Question

Reference:Oracle documentation “Oracle Database enforces all PRIMARY KEY constraints using indexes”

Q’s

1> I did not understand the reason behind having a constraint and a key(that would also implement the constraint i.e. in this case PRIMARY KEY constraint and PRIMARY KEY index)?

2> Should I implement a key or a constraint when building database tables?

3> Are there any scenarios where using one over the other would be a +ve/-ve factor?

Answer 1

I’m not sure that I understand the question.

In Oracle, when you declare a primary key constraint, Oracle will implicitly create an index to enforce that constraint if no suitable index already exists to enforce the constraint. It is not an “either/or” situation where you create one or the other– you’re always going to have both a primary key constraint and an index to enforce that constraint.

You do have a choice about whether to create the index separately prior to creating the constraint and telling Oracle to use the index to enforce the constraint or whether to allow Oracle to implicitly create the index. It generally doesn’t matter which approach you choose but there are things to be aware of.

1. If you create the index first, you have the ability to do things like specify the tablespace that the index will reside in. This won’t have a performance benefit but some DBAs prefer to organize tablespaces to separate tables and indexes.
2. If you create the index first, dropping the constraint in the future will not drop the index. That may be good or bad, it depends what the person that drops the constraint expects. It is useful, however, to be consistent so that if someone needs to drop constraints in the future that all constraints behave similarly.
3. If you create the index first, it is possible to use indexes that involve other columns to enforce the primary key. If you know, for example, that you want to have a composite index on your primary key and some other column, you can use that index to enforce the primary key rather than having potentially redundant indexes created, i.e.
4. If Oracle implicitly creates the index, the index name will match the constraint name. If you create the index first manually, the names may differ. It doesn’t technically matter whether the names match but you may have scripts that rely on a name match to link constraints to indexes.

An example that creates a non-unique composite index with a different name than the primary key constraint that is then used to enforce the primary key constraint.

SQL> create table foo( col1 number, col2 number );

Table created.

Elapsed: 00:00:00.01
SQL> create index idx_foo on foo( col1, col2 );

Index created.

Elapsed: 00:00:00.00
SQL> alter table foo
  2    add constraint pk_foo
  3        primary key( col1 )
  4        using index idx_foo;

Table altered.

Elapsed: 00:00:00.13