monitoring queries in pg_stat_activity

Hi all:

I have created a nosuperuser for monitoring, but I can't do a select on the
pg_stat_activity table, I get the follow message:

select datid,xact_start,query from pg_catalog.pg_stat_activity;
-[ RECORD 1 ]----+--------------------------------------------
datid | 204816
xact_start |
query | <insufficient privilege>

I have assigned these grants to my user:
GRANT ALL ON SCHEMA pg_catalog to myuser;
GRANT SELECT ON ALL TABLES IN schema pg_catalog TO myuser;

The user not should be a superuser. does there are other way for solve it?

thanks to all