,
but node3 waits until it receives the record from NODE2.
Can we always success it?

> Scenario 2) Create cascade replication like node1->node2->node3 using
> replication slots (attached cascade_3node_setup_with_slots has the
> script for this):
> Here, slot name was used as slot1 for node1 to node2 and slot2 for
> node2 to node3. Then I ran pg_createsubscriber by specifying primary
> as node1 and standby as node3. In this case pg_createsubscriber fails
> with the following error:
> pg_createsubscriber: error: could not obtain replication slot
> information: got 0 rows, expected 1 row
> [Inferior 1 (process 2623483) exited with code 01]
>
> This is failing because slot name slot2 is used between node2->node3
> but pg_createsubscriber is checked for slot1, the slot which is used
> for replication between node1->node2.
> Thoughts?

Right. The inconsistency is quite strange.

Overall, I felt such a case must be rejected. How should we detect at checking phase?

Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/