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Temporary Tablespaces [Oracle]

Data that is only used for the duration of a session is stored in a temporary tablespaces. Such data is for example the result of a sort (order by) operation. More specifically, the date is held in temporary segments.
It is normal if temporary tablespaces appear full after a while. This is because the extents are not managed in the data dictionary but in memory. The reason is simple: updating the data dictionary would be an expensive operation.
v$sort_usage and v$sort_segment can be used to find out who occupies the space in temporary Tablespaces.