FAST DUAL
DUAL is a virtual table created by the Oracle database in the SYS schema and available to all users.
Detailed Description
DUAL is a virtual table created by the Oracle database in the SYS schema and available to all users. The table has one column - DUMMY and contains one row with the value ‘x’.
DUAL is most useful for running selects that contain just a function call.
e.g. SELECT sysdate FROM dual; which returns the current system date and time filtered through the NLS_DATE setting.
It is know as ‘FAST’ DUAL because prior to Oracle 10g, selecting from DUAL involved a logical IO which could cause scaling issue in large complex queries.
Additional Links
- Oracle Magazine - How to read an execution plan
- Oracle Tuning Guide - Explaining and Displaying Execution Plans
- Oracle Tuning Guide - Comparing Execution Plans Tutorial
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