When analysing Oracle tkprof trace files I have noticed that there is sometimes a big difference between the cpu time and the elapsed time, and I don’t know what causes it.
For example:
call count cpu elapsed disk query current rows
------- ------ -------- ---------- ---------- ---------- ---------- ----------
Parse 1 0.00 42.09 0 0 0 0
Execute 1 0.01 0.01 0 0 0 0
Fetch 45 14.44 62.71 48664 505513 0 1871
------- ------ -------- ---------- ---------- ---------- ---------- ----------
total 47 14.45 104.82 48664 505513 0 1871
The wait statistics are like this:
Event waited on Times Max. Wait Total Waited
---------------------------------------- Waited ---------- ------------
SQL*Net message to client 46 0.00 0.00
SQL*Net message from client 46 0.19 1.68
buffer busy waits 559 0.23 8.59
db file scattered read 5204 0.21 7.49
db file sequential read 4240 0.20 13.49
latch free 215 0.11 3.62
I’m a software developer (not a DBA) so I’m generally looking these trace files to find inefficient queries or to see if an index could be used to stop a full table scan and so on. For this purpose I tend to go on the cpu time. In most cases the elapsed time is very similar to the cpu time.
I don’t have access to the database that generated the trace file (it’s from a client site) but I would like to understand what is going on so that I can make suggestions as to how to reduce the elapsed time.
The CPU time is the time your query actually needed; the rest is waiting for resources. On a busy server this could be largely due to waiting for CPU currently in use by other users; this doesn’t show up in the wait stats.