Oracle Database with very few foreign key constraints - oracle

Question

I've just started a new project and I am confronted with a production application Oracle 10g database that has just 3 foreign key constraints. I am not used to seeing databases with no foreign key constraints. I am guessing that there may be some performance/concurrency considerations to not using FKs. The reason is that in the logical database schema the architect has specified all the relationships, but these relationships are not implemented in the database as Foreign Key constraints.
Question: I read that I can define a Foreign Key Constraint with RELY NOVALIDATE that will not impact performance. Is it worth while to define RELY FK constraints on this database just so that the relationship can be easily seen? this application is not built using ORM, is it really worth while to do without foreign keys?
The database is denormalised with example below
Table 1 : FINProduct(ID (number), Description(varchar(5)), FINproductCode(varchar(10))...)
Table 2: FINProductCode(ID (number, FINproductCode(varchar(10)) , LastUpdated(datetime)...)
So instead of having a relationship between Tables 1 and 2 the FINproductCode column is just replicated in table 1.
It's too early to drink but I think i need one!

Answer 1

I would be very wary about assuming that the absence of foreign key constraints was a reasoned response to performance issues. There is an overhead to enforcing a foreign key constraint (particularly where appropriate indexes are missing) but it is incredibly unlikely that your application can validate the constraint more efficiently than Oracle can. So the question really is whether you want the small overhead of foreign key constraints or the near certainty that you will get invalid data inserted into the database. It would be extremely unlikely that this is a trade-off that you want to make-- I've yet to meet a business user that would be happy to capture incorrect and incomprehensible data even if doing so was a bit faster than capturing correct data.
Unless there is substantially more background, I would tend to create all the missing foreign key constraints. Creating RELY NOVALIDATE constraints is possible but it defeats the major benefit of foreign key constraints-- preventing invalid data from entering the database in the first place.

Answer 2

It depends on whether you want to add the FK only for documentation purposes or whether you want to prevent future INSERTs/UPDATEs with an invalid FK value.
If you want it only for documentation purposes, I'd create the FK constraint with RELY NOVALIDATE and DISABLE it afterwards - otherwise, Oracle will check it for future INSERTs / UPDATEs.
However: DON'T DO THIS UNLESS YOU ABSOLUTELY NEED IT!
I agree with Justin Cave: In most cases, you should just add "plain" FK constraints - this way, you can ensure that your existing data is correct.

Answer 3

I would try to create the constraints and report violations into a exception table. Fix the data and enable the constraint.
Create some test data
create table parent (pk integer
,data varchar2(1)
,CONSTRAINT PARENT_PK PRIMARY KEY (PK) ENABLE );
create table child (pk integer
,pk_parent integer
,data varchar2(1)
,CONSTRAINT CHILD_PK PRIMARY KEY (PK) ENABLE );
insert into parent values (1,'a');
insert into parent values (2,'b');
insert into child values (1,1,'a');
insert into child values (2,2,'b');
insert into child values (3,3,'c');
Create a foreign key constraint:
alter table child add constraint fk_parent foreign key(pk_parent) references parent(pk);
SQL Error: ORA-02298: Kan (ROB.FK_PARENT) niet valideren - bovenliggende sleutels zijn niet gevonden.
02298. 00000 - "cannot validate (%s.%s) - parent keys not found"
*Cause: an alter table validating constraint failed because the table has
child records.
*Action: Obvious
Create the foreign key with 'enable novalidate' option
alter table child add constraint fk_parent foreign key(pk_parent) references parent(pk) enable novalidate;
table CHILD altered.
insert into child values (4,4,'c');
SQL Error: ORA-02291: Integriteitsbeperking (ROB.FK_PARENT) is geschonden - bovenliggende sleutel is niet gevonden.
02291. 00000 - "integrity constraint (%s.%s) violated - parent key not found"
*Cause: A foreign key value has no matching primary key value.
*Action: Delete the foreign key or add a matching primary key.
No new data violating the FK can be inserted.
Now let's fix the data already in the table that violates the FK constraint
Create an exceptions table and try to enable the constraint:
create table exceptions(row_id rowid,
owner varchar2(30),
table_name varchar2(30),
constraint varchar2(30));
ALTER TABLE child ENABLE constraint fk_parent EXCEPTIONS INTO EXCEPTIONS;
Error report:
SQL Error: ORA-02298: Kan (ROB.FK_PARENT) niet valideren - bovenliggende sleutels zijn niet gevonden.
02298. 00000 - "cannot validate (%s.%s) - parent keys not found"
*Cause: an alter table validating constraint failed because the table has
child records.
*Action: Obvious
Check the exceptions table for problems:
select * from exceptions;
ROW_ID OWNER TABLE_NAME CONSTRAINT
------ ------------------------------ ------------------------------ ------------------------------
AABA78 ROB CHILD FK_PARENT
AAFAAA
Ow9AAC
select * from child where rowid = 'AABA78AAFAAAOw9AAC';
Fix the problem
delete from child where pk = 3;
1 rows deleted.
ALTER TABLE child ENABLE constraint fk_parent EXCEPTIONS INTO EXCEPTIONS;
table CHILD altered.
Constraint enabled and data correct