Subject Re: [ib-support] view on a table to speed up queries
Author Svein Erling Tysvær
low selectivity can be considered the opposite of unique. If your customers
table included a column for country, this would have low selectivity,
whereas phone number has high selectivity.

I think the reason for adding the primary key to the index for low
selectivity columns is simply to make it easier/quicker to identify each
individual row in the index.