V$SESSION_BLOCKERS displays the blocker sessions for each blocked session. Each row represents a blocked and blocker session pair. If a session is blocked by multiple sessions there will be multiple rows for that blocked session. The maximum number of blocker sessions displayed for a single blocked session is 30. If a session is not blocked by other sessions, then there will be no row in this view for that session.
Blocked session's Oracle session identifier
Blocked session's Oracle session serial number
A number identifying the wait for the blocked session
Resource or event number for which the blocked session is waiting
Resource or event for which the blocked session is waiting