Wait Event
Wait Event is an Oracle Database diagnostic indicator that identifies why a session is waiting rather than actively working. Whenever a process cannot proceed — because it is waiting for a block from disk, a lock, a network response or a CPU slot — Oracle records a specific wait event.
The most common #
| Wait Event | Meaning |
|---|---|
db file sequential read | Single-block read — typical of index access |
db file scattered read | Multi-block read — typical of full table scans |
log file sync | Waiting for commit to redo log |
enq: TX - row lock contention | Row lock conflict |
direct path read | Direct read (bypassing buffer cache) |
What they’re for #
Wait events are the foundation of Oracle’s diagnostic methodology. By analysing which events dominate DB time (via AWR or ASH) you can immediately identify the nature of the problem: I/O, contention, CPU or network.
Where to find them #
- Real-time:
V$SESSION_WAIT,V$ACTIVE_SESSION_HISTORY - Historical: AWR reports (Top Timed Foreground Events section),
DBA_HIST_ACTIVE_SESS_HISTORY
The DBA’s rule: don’t guess what’s slowing the database down — look at the wait events.