Re: Track in pg_replication_slots the reason why slots conflict?

Hi,

On 2023-12-21 09:21:04 +0900, Michael Paquier wrote:
> While listening at Bertrand's talk about logical decoding on standbys
> last week at Prague, I got surprised by the fact that we do not
> reflect in the catalogs the reason why a conflict happened for a slot.
> There are three of them depending on ReplicationSlotInvalidationCause:
> - WAL removed.
> - Invalid horizon.
> - Insufficient WAL level.

It should be extremely rare to hit any of these other than "WAL removed", so
I'm not sure it's worth adding interface complexity to show them.


> ReplicationSlotCtl holds this information, so couldn't it be useful
> for monitoring purposes to know why a slot got invalidated and add a
> column to pg_get_replication_slots()?  This could just be an extra
> text conflicting_reason, defaulting to NULL when there's nothing to
> see.

Extra columns aren't free from a usability perspective. IFF we do something, I
think it should be a single column with a cause.

Greetings,

Andres Freund