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Don Burleson Blog 







Foreign key indexing tips

Oracle Database Tips by Donald BurlesonJanuary 6, 2015

Question:  I understand that enforcing referential integrity is important in Oracle and foreign key constraints are critical to maintaining parent child relationships.  However, I see discussion where they indicate that the rules for indexing foreign keys of a child table have changed over the releases of Oracle.

I am unclear why the rules for indexing a foreign key would change because of the release of Oracle?

Answer:  The changes in Oracle best practices for indexing foreign keys relates directly to Oracle's internal locking mechanism, specifically the duration of the lock, which may cause contention during heavy DML activity on the parent table.

Also see this script to find foreign keys with no indexes.

Even though Oracle allows you to define a foreign key without an index, it's the SQL workload that determines whether an index is required.  A "missing" foreign key index will manifest itself with unnecessary large-table full-table scans against the child table.

As a general rule, the need to define a foreign key implies that there will be SQL statements that require a join of the parent and child tables.  Without a foreign key index, this SQL could not perform a nested loops join, and a more expensive sort merge join might be required:


   cust_name, order_details
   cust  c,
   ordor o,
   c.cust_id = o.cust_id;

In general, an index is required on a foreign key when:

  • You see boatloads of "TABLE ACCESS FULL' operations against the child table when you perform DML against the parent table.
  • You update/delete against the primary key in the parent table

When you create a foreign key constraint, you can choose the "on delete restrict" option to ensure that a parent rows cannot be deleted while a child row still exists.  You can also implement on delete cascade to delete all child rows when a parent row is deleted.

Obviously, if you are using "on delete cascade" and you do not create an index on the child parent key the deletion of a parent row would require a full-table scan of the child table, to find and delete the child rows.  Likewise, and "on delete restrict" without an index would force Oracle to perform a full-table scan against the child table to see if any children exist.

Locking and foreign key indexes

Without an index on the child table's foreign key, table-level locking may occur when a parent row is updated.  Rampant author Jeff Hunter notes that there is a table-level locking issue prior to release 11g:

FOREIGN KEY (deptno)
REFERENCES dept (deptno);

Once this constraint is enabled, attempting to insert an "EMP" record with an invalid DEPTNO, or trying to delete a DEPTNO row that has matching "EMP" records, will generate an error.

However, in order to preserve integrity during the operation, Oracle needs to apply a full "table-level" lock (as opposed to the usual row-level locks) to the child table when the parent table is modified.

By creating an index on the foreign key of the child table, these "table-level" locks can be avoided. (for instance, creating a foreign key on "EMP.DEPTNO").

ON emp(deptno)

Keep in mind that you will often be creating an index on the foreign keys in order to optimize join and queries.

|However, if you fail to create such a foreign key index and if the parent table is subject to updates, you may see heavy lock contention. If ever in doubt, it's often safer to create indexes on ALL foreign keys, despite the possible overhead of maintaining unneeded indexes.


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