[sql] How can you get the active users connected to a postgreSQL database via SQL?

How can you get the active users connected to a postgreSQL database via SQL? This could be the userid's or number of users.

Using balexandre's info:

SELECT usesysid, usename FROM pg_stat_activity;

OP asked for users connected to a particular database:

-- Who's currently connected to my_great_database?
SELECT * FROM pg_stat_activity 
  WHERE datname = 'my_great_database';

This gets you all sorts of juicy info (as others have mentioned) such as

• userid (column usesysid)
• username (usename)
• client application name (appname), if it bothers to set that variable -- psql does :-)
• IP address (client_addr)
• what state it's in (a couple columns related to state and wait status)
• and everybody's favorite, the current SQL command being run (query)

Using balexandre's info:

SELECT usesysid, usename FROM pg_stat_activity;

OP asked for users connected to a particular database:

-- Who's currently connected to my_great_database?
SELECT * FROM pg_stat_activity 
  WHERE datname = 'my_great_database';

This gets you all sorts of juicy info (as others have mentioned) such as

• userid (column usesysid)
• username (usename)
• client application name (appname), if it bothers to set that variable -- psql does :-)
• IP address (client_addr)
• what state it's in (a couple columns related to state and wait status)
• and everybody's favorite, the current SQL command being run (query)

Using balexandre's info:

SELECT usesysid, usename FROM pg_stat_activity;