What is a mutating and constraining table?
“Mutating” means “changing”. A mutating table is a table that is currently being modified by an update, delete, or insert statement. When a trigger tries to reference a table that is in state of flux (being changed), it is considered “mutating” and raises an error since Oracle should not return data that has not yet reached its final state. Another way this error can occur is if the trigger has statements to change the primary, foreign or unique key columns of the table off which it fires. If you must have triggers on tables that have referential constraints, the workaround is to enforce the referential integrity through triggers as well. There are several restrictions in Oracle regarding triggers: • A row-level trigger cannot query or modify a mutating table. (Of course, NEW and OLD still can be accessed by the trigger). • A statement-level trigger cannot query or modify a mutating table if the trigger is fired as the result of a CASCADE delete. • Etc.
