How to use pg_stat_activity correctly ?

Hi All,

I try to query on pg_stat_activity but the returned result is only like this:

teramedv2=> select * from pg_stat_activity where datname='teramedv2';
datid | datname | procpid | usesysid | usename | current_query | query_start
-------+-----------+---------+----------+---------+---------------+-------------
17178 | teramedv2 | 32727 | 103 | test | |
17178 | teramedv2 | 26848 | 103 | test | |
17178 | teramedv2 | 563 | 103 | test | |
17178 | teramedv2 | 26799 | 103 | test | |
(4 rows)

the field current_query and query_start seems to me always null ... I
try to do a long run select query on the other console and see the
returned result from pg_stat_activity but the returned result is only
new row with datid,datname,procpid,usename fields is filled with the
correct value but current_query and query_start fields is always null.
Is this an expected result ?