The unaccounted for time of 3 seconds is important to understand. There are many reasons for this time. Because the session was not being traced, the data collection was manually started and stopped. The manual collection can and does produce some error, but this can be minimized by familiarizing yourself and your user with the script. There are also issues related with the posting for SNMFC wait time because v$session_event is updated perhaps only every 3 seconds or when the wait situation changes. So from a script development perspective, you need to be aware of this.

The final cause for the unaccounted for time is simply because there could be 3 seconds of time between the Oracle client process and the user's experience, which is the missing component of the true end-to-end user time.

In a client/server architecture, there should be no time between the Oracle client process and the user's experience. However, with modern web architectures, this is not the case. Referring to Figure 5-13 again, the time between the Oracle client process and the user's web browser would be represented by unaccounted for time. This is your clue that the true performance issues reside outside Oracle's realm.