Question

After you finish converting entities in mike physical ERD into tables, it's necessary to evaluate where to apply unique indexes. in what case is it necessary to create unique indexes as constraints?

To improve query performance on tables with a lot of rows.

To enforce uniqueness on tables with surrogate primary keys

Answer 1

Unique indexes provide additional information helpful to the query optimizer.

Query optimizer uses this information to produce more efficient execution plans.

Unique index constarints are necessary to improve query performance on tables with a lot of rows

whereas surrogate primary key itself enforces uniqueness on tables. It identifies each rows of the table uniquley. No additional unique index constraint is needed for this.