By Carlos M. Orozco -Oracle on Mar 22, 2014
Slowness could be perceived as taking long and not always what you think. Taking a look at your resource usage, CPU, Memory and I/O can be done with Enterprise Manager. I can view historical data to see where there may have been a high demand for any of these resources. Enterprise Manager will also allow you to drill down into the AWR snapshots and report back on SQL executing at that time. You can locate resource consuming SQL identified by the Automatic Database Diagnostic Monitor (ADDM) which analyzes the AWR data on a regular basis, then locates the root causes of performance problems, provides recommendations for correcting any problems. Sometimes, workloads take longer due to sub optimal execution plans. Something that may have completed in a minutes or seconds may now be taking hours or days due to changes in the plan. Plan is determined by the optimizer and if the data load process did not collect stats or potentially the data was loaded after a stats job runs, you may be a seeing poor cardinality estimates from the optimizer on the stats collected. If you note a big discrepancy between estimated rows and actual rows processed, you have to deal with it by collecting stats. It may required extended stats as well. I would explore using SQL Monitor to look at any SQL have identified as taking longer. Another tool you can use is SQLT available from Oracle support at:https://support.oracle.com/epmos/faces/DocumentDisplay?id=215187.1
The reports may include some content provided by the Oracle Diagnostic and/or the Oracle Tuning Packs (in particular SQL Tuning Advisor "STA", SQL Tuning Sets "STS", SQL Monitoring and/or Automatic Workload Repository "AWR"). Be aware that using this extended functionality requires a license for the corresponding pack. If you need to disable SQLT access to one of these packages, please execute one of the following commands connected as SQLTXPLAIN: SQL> EXEC sqlt$a.disable_tuning_pack_access; or SQL> EXEC sqlt$a.disable_diagnostic_pack_access;
SQLT was written by Carlos Sierra who worked in Oracle's support division and later moved on to Enketec https://www.enkitec.com/about/bios/carlos.sierra - you can also look for a great book titled "Oracle SQL Tuning with Oracle SQLTXPLAIN, author Stelios Charalambides"