How to log my user's activities by piwik

Support & Bugs
1

I want to know the online time of my user, such as user1, how to log the ‘user1’ into the database? Or can I identify the time by the relation between the cookie,session and the user1?
Anyone can help me? I prefer not to hack the piwik’s code.
BTW, is there any doc about detail description for every column of piwik?

2

there is currently no such feature, but a custom plugin could add this field “custom” in the log_visit table, that would be tied to a visitor. You could set this value from the piwik javascript tag

piwik.setCustomVar(‘user2’);

or we could add this to core, like Google analytics provides.