The following figure shows the onpload unload
process. In the unload process, the behavior of onpload parallels
the behavior described in Threads that the onpload utility uses and Threads that the database server uses, except that the threads
are unloading the data instead of loading it. Figure 1: The unload procedure
The ulstrm (unload-stream) thread packages data for output
to the onpload client from the query plan. The
SQL optimizer creates the query plan. The query plan behaves like
a query plan running from any other client, such as DB-Acess. The
exchange operator distributes the resulting data to the ulworker threads
in a round-robin fashion, and onpload unloads the
data onto tapes or files.
Parallelism with respect to the output device, the source table
fragments, and the flow of the data is evident in the preceding figure.