Description: Selectivity in the context of SQL query optimization refers to the ability of a query to effectively filter rows from a table. A query with high selectivity returns a reduced number of rows compared to the total number of rows available, meaning it is more efficient in terms of performance. Selectivity is generally measured as the ratio of returned rows to the total rows in the table. The higher this ratio, the lower the selectivity. This concept is crucial for database design and writing SQL queries, as it influences index choice and how queries are executed. High selectivity allows database engines to use indexes more effectively, resulting in faster response times and more efficient use of system resources. Conversely, queries with low selectivity may lead to a full table scan, which can be costly in terms of time and resources. Therefore, understanding and applying the concept of selectivity is fundamental to optimizing database performance and improving the efficiency of applications that rely on them.
