Oracle maintains all Explain Plans in DBA_HIST_SQLPLAN. Here's how to analyze that table to see which indexes are used and how
Here is one person's approach to reading an Oracle Automated Workload Repository (AWR) report. It is not intended to be a complete explanation of all the information contained within the report, just a list of places to start.
Click on this link Reading an Oracle AWR report to open the PDF in a new window