The trap begins when we hold the report in our hands and we think we hold both the cause and effect before us. We start believing that the SQL was run in isolation and is unaffected by anything and everything else occurring on the system. For this to be true, the database server would need to be completely dedicated to that specific SQL. All other database server activity and operating system activity would need to halt while your SQL ran.

Here's a very simple yet crystal-clear example. When a SQL statement reads 1,000 blocks from disk, is it because the SQL statement is not tuned, the buffer cache is too small for the table, or other SQL statements replaced cached blocks with other blocks our SQL is not interested in? The answer is possibly, possibly, and possibly.

To think that a single SQL statement is unaffected by everything else on the system goes against the very essence of why Oracle is an awesome database and why operating systems were invented. Both Oracle and operating systems exist, in part, to share scarce resources. Somehow, the technology is supposed to cause balance and fairness for all the requests for computing resources. But when we look at a profiled session, we start forgetting all of this and begin to think in isolation.