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

On Tue, Apr 06, 2021 at 08:05:19PM +0530, Nitin Jadhav wrote:
>
> 1.
> +void
> +pgstat_report_queryid(uint64 queryId, bool force)
> +{
> + volatile PgBackendStatus *beentry = MyBEEntry;
> +
> + if (!beentry)
> + return;
> +
> + /*
> + * if track_activities is disabled, st_queryid should already have been
> + * reset
> + */
> + if (!pgstat_track_activities)
> + return;
>
> The above two conditions can be clubbed together in a single condition.

Right, I just kept it separate as the comment is only relevant for the 2nd
test.  I'm fine with merging both if needed.

> 2.
> +/* ----------
> + * pgstat_get_my_queryid() -
> + *
> + * Return current backend's query identifier.
> + */
> +uint64
> +pgstat_get_my_queryid(void)
> +{
> + if (!MyBEEntry)
> + return 0;
> +
> + return MyBEEntry->st_queryid;
> +}
>
> Is it safe to directly read the data from MyBEEntry without
> calling pgstat_begin_read_activity() and pgstat_end_read_activity(). Kindly
> ref pgstat_get_backend_current_activity() for more information. Kindly let
> me know if I am wrong.

This field is only written by a backend for its own entry.
pg_stat_get_activity already has required protection, so the rest of the calls
to read that field shouldn't have any risk of reading torn values on platform
where this isn't an atomic operation due to concurrent write, as it will be
from the same backend that originally wrote it.  It avoids some overhead to
retrieve the queryid, but if people think it's worth having the loop (or a
comment explaining why there's no loop) I'm also fine with it.