Question

I have created script to find selectivity of each columns for every tables. In those some tables with less than 100 rows but selectivity of column is more than 50%.
where Selectivity = Distinct Values / Total Number Rows

So, is those column are eligible for index?
Or, can you tell, how much minimum rows require for eligibility for creating index?

Answer 1

I think I understand what you are trying to accomplish by calculating a ‘Selectivity’ value for your data but you cannot apply the rule blindly.

In fact in for certain queries the ‘Selectivity’ value might be really low an index will still be very beneficial. For example:
Assume a ‘inbox’ table with millions of rows, these rows have a ‘Read‘ boolean field. In this case the distinct values over the number of rows will be really low. If most items are read most of the time then finding unread items with an index on this field will be very efficient.

Creating indexes index come at a cost. Although you get the benefit for reads, you pay for writes and disk usage.

I would rather recommend you profile your queries and index accordingly. You can also look at the data from sys.dm_db_missing_index_group_stats and other Dynamic management views that will give you insight on indexes usage (or missing) ones.