Oracle 10053 trace event provides information on how the Cost Based Optimizer (CBO) came to its decision (the final execution plan). This is often helpful troubleshooting "bad" SQL execution plan. Unfortunately, 10053 trace files can be enormous. This free viewer will help you navigate quickly to the most important sections of the trace file. Also it highlights section headers and formats the transformed final query for enhanced readability,
Tip. Starting from version 11g 10053 trace can show the final query used after all views are expanded and the query is transformed. This can be useful to see the SQL text actually executed by Oracle before troubleshooting performance issues.
Unfortunately 10053 traces are not documented. Some information about them can be obtained from Oracle support web site or Internet. Using 10053 traces can be overwhelming, but can provide explanation about optimizer decision when all other ways failed to diagnose bad execution plan.