I'm having a discussion with our DBA about adding an index and we seem to have different ideas about what the term 'selectivity' means.

He argues that because the index will have many duplicate values, it will not be selective. (In a table with 5M rows, only 2000 rows will have a unique value for that column, the rest will be NULL.)

I think it will be selective, because any query on that column will return only a single row.

I am trying to speed up an update query that uses the column in the where clause.

This is on a table with a lot of inserts and updates, so I understand there will be some overhead associated with index maintenance.

Any thoughts? Thanks.