Constraints, indexes, and triggers are enabled by default. The CREATE TABLE, ALTER TABLE, CREATE INDEX, and CREATE TRIGGER statements create database objects in enabled mode, unless you specify another mode. When a database object is enabled, the database server recognizes the existence of the database object and takes the database object into consideration while it executes an INSERT, DELETE, or UPDATE statement (or for Select triggers, a SELECT statement). Thus, an enabled constraint is enforced, an enabled index updated, and an enabled trigger is executed when the trigger event takes place.
When you enable constraints and unique indexes, if a violating row exists, the data manipulation statement fails (that is, no rows are changed) and the database server returns an error message.
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