Query to pg_stat_activity

Hello.

I want to query pg_stat_activity to see the current_query field.  I do:

mdbs=# select current_user;
 current_user
--------------
 postgres
(1 row)

mdbs=# select * from pg_stat_activity;
 datid | datname | procpid | usesysid |    usename     | current_query
-------+---------+---------+----------+----------------+---------------
 16976 | mdbs    |    8537 |      121 | mdbs_app_owner |
 16976 | mdbs    |   12378 |      121 | mdbs_app_owner |
 16976 | mdbs    |   14375 |        1 | postgres       |
 16976 | mdbs    |   14337 |      121 | mdbs_app_owner |

but, "current_query" is null.  Why?

Thanks.

CG