Explain the Index Only concept and how it affects query execution in databases.

The Index Only concept means that a database query can be executed using only data from the index, without accessing the main table data.

This is possible if all the columns needed for the query are contained in the index (for example, in a covering index).

Impact on query execution:

• Significantly speeds up execution, as the number of disk read operations is reduced.
• Reduces system load, as access to the main table is not required.

Example: if you have an index on columns (id, name), and the query only selects these fields, then the DBMS can use an Index Only Scan.