What is index selectivity?

The selectivity of an index is a measure of the uniqueness of the values in the indexed column or set of columns. It indicates what proportion of the table's rows are covered by a unique index value.

High selectivity means the index contains many unique values, which allows for efficient data filtering and faster queries. Low selectivity occurs when there are many repeating values, making the index less useful, as queries using it return many rows.

For example, if a table has 1000 rows and an index on the "gender" column (male/female), the selectivity will be low (around 0.5), since there are only two values. If the index is on a unique identifier, the selectivity is close to 1.

In Go, this is important when working with databases and query optimization, to choose indexes with high selectivity to speed up searches.