From Oracle FAQ
The User Global Area (UGA) is a memory area (RAM) that holds session-based information.
When running in Dedicated Server mode (one session = one dedicated process), the UGA is stored in the PGA (process global area).
When running in Shared Server mode (MTS with shared servers and dispatchers), sessions can be served by multiple server processes. As a result, the UGA cannot be stored in the PGA, and is moved to the SGA (shared global area).
Monitoring the UGA
SELECT s.sid, n.name, s.value/power(1024,2) MB FROM v$sesstat s, v$statname n WHERE s.statistic# = n.statistic# AND n.name LIKE '%uga%';
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