Question

While reading this link, I did not understand the point on avoiding table locks by adding indexes on the foreign key column. Can anyone please help me understand the point?

Additionally, I never knew of the importance of adding indexes to foreign key before reading the same link(thanks to Thomas Kyte and the forum!). Can anyone please point me to more such important implementations that I need to consider when implementing foreign keys.

Answer 1

Assume two tables, PARENT and CHILD. Assume that there is a foreign key constraint on CHILD.PARENT_ID referencing PARENT.PARENT_ID and that CHILD.PARENT_ID is a required (not null) column, and that no index exists on CHILD.PARENT_ID.

When updating PARENT_ID or deleting records from PARENT, Oracle needs to prevent new transactions from concurrently adding new records in CHILD with the values of PARENT_ID being removed. Oracle could use an index against CHILD.PARENT_ID to check to see what records, if any, may be affected – and to block new rows using the PARENT_ID being affected from being committed until the activity against PARENT from committing or rolling back.

However, no such index exists for this concurrency issue. So Oracle uses the only tool it has – locking the CHILD table completely until the operation against PARENT commits or rolls back.