Re: Track the amount of time waiting due to cost_delay

>> This sounds like useful information to me.

> Thanks for looking at it!

The VacuumDelay is the only visibility available to
gauge the cost_delay. Having this information
advertised by pg_stat_progress_vacuum as is being proposed
is much better. However, I also think that the
"number of times" the vacuum went into delay will be needed
as well. Both values will be useful to tune cost_delay and cost_limit.

It may also make sense to accumulate the total_time in delay
and the number of times delayed in a cumulative statistics [0]
view to allow a user to trend this information overtime.
I don't think this info fits in any of the existing views, i.e.
pg_stat_database, so maybe a new view for cumulative
vacuum stats may be needed. This is likely a separate
discussion, but calling it out here.

>> IIUC you'd need to get information from both pg_stat_progress_vacuum and
>> pg_stat_activity in order to know what percentage of time was being spent
>> in cost delay. Is that how you'd expect for this to be used in practice?

> Yeah, one could use a query such as:

> select p.*, now() - a.xact_start as duration from pg_stat_progress_vacuum p JOIN pg_stat_activity a using (pid)

Maybe all progress views should just expose the "beentry->st_activity_start_timestamp "
to let the user know when the current operation began.

Regards,

Sami Imseih
Amazon Web Services (AWS)

[0] https://www.postgresql.org/docs/current/monitoring-stats.html