What does a high degree of selectivity in database queries mean?

High selectivity in database queries means that the query filters data very efficiently, returning a small subset of records from the total.

For example, if a table has a million records and a query with a WHERE condition returns only 10 records, then the selectivity is high (10/1,000,000 = 0.001%).

High selectivity is important for query optimization because it allows the database to use indexes and quickly find the needed data, reducing load and execution time.

Low selectivity — when a query returns a large part of the table — usually leads to a full table scan and decreased performance.