Re: Expose lock group leader pid in pg_stat_activity

Hello

> As I understand it, lock group is some infrastructure that is used by
> parallel queries, but could be used for something else too. So if
> more documentation is needed, we should say something like "For now,
> only parallel queries can have a lock group" or something like that.

If lockGroupLeader will be used in some way for non-parallel query, then the name leader_pid could be confusing. No?
I treat pg_stat_activity as view for user. We have document somewhere what is "lock group leader" (excepts README in
sourcetree)? I meant user going to read documentation, "ok, this field is process ID of the lock group leader, but what
isit?". Expose a leader pid for parallel worker will be clear improvement for user. And seems lockGroupLeader->pid is
exactlythis stuff. Therefore, I would like to see such description and meaning of the field.
 

> The fact that leader_pid == pid for the leader and different for the
> other member should be obvious, I'm not sure that it's worth
> documenting that.

It may be not obvious that leader_pid is not null in this case. But ok, no objections.

regards, Sergei