If all the data requested are in the index, Derby does
not have to go to the table at all. (See Covering indexes.)
For operations that require a sort (ORDER BY), if Derby uses
the index to retrieve the data, it does not have to perform a separate sorting
step for some of these operations in some situations. (See About the optimizer's choice of sort avoidance.)
Note: Derby does
not support indexing on columns with data types like BLOB, CLOB, and XML.