pgsql: Optimize check for pending backend IO stats

Optimize check for pending backend IO stats

This commit changes the backend stats code so as we rely on a single
boolean rather than a repeated check based on pg_memory_is_all_zeros()
in the code, making it cheaper should PgStat_PendingIO get bigger in
size.

The frequency of backend stats reports is not a bottleneck, but there is
no reason to not make that cheaper, and the logic is simple as the only
entry points updating backend IO stats are pgstat_count_backend_io_op()
and pgstat_count_backend_io_op_time().

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/Z8WYf1jyy4MwOveQ@ip-10-97-1-34.eu-west-3.compute.internal

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6d3ea48ff1aea5fb1ccfed69424bf93a8643b4a4

Modified Files
--------------
src/backend/utils/activity/pgstat_backend.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)