Performance Diagnosis - Understanding the core SQL statments

From:
http://download-west.oracle.com/docs/cd/B10501_01/server.920/a96532/ch2.htm


Understanding the core SQL statements in an application is key to performance diagnosis.


For example, a core business transaction is required to run in a subsecond response time. Initial investigation of the transaction shows that this transaction performs 200 logical reads, of which 40 are always obtained from disk. Taking a disk response time of 20 milliseconds, the likely I/O time is 40 x .02 = 0.8 seconds, which probably fails the response time target. The DBA requests that the transaction be rewritten, and the number of logical I/Os is reduced to 80, with an average of five coming from disk.