Henrietta Dombrovskaya continues a series on indexing in PostgreSQL:
In the previous article we learned that the most helpful indexes are indexes with the lowest selectivity, which means that each distinct value in an index corresponds to a small number of rows. The smallest number of rows is one, thereby, the most useful indexes are unique indexes.
The definition of a unique index states just that: an index is unique if for each indexed value there is exactly one matching row in the table. PostgreSQL automatically creates a unique index to support any primary key or unique constraint on a table.
Read on for more about unique indexes, compound indexes, and bitmaps.