Difference between non deferrable constraint and initially immediate defferable constraint

Question

Hi,
what is the difference between nondeferrable and initially immediate deferrable constraint. In concepts11g guide it's mentioned that database apply constraint at the end of statement.
thanks,
alok

Answer 1

There are two concepts here.
First whether or not the constraint can be deferred, That is what the deferrable clause controls.  By default all constraints are enforced at the time the statement is issued, that is, when you issue an insert statement, all constraints are checked before the data is actually inserted, and if any constraint checks fail you will receive an error.  When a constraint is declared as deferrable, that means that you can put off the constraint checking until the end of the trasnaction (i.e. commit time).
The initially clause determines what the "default" behaviour for a deferrable constraint will be.  If you say initially immediate, then it will behave exactly the same as a non-deferrable constraint.  If you say initially deferred, then constraint checking will be postponed until the end of the transaction (which could be several different SQL statements).  If you have a deferrable constraint, either initially deferred or immediate, you can alter the behaviour for a particular transaction by issueing an alter constraint statement to set either one constraint (by specifiying the constraint name) or all deferrable constraints (by specifying all) to either immediate or deferred.  The behaviour will revert back to whatever was defined at the time the constraint was created when you issue a commit ot rollback.
John