Which statement about NOT ENFORCED unique constraints is TRUE?
Which statement about NOT ENFORCED unique constraints is TRUE?
NOT ENFORCED unique constraints are informational constraints that help the query optimizer to select an optimal data access plan. Although they are not enforced, they provide valuable information to the optimizer about the uniqueness of the data, which can influence the selection of an optimal access plan.
The correct answer is B. In DB2® Version 10.5, support is added for NOT ENFORCED primary key and unique constraints. These informational constraints help the query optimizer to select optimal data access plans when index access to the data provides no additional benefit. If you know that the data conforms to these constraints, you can use the NOT ENFORCED capability to help achieve two goals: - Improve performance, primarily in insert, update, and delete operations on the table - Reduce space requirements that are associated with enforcing a primary key or unique constraint Reference : https://www.ibm.com/support/knowledgecenter/en/SSEPGG_10.5.0/com.ibm.db2.luw.wn.doc/doc/c0061153.html