In the event that the column AGE and CITY are not
indexed, MYSQL will have to scan the whole table for AGE.
Then, it scans the whole table to do sorting GROUP BY
CITY. This makes the selecting and sorting processes a slow
one. Since index is always sorted, similar to that of an index in
an encyclopedia, the MYSQL does not need to do a whole
table look up. Therefore, an index on AGE and CITY will
speed up the process.
However, we need to understand that different storage
engines offer different indexing strategies which are
summarized below: