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ops$tkyte%ORA11GR2> update parent set pk = 2; update parent set pk = 2 * ERROR at line 1: ORA-02292: integrity constraint (OPS$TKYTECHILD_FK_PARENT) violated - child record found Since the constraint is in IMMEDIATE mode, this UPDATE fails We ll change the mode and try again: ops$tkyte%ORA11GR2> set constraint child_fk_parent deferred; Constraint set ops$tkyte%ORA11GR2> update parent set pk = 2; 1 row updated Now it succeeds For illustration purposes, I ll show how to check a deferred constraint explicitly before committing, to see if the modifications we made are in agreement with the business rules (in other words, to check that the constraint isn t currently being violated).

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It s a good idea to do this before committing or releasing control to some other part of the program (which may not be expecting the deferred constraints): ops$tkyte%ORA11GR2> set constraint child_fk_parent immediate; set constraint child_fk_parent immediate * ERROR at line 1: ORA-02291: integrity constraint (OPS$TKYTECHILD_FK_PARENT) violated - parent key not found It fails and returns an error immediately as expected, since we knew that the constraint had been violated The UPDATE to PARENT was not rolled back (that would violate the statement-level atomicity); it is still outstanding Also note that our transaction is still working with the CHILD_FK_PARENT constraint deferred because the SET CONSTRAINT command failed Let s continue now by cascading the UPDATE to CHILD: ops$tkyte%ORA11GR2> update child set fk = 2; 1 row updated ops$tkyte%ORA11GR2> set constraint child_fk_parent immediate; Constraint set ops$tkyte%ORA11GR2> commit; Commit complete And that s the way it works.

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Note that to defer a constraint, you must create them that way you have to drop and re-create the constraint to change it from nondeferrable to deferrable That might lead you to believe that you should create all of your constraints as deferrable initially immediate, just in case you wanted to defer them at some point In general, that is not true You want to allow constraints to be deferred only if you have a real need to do so By creating deferred constraints, you introduce differences in the physical implementation (in the structure of your data) that might not be obvious For example, if you create a deferrable UNIQUE or PRIMARY KEY constraint, the index that Oracle creates to support the enforcement of that constraint will be a non-unique index.

Normally, you expect a unique index to enforce a unique constraint, but since you have specified that the constraint could temporarily be ignored, it can t use that unique index Other subtle changes will be observed, for example, with.

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NOT NULL constraints In 11 Indexes, we ll see how an index on a NOT NULL column can be used in many cases where a similar index on a NULL column can t be If you allow your NOT NULL constraints to be deferrable, the optimizer will start treating the column as if it supports NULLs because it in fact does support NULLs during your transaction For example, suppose you have a table with the following columns and data: ops$tkyte%ORA11GR2> create table t 2 ( x int constraint x_not_null not null deferrable, 3 y int constraint y_not_null not null, 4 z varchar2(30) 5 ); Table created ops$tkyte%ORA11GR2> insert into t(x,y,z) 2 select rownum, rownum, rpad('x',30,'x') 3 from all_users; 45 rows created ops$tkyte%ORA11GR2> exec dbms_statsgather_table_stats( user, 'T' ); PL/SQL procedure successfully completed In this example, column X is created such that when you COMMIT, X will not be null.

However, during your transaction X is allowed to be null since the constraint is deferrable Column Y, on the other hand, is always NOT NULL If you were to index column Y: ops$tkyte%ORA11GR2> create index t_idx on t(y); Index created.

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