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

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@momjian.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.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EnterpriseDB                             https://enterprisedb.com

  The usefulness of a cup is in its emptiness, Bruce Lee