Question

Sorry for weird title, don’t know how to name the Q better. So:

I have 3 tables. EntityA, EntityB, AB. The classical many-to-many implementation.

Is there any possibility to create constraint that mandates ALWAYS to have at least one relation between A and B.

Example workflow:

a) Insert A, Insert B, insert relation, commit; SUCCESS
b) Insert A, Insert B, commit; FALSE

So the question is: is there any on commit trigger? Or something similar to.

Answer 1

There is no such thing as an ON COMMIT trigger. However, you can generally simulate the behavior of an ON COMMIT trigger using materialized views. In your case, you could

• create materialized view logs on the three tables
• create a fast-refreshable materialized view that joins the three tables
• create a constraint on the materialized view that raises an exception if there are any rows that fail the validation

When you commit, the materialized view refresh takes place. If a constraint on the materialized view fails, the commit fails.