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When a session runs a statement for a table with parallel degree of n (4 degrees for example), the Oracle engine creates n new sessions to serve it (these sessions are called: "Parallel Server Agents"). Each of the parallel server agents runs as an individual entity and analyzes part of the requested data, while the original session (the one that you logged in with) is called the coordinator session. The last goal is to receive all the information from the parallel server agents and summarize (sort/filter/calculate/join) it. Usually the coordinator session will not perform an "In Oracle" operation. In fact it is waiting "All the time" when the parallel server agents are working. These parallel server agents can perform I/O / Locks / RAC operations - like any other regular session. These sessions (4 in our example) have the same session identifiers as the coordinator session. Therefore in the collector's aggregation phase all the sessions are aggregate into one row. For example, if a specific statement runs for 1 hour (wall clock time) you can see in the Activity tab that the statement consumed 4 hours "In Oracle" time. You can also find the parallel degree of the statement within the Activity tab.
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Precise for Oracle captures standard Oracle statistics that you can see in the V$SESSTAT table. However, the Collector reads the values in a low-overhead fashion directly from the SGA memory that the V$ tables map onto. Where necessary, the statistics are divided between the sessions executing each SQL statement and then aggregated to the user, program, and instance levels. You will see these statistics in the Current and Activity tabs, displayed as name-value pairs. Each statistic represents a different counter and has its own units. See Oracle’s documentation for more information on these statistics.
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