Explain how index Scan can execute query
Index scan is an operation performed by a database query optimizer to execute a query efficiently. It involves using an index, which is a data structure that stores the values of a specific column or columns in a table, along with the reference to the corresponding rows in the table.
When an index scan is executed, the database engine follows these steps:
1. The query optimizer determines the best index to use based on the query conditions and available indexes. It considers factors such as selectivity (the proportion of rows that satisfy a condition), index size, and column cardinality (the number of unique values in a column).
2. The database engine retrieves the needed index pages from disk into memory. This is known as index access.
3. The query optimizer searches the index for the required data based on the query conditions. It uses techniques like binary search or B-tree traversal to locate the relevant index entries efficiently.
4. As the index scan progresses, the database engine retrieves the corresponding rows from the table based on the index entries. This step is known as data retrieval.
5. The retrieved rows are then processed and returned as the result of the query.
Index scans are advantageous because they can significantly reduce the amount of data that needs to be read from the table. By utilizing the index, the engine can quickly find and retrieve the required data without having to scan the entire table. This results in faster query execution times and improved overall performance.
However, index scans may not be suitable for every query. In some cases, a full table scan (scanning the entire table sequentially) may be more efficient, especially if the query needs to fetch a large portion of the table's data or if the index is not selective enough. The query optimizer considers these factors and chooses the most appropriate scan method based on the query and available resources.