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Oracle fast dual

Oracle Database Tips by Donald Burleson

Arup Nanda has an excellent description of the 10g "Fast dual" feature, whereby select from the "dual" fake table are now performed faster, since fast dual does not perform logical I/O (consistent gets):

How many developers (and DBAs, too) do you think use this command often?

select USER into <some variable> from DUAL

Far too many, probably. Each call to the DUAL creates logical I/Os, which the database can do without. In some cases the call to DUAL is inevitable as in the line <somevariable> := USER. Because Oracle code treats DUAL as a special table, some ideas for tuning tables may not apply or be relevant.

Oracle Database 10g makes all that worry simply disappear: Because DUAL is a special table, the consistent gets are considerably reduced and the optimization plan is different as seen from the event 10046 trace.

In Oracle9i
Rows Execution Plan
------- ---------------------------------------------------

In 10g
Rows Execution Plan
------- ---------------------------------------------------

Notice the use of the new FAST DUAL optimization plan, as opposed to the FULL TABLE SCAN of DUAL in Oracle9i. This improvement reduces the consistent reads significantly, benefiting applications that use the DUAL table frequently.

Note: Technically these fast DUAL improvements are implemented in the SQL Optimizer, but of course for many users SQL*Plus is the primary tool for manipulating SQL.

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