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.