Gauri Shiv Enterprises

  SCN can refer to:

    System Change Number - A number, internal to Oracle that is incremented over time as change vectors  
                           are generated, applied, and written to the Redo log.

  While user is issuing DML transaction the  redo entrie for this transaction Is store to the buffer in to SGA. And  after commit issue redo data written to the  redo log file  which physical located on  hard disk. This is normal practice and it is called synchronous Commit.  Oracle does not return control until the transaction physically written.

But this practice not very much acceptable for  high speed Transition.   this point of view oracle introduce Asynchronous commit  in 10g release 2.

Query Optimizer A sql statement can be executed in many different ways such as full table scans,index scans,nested loops and hash joins.