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DUAL Table

The DUAL table was created by Charles Weiss of Oracle Corporation to provide a table for joining in internal views:
I created the DUAL table as an underlying object in the Oracle Data Dictionary. It was never meant to be seen itself, but instead used inside a view that was expected to be queried. The idea was that you could do a JOIN to the DUAL table and create two rows in the result for every one row in your table. Then, by using GROUP BY, the resulting join could be summarized to show the amount of storage for the DATA extent and for the INDEX extent(s). The name, DUAL, apt for the process of creating a pair of rows from just one.[1]
The original DUAL table had two rows in it (hence its name), but subsequently it only had one row.

Optimization

Beginning with 10g Release 1 Oracle Database have beenOPTIMIZED[2] and the database no longer performs physical or logical IO on the DUAL table even though the DUAL table still actually exists.
DUAL is readily available for all the users in database.


Courtesy: Wikipedia

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