查询某用户下当前会话的连接数 问题

查询某用户下当前会话的连接数

select * from v$session where username='xxx'


可是
select OSUSER, MACHINE, TERMINAL, PROCESS, PROGRAM  from v$session a, v$sqlarea b
where a.sql_address =b.address order by cpu_time/executions desc  

a.sql_address =b.address  这个是啥意思呢？