pg_stat_activity

Hello,

question in 2 times :

1 - "select * from pg_stat_activity ;" gives me serverals lines but all
lines have <command string not enable> in current_query
I have set "stats_command_string" to true...

2 - I wanna to log  users activity : connections, disconnection,
queries, ... I assume there is a good way to do that .....

Thanks a lot

cédric