Oracle’s extended SQL trace data stream contains a linear sequential record of every database call and every operating system call that the Oracle kernel executes in response to the code that you write.
The trace file contains timings that enable you to precisely measure how long your code takes to run and why. The detailed information about individual database calls and operating system calls is vital to building scalable applications (as well as troubleshooting them in production). The information in there is a gold mine, and some of it is available nowhere else.
This paper, published in February 2011, is the inspiration for chapters 3 and 4 in The Method R Guide to Mastering Oracle Trace Data, 2nd Edition. The author has significantly expanded and enriched the material for the book.