Re: Feature improvement: can we add queryId for pg_catalog.pg_stat_activity view?

On Fri, Oct 16, 2020 at 11:04 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
>
> On Thu, Oct 15, 2020 at 11:41:23AM +0800, Julien Rouhaud wrote:
> > On Wed, Oct 14, 2020 at 10:40 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> > > There is that, and log_line_prefix, which I can imaging being useful.
> > > My point is that if the queryid is visible, there should be a reason it
> > > defaults to show empty.
> >
> > I did some naive benchmarking. Using a custom pgbench script with this query:
> >
> > SELECT *
> > FROM pg_class c
> > JOIN pg_attribute a ON a.attrelid = c.oid
> > ORDER BY 1 DESC
> > LIMIT 1;
> >
> > I can see around 2% overhead (this query is reported with ~ 3ms
> > latency average). Adding a few joins, overhead goes down to 1%.
>
> That number is too high to enable this by default. I suggest we either
> improve the performance of this, or clearly document that you have to
> enable the hash computation to see the pg_stat_activity and
> log_line_prefix fields.

I realize that I didn't update the documentation part to reflect the
new GUC. I'll fix that and add more warnings about the requirements
to have values displayed in pg_stat_acitivity and log_line_prefix.